Guidebook for Post IEC

Post-conference field excursion 8th IEC, 2009 Xining, China

Ultrahigh-pressure metamorphism and tectonic evolution

from oceanic subduction to continental collision in the North

Qaidam, northwestern China

Guidebook for the post-conference field trip of the 8th IEC August 31 to September 3, 2009

Edited by Shuguang Song

Department of Geology, Peking University, Beijing 100871, China

Field trip leaders: Liang Liu (Northwest University, Xi’an) Danling Chen (Northwest University, Xi’an) Guibin Zhang (Peking University, Beijing) Jianxin Zhang (Institute of Geology, CAGS, Beijing) Shuguang Song (Peking University, Beijing) Jianjun Yang (Institute of Geology and Geophysics, CAS, Beijing)

UHP metamorphism and tectonic evolution of the N. Qaidam UHP belt, NW China

- 1 -

1. Introduction High-pressure metamorphic rocks within orogenic belts record dynamic Earth processes of subduction and exhumation of both oceanic and continental lithospheric materials. Paleo-subduction zones identified within continents may be conveniently divided into oceanic-type and continental-type in terms of lithological assemblages (Song et al., 2006), which are equivalent to the Pacific-type and Alpine-type (Ernst, 2001) or B-type and A-type (Maruyama et al., 1996) subduction zones in the literature. The general notion is that, after oceanic lithosphere is totally consumed, the continental portion of the same lithosphere (i.e., in the case of passive continental margins) continues to subduct to depths greater than 80 km before exhumation as a result of oceanic lithosphere broke-off at depth. The Early Paleozoic Qilian–Qaidam orogeny (e.g. Song et al., 2006) are interpreted as examples of such processes. Important new occurrences include the Qilian lawsonite blueschist and eclogites which required an extremely cold subduction of the Early Paleozoic oceanic lithosphere (Pre-conference field trip), and the mixing of oceanic and supracrustal protoliths in the Qaidam continental subduction channel. These two Early Paleozoic sutures in northern Tibet provide additional constraints on the regional tectonic evolution of Paleo-Tethys in south-central Asia.

The North Qaidam is one of the newly-recognized ultrahigh-pressure (UHP) metamorphic belts in the world (e.g., Yang J.J. et al., 1994; Yang J.S., 1998, 2001, 2002; Song et al., 2003a,b, 2004, 2005a,b, 2006; Zhang et al., 2006; Mattinson et al., 2006, 2007). This orogenic belt has received much attention over the past decade. A significantly improved understanding of the tectonic evolution of the orogenic belt is now emerging thanks to detailed mineralogical, petrologic, geochemical and geochronological studies as well as field observations carried out over the past few years. This allows us to fully understand the tectonic evolution from oceanic subduction to continental subduction/collision in the Early Paleozoic time.

A special issue entitled “Tectonics and HP-UHP Metamorphism of Northern Tibet”, which was published in Journal of Asian Earth Sciences (Volume 35, Issues 3-4, Pages 191-376, July 2009) and edited by J.G. Liou, W. Gary Ernst, S.G. Song and B-M. Jahn, has been prepared for the 8th International Eclogite Conference (IEC), August 2009 Xining, China. Most papers in this issue summarize results of many current petrological-geochemical-geochronological and tectonic studies of both Qilian Pacific-type and Qaidam Alpine-type lithotectonic assemblages.

This guidebook has been prepared for the field excursion to the North Qaidam

Post-conference field excursion 8th International Eclogite Conference, 2009 Xining, China

- 2 -

UHP metamorphic belt, NW China coming after the 8th International Eclogite Conference in Xining. The purpose of this field trip is to examine the well-preserved and wide-spread ultrahigh-pressure metamorphic rocks including eclogite, pelitic and granitic gneisses and garnet peridotite, as well as associated post-collisional granites in this belt, and further understanding the evolutional history of the continental deep subduction and exhumation in the Early Paleozoic in the northern Tibetan Plateau. Meanwhile, this trip will play an important role in promoting the geological studies more profoundly in this region.

This trip will focus on the petrology and rock assemblages and ultrahigh-pressure metamorphism of the continental subduction belt in the North Qaidam region. Six major aspects in five stops are planned: (1) UHP metamorphic oceanic ophiolite sequence including UHP metamorphosed abyssal harzburgite, banded kyanite eclogite (cumulate gabbro) and massive eclogite within the continental subduction terrence (Stop 1); (2) granitic gneiss with protolith forming ages of 900-1000 Ma (Stops 1 and 5); (3) UHP pelitic gneiss (Stops 1, 3 and 5); (4) eclogite blocks within gneisses of various protoliths (Stops 1, 3 and 5); (5) UHP garnet peridotite and garnet pyroxenite; (Stop 4); and (6) Post-collisional I-type granodiorite and granite (Stop 2).

The examined area is located in territory of Qinghai Provence, the north margin of the Qaidam Basin, northern Qinghai-Xizhang (Tibet) Plateau. Its geographical coordinate is about 94°20′--101°00′ east longitude and 36°20′--39°50' north latitude. The trip will start at Xining City, via Qinghai Lake (the largest inland salt lake in China), Chaka, Dulan, Golmud, Da Qaidam, and will end in Dunhuang City, Gansu Provence (Fig. 1). The total length of the journey is over 1500 km. The general height Altitude along this trip ranges from ca. 2000 to 3800 meter high above sea level and most between 3000~3500 m. The weather will be dry and cool in the early September with strong sunshine, but be cold if raining, so a coat would be needed in the field. Stop 1 is along the main road, but other stops involve extended walks, where field boots are necessary.

From Xining to Qaihai Lake, the landscape is beautiful for the vast pasture land and mountains dotted with white sheep and dark yaks. But from Dulan to Golmud and further to Altyn Tagh (mountains), the trip will go along the north margin of the Qaidam Basin and the land view is mainly inanimate Gobi with sand and stone.

The itinery of the field trip is summarized below, and the route of the field trip is

shown in Figure 1.


- 3 -

Fig. 1. Field trip map showing the route and stops

August 31, 2009. The trip will depart from Xining in the early morning of the 31th August, via the 100-km-long Qinghai Lake, Chaka to Dulan. UHP metamorphic oceanic ophiolite sequence and various gneisses (Stop 1) will be examined on the route to Dulan. Night in Dulan county-town, Qinghai Province.

Sepember 1sh 2009. Depart from Dulan in the morning at about 8:30. The trip will go back to Stop 2 to examine the post-collision granodiorite and granite between the north Dulan belt and the south Dulan belt. Night in Golmud City, Qinghai Province.

Sepember 2nd 2009. Depart from Golmud at 8:30 in the morning. Drive towards north across the 10-km-long salt-paved road to Xitieshan and Luliangshan. Eclogite and garnet-kyanite schist (Stop 3) and garnet peridotite (Stop 4) will be examined. Back to Golmud in the afternoon.

Sepember 3nd 2009. Depart from Golmud at 8:30 in the morning on the same route towards Yuka River. Eclogite blocks and granitic and pelitic gneisses (Stop 5) will be examined. Continue northward across the Dangjinshan Pass (3680 m) and the famous Altyn Tagh Fault (the largest strike-slip fault system in NW China) to Dunhuang City, the end of the post-conference field excursion. Night in Dunhuang.


- 4 -

2. Regional Geologic Setting The Tibetan Plateau has long been considered to have formed as a result of the

subduction, collision and postcollisional convergence of the Paleo- + Neo-Tethys plate and its overriding continental lithosphere (e.g., Tapponnier et al., 2001; Royden et al., 2008). Successive accretions have shortened the crust at least 80 km and created the magnificent mountain ranges in south-central Asia. As the “Roof of the World”, the Tibetan Plateau stands 5 km high over a region of approximately 3 million km2. The geologic-tectonic evolution related to the flow of crust and mantle beneath the plateau and its effect on the Neogene climate has been the subject of many scientific investigations (e.g., Clift et al., 2008; Liou et al., 2009).

Fig. 2. Distribution of major tectonic units in the Tibetan Plateau; these include various continental

blocks, HP and UHP suture zones and metamorphic belts (After Liou et al., 2009).

As shown in Fig. 2, the Altun-Qilian-Qaidam orogenic system at the northern border of the Himalayan-Tibet Plateau is bounded by the Qaidam Basin to the south, the Tarim Basin to the west, and the Sino-Korean craton to the east. This region includes several orogenic belts truncated by the Altyn Tagh fault, one of the largest strike-slip fault systems in the world. According to the similarities in many aspects between the blocks on the opposite sites of the Altyn Tagh fault, including disposition of lithologic units, occurrences of both Pacific- and Alpine-type HP and UHP belts, and ages of HP-UHP metamorphism, it has been suggested that ~400 km of left-lateral


- 5 -

displacement occurred along this profound break (e.g., Yue and Liou, 1999; Yang et al., 2001, Zhang et al., 2001). To the east, the Qilian orogen, > 300 km wide, extends from the Altyn Tagh fault southeastward for ~1000 km to the Qinling-Dabie orogen, and forms a major geographic-tectonic boundary between North and South China. Both Qilian Pacific-type and Qaidam Alpine-type sutures are main topics of this special issue of papers dealing with tectonic and metamorphic characteristics of the HP-UHP rocks.

Fig. 2. Schematic maps showing major tectonic subunits of the Qilian–Qaidam orogenic belts in the

northern Tibetan Plateau (b) (After Song et al. 2006)

The Qilian–Qaidam Mountain region consists of the earliest subduction belts in the northern Qinghai-Tibet Plateau that may represent tectonic evolution of Paleo-Tethys from oceanic subduction to continental subduction/collision. From north to south, five tectonic units extend subparallel in E-W trending (Fig. 3); they are (1) the Alashan Block, (2) the North Qilian oceanic-type Suture Zone, (3) the Qilian Block, (4) the North Qaidam continental-type UHPM Belt, and (5) the Qaidam Block.

2.1. Alashan Block The Alashan Block in the western part of the North China Craton consists

predominantly of early Precambrian basement with 1.9 Ga granitic gneiss (Xiu et al., 2004) and 1.7-2.7 Ga detrital zircons (Geng et al., 2007; Tung et al., 2007), and is overlain by Cambrian to middle Ordovician cover sequences (Bureau of Geology and Mineral Resources of Ningxia Province, 1990). It was intruded by a ~ 830 Ma Cu-Ni-bearing ultramafic body (Li et al., 2005). The Qilian Block, located between the


- 6 -

North Qilian Suture Zone and the North Qaidam UHPM Belt, is an imbricate thrust belt of Precambrian basement overlain by Paleozoic sedimentary sequences. The basement consists of granitic gneiss, marble, amphibolite and minor granulite. Granitic gneisses from the Qilian Block have protolith ages of 880–940 Ma (Wan et al., 2001; Wu et al., 2007; Tung et al., 2007), similar to ages of the granitic gneisses in the North Qaidam UHPM Belt. Some Paleoproterozoic granitic gneisses of ~ 2470±20 Ma have been recognized recently in the Qilian Block (Chen et al. 2007).

2.2. North Qilian suture zone The North Qilian Suture Zone is an elongate, NW-trending belt that lies between

the Alashan Block (north) and the Qilian Block (south) (Fig. 2). This suture zone contains Early Paleozoic ophiolite sequences, HP metamorphic belts, island-arc volcanic rocks and granitic plutons, Silurian flysch formations, Devonian molasse, and Carboniferous to Triassic sedimentary cover sequences (Wu et al., 1993; Feng and He, 1995; Song, 1996, 1997).

2.3. Qilian Block The Qilian Block, located between the North Qilian Suture Zone and the North

Qaidam UHP Belt, is an imbricate thrust belt of Precambrian basement overlain by Paleozoic sedimentary sequences. The basement consists of felsic gneiss, marble, amphibolite and localized granulite. Wan et al. (2001) reported 910–940Ma single-zircon ages in granitic gneisses from different regions of the north part of the Qilian Block, which were interpreted as protolith formation ages and are consistent with the ages of granitic gneisses from the North Qaidam UHP Belt (see below).

2.4. North Qaidam UHP belt The North Qaidam UHPM belt is located between the Qilian Block in the north

and Qaidam Block in the south. It trends NWW and extends from Dulan northwestward, through Xitieshan and Lüliangshan, to Yuka for about 400 km (see Fig. 1) where it is offset from the equivalent Altun UHPM belt by the Altyn Tagh Fault, a large NE-striking left-lateral strike-slip fault in western China. The North Qaidam/Altun UHPM belt mainly consists of granitic and pelitic gneisses intercalated with blocks of eclogite and varying amounts of ultramafic rocks, especially garnet peridotite. The rock assemblages suggest that this belt is typical of a continental-type subduction zone (see Yang et al., 2002; Song et al., 2003a,b, 2005, 2006, 2009; Zhang et al., 2006; 2008; Mattinson et al., 2006, 2007, 2009), which differs from the “cold”, oceanic-type subduction of the North Qilian suture zone (Wu et al. 1993; Song et al.,


- 7 -

2007, 2009; J. Zhang et al., 2007). Coesite inclusions have been identified in zircon and garnet from metapelite and

eclogite at Dulan (Yang et al., 2001, 2002; Song et al., 2003a,b; Zhang G. et al., 2009; Zhang J. et al., 2009) and diamond in zircon from the garnet peridotite at Lüliangshan (Song et al., 2005a), respectively. P-T estimates of the enclosing eclogite and garnet peridotite establish the North Qaidam eclogite belt as an Early Paleozoic UHPM terrane exhumed from depths >100-200 km.

2.5. Qaidam Block The Qaidam Block to the south is a Mesozoic intra-continental basin with strata

deposited on the Precambrian crystalline basement, which may have an affinity with the Yangtze Craton (Zhang et al., 2003).

3. Dulan UHP terrane

The Dulan UHP terrene is located at the southeast end of the North Qaidam UHP belt. It has been extensively studied in terms of petrology, geochemistry, and zircon geochronology by diference groups of authors in the last decade (Zhang X., 1999; Yang et al., 2000, 2001; Song & Yang 2001; Song et al., 2003a,b; 2006, 2009; Mattinson et al., 2006, 2007, 2009; Zhang G. et al., 2005, 2008, 2009; Zhang J. 2009). This terrane is the first place where coesite inclusions in zircon grains from metapelite and coesite-pseudomorphs in eclogite were identified (Yang et al., 2001, 2002; Song et al., 2003a,b), which corroborated that the North Qaidam is a typical continental subduction complex exhumed from depths > 80–100 km.

On the basis of spatial relations, and the parageneses and compositions of eclogitic minerals, two distinct sub-belts, namely the South Dulan Belt (SDB) and the North Dulan Belt (NDB), are recognized (Song et al., 2003a; Fig. 3). The northern belt crops out in an area of about 100 km2; and is partly covered by Quaternary sediments in the east and intruded by a large diorite-granite pluton in the west. The south belt, extending for more than 30 km, is restricted to a belt of 2–5 km wide and bounded by two thrust faults. The Dulan eclogite-bearing terrane consists of eclogite, garnet amphibolite (retrograde eclogite), mafic granulite (granulitized eclogite), ortho- and para-gneisses, marble, garnet-free peridotite and garnet-bearing pyroxenite. They constitute ophiolite suites in two cross-sections described below. Eclogite blocks in the NBD are well preserved but most are difficult to access except for the Shaliuhe section (Stop 1) along the main road to Lhasa.


- 8 -

Fig. 3. Geological map of the Dulan UHP terrane (Modified after Song et al., 2003)

3.1. Rock assemblage and petrography 3.1.1. Eclogite

Eclogite in the North Dulan belt occurs as lens-shaped blocks of various-sizes within both granitic and Al-rich pelitic gneisses; some larger ones are very fresh but most have retrogressed during exhumation and fluid alteration. The largest block is up to 300×1000 m in size, intercalated in pelitic gneisses in the NDB. The NDB eclogites consist of peak assemblages of Grt + Omp + Rt + Phn + Coe (Qtz pseudomorph) + Zrn + Ap and Grt + Omp + Ep + Ttn + Cal (Arg) + Zrn, and display distinct, retrograde low Jd-content CpxII + Ab (symplectite) and amphibolite facies assemblages of parasitic Hbl + Pl + Ep ⁄ Czo in some blocks. Most fresh eclogites show granoblastic texture, but the modal content of minerals varies in different blocks. 3.1.2. Pelitic and granitic gneisses

Paragneiss consists of garnet- (kyanite-) bearing muscovite-biotite schist, garnet-free muscovite–biotite schist and minor muscovitebearing quartzite. Large eclogite blocks are always intercalated with meta-pelitic rocks. The pelitic gneiss or


- 9 -

schist contains garnet (5–8%), biotite (5–10%), muscovite (10–40%) in addition to quartz and sodic plagioclase. Garnet is Alm-rich (up to 80 mol%); kyanite appears in many pelitic gneisses in both sub-belts and is replaced by muscovite. REE-rich allanite also occurs in some pelitic gneiss.

Fig. 4. Eclogite blocks and pelitic and granitic gneisses in the north Dulan belt. But these eclogite

outcrops are difficult to approach.

The granitic gneiss is a major rock assemblage and occupies about 60–80% of the

Dulan eclogite-gneiss UHPM terrane. The gneisses are white to pale grey and show medium- to coarse-grained granoblastic texture with strong foliation. They consist of K-feldspar, plagioclase, quartz, muscovite, and tourmaline, and lack garnet.


- 10 -

3.1.3. Maffic granulite in the SDB All SDB eclogites have been overprinted by granulite-facies metamorphism and

formed maffic granulite (garnet-clinopyroxinite) and tonalitic melts. The maffic granulite generally shows homogenous, medium- to coarse-grained granoblastic texture. Based on reaction textures, three main metamorphic stages are recognized: (1) a peak eclogite facies stage with an assemblage of Grt + Omp + Rt + Ky; (2) a granulite facies stage with development of Grt + CpxII + Pl ± Scp; and (3) an amphibolite facies stage with Hbl + Czo + Bt + Ab.

3.2. UHP metamorphosed ophiolitic sequence in the NBD

3.2.1 Ophiolite-like sequence in Yematan section (Section A) Garnet-free, strongly serpentinized peridotites also occur in the North Qaidam

UHPM belt, as blocks of varying size. Song et al. (2003b) noted that an 80-meter-thick garnet-free peridotite block occurs with garnet-bearing pyroxenite and eclogite in the Yematan section (Fig. 5). The garnet pyroxenite was interpreted to be an ultramafic cumulate with high MgO (18.8 wt %), Cr (1095 ppm) and Ni (333 ppm), while eclogite blocks are geochemically similar to present-day N-type to E-type MORB. The rock assemblage most likely represents segments of an oceanic lithosphere (i.e., ophiolitic) from mantle peridotite to Mg-rich cumulate, Ca-rich gabbro, and to basaltic lavas. The garnet-free peridotite block is strongly serpentinized with no relics of primary minerals preserved.

Fig. 5. Two representative cross‐sections showing fragments of eclogite‐facies metamorphic

ophiolite sequence.


- 11 -

3.2.2 Ophiolite-like sequence in Shaliuhe Section (Section B, Stop 1) We have reported another large ultramafic block (~ 400×800 m in size) along the

Shaliuhe cross-section in the south Dulan belt (Zhang et al., 2005, 2008). Figure 5 shows that the northern part of Section B consists of three rock types: (1) kyanite-eclogite, (2) serpentinized harzburgite and (3) garnet-bearing pyroxenite and olivine pyroxenite. The peridotite block is dark-colored, strongly serpentinized and is apparently conformable with kyanite-eclogite and pyroxenites (Figs. 6a-c). Most samples in this peridotite block were entirely serpentinized/altered and made up of serpentine, talc, anthophyllite etc. Magnetite occurs at boundaries of olivine pseudomorphs (Fig. 6d). Relic olivine, opx/opx pseudomorphs and chromite were found in some samples (Fig. 6d-f).

Two generations of olivine have been recognized in the Shaliuhe peridotite (Zhang et al., 2005): relict olivine (Ol1) and metamorphic olivine (Ol2). The first generation of olivine (Ol1) occurs as small relict crystals among serpentines, and some crystals retain clear kink-bands (Fig. 6e). EMP analysis shows that the relict olivine has a narrow range of Fo content from 0.883 to 0.915 and relatively high NiO content from 0.28 to 0.46, which is consistent with the olivine compositions from the present-day abyssal peridotite (Fig. 9a). The second generation (Ol2) occurs as large and dirty crystals with a dense cluster of tiny fluid inclusions. EMP analysis shows these olivines have extremely high Fo content of 0.943−0.966 but low NiO content (0.21−0.35 wt%) (Fig. 9a), which is consistent with an origin through recrystallization of serpentines during subsequent metamorphism (i.e., eclogite-facies?; Zhang G. et al., 2008). Olivine in the cumulate olivine-pyroxenite has a narrow Fo range (0.88−0.90) and low NiO content (0.25−0.39 wt %).

Opx in the Shaliuhe harzburgite occurs as relic crystals surrounded by talc (Tc) + anthophyllite (Ant) corona (Fig. 6f) and some occur as Opx pseudomorphs completely replaced by talc and serpentine (Fig. 6e). High concentrations of Cpx lamellae occur in the relic Opx crystals, suggesting high CaO-content in the original host Opx that should be stable at magmatic conditions (> 1100 °C, Lindsley, 1983; Niu, 1999). EMP analysis shows that Opx from the Shaliuhe harzburgite has high Al2O3 (2.69−4.63 wt%) and high and constant Mg# (0.908−0.917), which is within the compositional range of Opx from present-day abyssal peridotites, but differs from Opx in the Lüliangshan garnet peridotite (Fig. 9b).

The kyanite-eclogite retains a banded structure that is most probably at least partly inherited from original gabbroic cumulate bands (Fig. 7a). Geochemical analyses


- 12 -

further reveal that this banded kyanite-eclogite has characteristics of cumulate gabbro (or more troctolitic) by high contents of Al2O3 (17.2–22.7 wt%), CaO (12.5–13.5 wt%), MgO (7.2–13.5 wt%), Cr (422–790 ppm), Ni , Sr, and low TiO2 and strong positive Eu anomalies (Eu* 1.51–2.08) (Zhang et al., 2008).

Fig. 6. Photographs showing ophiolitic harzburgite and cumulate in the Shaliuhe cross‐section. (a)

Field occurrence of serpentinized harzburgite, banded kyanite (Ky)‐eclogite and olivine pyroxenite

in the north end of the Shaliuhe section. (b) Field occurrence of serpentinized harzburgite,

banded kyanite (Ky)‐eclogite and garnet pyroxenite in the middle part of the Shaliuhe section. (c)

Serpentinized harzburgite. (d‐f) photomicrographs showing relic olivine (Ol1) and orthopyroxene

(Opx) and its pseudomorphs (Opx‐ps).

Olivine pyroxenite and garnet-bearing pyroxenite also show banded structure (Fig. 7b). The garnet-bearing pyroxenite has retrograded into garnet amphibolite without plagioclase. The olivine pyroxenite shows massive coarse-grained inequireanular, cumulate-like textures with olivine occurring as intercumulus between cpx grains (Fig.


- 13 -

7c,d). The major primary minerals are cpx, opx and olivine. The cpx is overprinted by amphibole and the opx crystals are completely replaced by amphibole and talc. These rocks are best interpreted as an Mg-rich cumulate that forms the lower part of an ophiolitic cumulate sequence.

Fig. 7. Photographs of cumulate units of the Shaliuhe ophiolite sequence. (a) Banded structure of

kyanite‐eclogite; (b) Banded structure of garnet‐bearing and olivine pyroxenites. (c and d)

micro‐texture of olivine pyroxenite showing relation between olivine (Ol) and pyroxenes.

3.3. Metamorphic evolution and P–T estimations Textural relations and mineral paragenesis indicate that eclogites in both sub-belts of the Dulan region experienced multistage metamorphism, but their P–T paths are very different. P–T conditions for peak stage of HP-UHP metamorphism were estimated by geothermobarometers and established phase equilibia. 3.3.1. P–T conditions of the NDB Identification of coesite and graphite inclusions in zircon separates from the NDB paragneiss and garnet from the eclogite blocks constrains the pressure limit of peak metamorphism in a range of 2.7-3.5 GPa between the equilibrium curves for the coesite – quartz (Hemingway et al., 1998) and the diamond – graphite (Bundy, 1980) polymorphic transformations.

The peak stage assemblages in the NDB eclogites are (1) Grt + Omp + Phe + Rut


- 14 -

± Coe, and (2) Grt + Omp + Ep + Arg. P–T estimates using geothermobarometry of Ravna & Terry (2004) yielded peak metamorphic conditions at T = 631–687°C and P = 2.87-3.17 GPa. All peak-stage minerals in the NDB eclogites have experienced various degrees of retrograde recrystallization from UHP eclogite to amphibolite-facies conditions. The retrograde reactions include: coesite replaced by quartz, omphacite replaced by Cpx + Ab and further by Hbl + Ab symplectites, garnet and/or omphacite by amphibole and plagioclase, phengite by biotite and plagioclase, and rutile by titanite. The Cpx + Ab vermicular symplectite rims around omphacite also indicate decompression breakdown under ‘dry’ conditions. Temperature calculated by Grt-Cpx Fe-Mg exchange thermometer is about 638 °C for sample 99Y102, which is not significantly lower than the peak condition. The latter assemblage Hbl + Pl + Qtz ± Bi ± Ttn suggests that eclogites were subjected to amphibolite facies metamorphism and constrains P–T conditions around 600 °C and < 1.0 GPa. 3.3.2. P–T conditions of the SDB

Peak eclogite stage. All eclogite in the SDB are strongly overprinted by granulite facies metamorphism and eclogite-facies minerals, such as omphacite (Jd45-48) are preserved as inclusions in kyanite, suggesting similar UHP metamorphic conditions to the NDB eclogite.

Granulite overprint stage. Granulite stage metamorphism is characterized by the occurrence of coarse-grained plagioclase, low Jd-content clinopyroxene (CpxII) and SO4-bearing scapolite with equilibrium texture. Experiment suggests that meionite (3CaAl2Si2O8·CaCO3) is stable only above 875°C; and sulfate meionite, also stable only at elevated temperature, is favored by high pressure (Goldsmith & Newton, 1977). The sulfur-rich scapolites in metamorphic rocks are, in general, considered as characteristic and diagnostic of granulite-facies assemblages (Lovering & White, 1964, 1969).

Two Fe2+-Mg exchange geothermometers (Powell, 1985; Krogh, 1988), based on the Fe2+-Mg partitioning (KD) between garnet and clinopyroxene, are used to estimate the temperature. Fe3+ is calculated by stoichiometric charge balance of Droop (1987), because the minimum estimate for Fe3+ (= Na–Al–Cr) cannot be applied to clinopyroxene with significant Al2R–1Si–1 such as clinopyroxenes of the granulite facies (Banno et al., 2000). Garnet is assumed in equilibrium with CpxII; it contains lower MgO and higher FeO contents than the peak eclogitic garnet (Fig. 6A). These two thermometers yield the same temperature, in the range 874–947 °C (Krogh, 1988)


- 15 -

and 882–948 °C (Powell, 1985) at 2.0 GPa, which are higher than the maximum temperature of the peak eclogite stage. The GADS barometers (Newton & Perkins, 1982; Eckert et al., 1991), based on the equilibrium reaction 2Grs + Prp + 3Qtz = 3An + 3Di, were used to evaluate pressures for granulite-facies recrystallization; they yield concordant results of P = 1.86–1.98 GPa.

Amphibolite facies retrogression stage. The amphibolite facies retrogression is marked by various degrees of amphibolitization in all eclogite blocks and by the retrograde assemblage Hbl + Ep/Czo + Pl + Bi + Qtz overprinted on the earlier assemblages. Garnet coexisting with these minerals possesses low MgO and high FeO contents on the rims. Using the Grt–Hbl thermometry of Graham and Powell (1984) and Grt–Hbl–Pl–Qtz barometer of Kohn & Spear (1990), we estimated temperature in the range 660-695°C and pressure 0.7-0.9 GPa.

Fig. 8. P–T paths of the NDB eclogites (a) and the SDB eclogites (After Song et al., 2003a)

3.4. Protoliths and metamorphic ages for the UHP rocks 3.4.1. Protolith and metamorphic ages of the Shaliuhe ophiolite

Zircon grains from the banded kyanite-eclogite (Stop 1) possess well-preserved magmatic cores and these cores gave 550-500 Ma with a mean age of 516±8 Ma by U-Pb SHRIMP dating (Zhang et al., 2008). This age should represent the protolith forming age of the oceanic crust, most probably the fragments of the Paleo-Tethys Ocean. The eclogite-facies metamorphic age of by the major mantle domains gave a mean age of 450 ± 7 Ma and 5 zircon rims gave an age of 426 ± 13 Ma (Fig. 9).


- 16 -

Fig. 9. Tera–Wasserburg (TW) diagrams for zircons from Shaliuhe kyanite eclogite

(Data from Zhang G. et al., 2008)

3.4.2. Metamorphic ages of the NDB gneisses and eclogite in Yematan Through dating the coesite-bearing zircon grains in the NDB pelitic gneiss from

the Yematan area by Using CL and SHRIMP technology, Song et al. (2006) determined a peak UHP metamorphic ages 423 ± 6 Ma and a retrograde age of 403 ± 9 Ma. Mattinson et al. (2009) also reported similar metamorphic ages of 426±4 and 430±5 Ma for two metapelite samples, similar to the result by Song et al. (2006). Plotting all zircon analyses in the Histograms diagram (Fig. 10a), a major peak at 426 Ma and weak peak at 407 Ma are obtained, which represent the peak UHP metamorphic age and retrograde age, respectively.

Dating of zircon separates from the NBD eclogite, on the other hand, is more complete than dating of zircons from metapelites. Song et al. (2006) reported zircon U-Pb analyses by SHRIMP: 15 zircon grains yield 206Pb/238U ages ranging from 440 to 482Ma with a mean at 457 ± 7Ma (MSWD = 0.91, Fig. 5b). This age is consistent with the whole-rock–garnet–omphacite Sm–Nd isochron ages (458 ± 10Ma and 459 ± 3 Ma). Eclogitic mineral inclusions such as garnet, omphacite and rutile, as well as


- 17 -

CL images, suggest that this age represents eclogite-facies metamorphism ~30-35 myr earlier than coesite-bearing UHP metamorphic ages.

Fig. 10. Histograms of zircon apparent 206Pb/238U ages of eclogite and ceosite‐bearing paragneiss

from Dulan terrane (data from Song et al., 2006; Mattinson et al., 2006, 2009).

Meanwhile, Mattinson et al. (2006) reported zircon SHRIMP ages for three eclogite samples from the Yematan area: one sample gave a mean at 452 ± 4 Ma, the second yielded ages ranging from 418-459 Ma with two age groups of 442±4 Ma and 432.5±5 Ma, and the third gaveages of 415-437 Ma with a mean of 423 ± 4 Ma. Therefore, two age groups can be distinguished in the histograms of all analyzed data:


- 18 -

473-443 Ma and 434-421 Ma (Fig. 10). These data allow us to infer that the North Qaidam UHPM belt may have experienced an evolutionary history from oceanic subduction before 440 Ma to continental subduction to depths >100 km at c. 423 Ma and exhumed at c. 403 Ma (zircon rim ages in pelitic gneiss) (Song et al., 2006).

3.5. Post-collision diorite-granodiorite-granite sequence

Granitic rocks can be generated in the various stages, e.g., the pre-, syn- and post-collision of the orogeny, and thus are the key to understanding history of plate subduction and exhumation. In the Dulan UHPM terrane, a zone of diorite-granodiorite-granite plutons occurs between the north and south Dulan belts (Fig. 3). These plutons show homogeneous medium to coarse-grained granitic texture without any deformation. Rock assemblage includes amphibole diorite, tonalite, grano-diorite and monzonitic granite, similar to magmatic assemblage in the active continental margin. In the TAS diagram, these rocks are sub-alkaline series and compositionally vary from andesite, dacite to rhyolite.

Pc B O1 O2 O3

F

U1

U2

U3

Ph

S1

S2

S3

T R

Ir

0

2

4

6

8

10

12

14

35 45 55 65 75

SiO2

Na2

O+K

2O

Fig. 11. TAS diagram showing the compositional variation of the post‐collision granite

3.5.1. Amphibole diorite As shown in Fig. 3, the amphibole diorite is a large pluton and consists of

amphibole (30 %), plagioclase (60), diopside (5-8%). Geochemistry of this pluton has not been well-studied and only one analysis (DL55) is available so far. This rock is characterized by low SiO2 (52 wt%), K2O (1.0 wt%) with high Mg-number [Mg/(Mg+Fe)] of 0.64. Zircon U-Pb dating by IA-ICP-MS gave ages ranging 350-367 Ma with a mean of 360 ± 3 Ma, much younger than the UHP metamorphic ages (423-432 Ma).


- 19 -

3.5.2. Tonalite Tonalite occurs as small plutons together with the grano-diorite and shows

pale-grey or offwhite colour. The rock consists of plagioclase, quartz, biotite, apatite and zircon. Geochemical analyses reveal that they have relative low SiO2 (56-66 wt%), TiO2 (0.46-0.84 wt%), K2O (1.4-1.6 wt%) and K2O/Na2O (0.34-0.50). Trace elements shows that they are typical arc rocks with Sr content of 278-344 ppm and Y 10.4-20.1 ppm without depletion of HREE, rather than adakitic magma (Fig. ). Zircon U-Pb dating by IA-ICP-MS gave two age groups: the major age group is ranging 391-407 Ma with a mean of 397 ± 5 Ma for tonalitc magma generation, and the other group is inherited zircon ages ranging from 884-2527 Ma (207Pb/206Pb age) that represent the Precambrian basement (Fig. ).

Typica l ARCRocks

Adaki te

0

100

200

300

400

500

0 10 20 30 40 50

Y

Sr/Y

Fig. 12. Y—Sr/Y diagram for the post‐collision granites

3.5.3. Granodiorite The granodiorite is the major component of the granitic pluton. It shows medium

to coarse-grained texture with large quantity of dark-coloured intermediate to basic amphibole-rich xenoliths. Geochemical analyses show that it has rather uniform contents of SiO2 (~ 65-66 wt%), TiO2 (0.5-0.65 wt%) and K2O/Na2O ratios (0.6-0.9). Mineralogy and geochemistry suggest that this granodiorite is the typical I-type granite. Zircon U-Pb dating by IA-ICP-MS for two samples gave ages of 380 ± 2 Ma and 393 ± 3.6 Ma, and zircon from diorite xenolith yielded 381 ± 2 Ma. 3.5.4. Monzonitic granite

The monzonitic granite is reddish-coloured and intrudes as small bodies into the


- 20 -

granodiorite pluton. It shows homogenous coarse-grained texture with two feldspars, quartz, biotite and amphibole. Two chemical analyses show strong Eu negative anomaly and very low contents of Y and HREE, and plot in the adakite region in the Y-Sr/Y diagram. Zircon U-Pb dating by IA-ICP-MS for two samples gave ages of 368 ± 2 Ma.

1.0

10.0

100.0

1000.0

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Roc

k/C

hond

rite

0.1

1.0

10.0

100.0

1000.0

Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

Rock/Prim

itive Mantle

Fig. 13. Chrondite‐normalized REE patterns and primitive‐mantle‐normalized spidergrams for the

post‐collision granite sequence

Red circle: amphibole diorite; diamond: granodiorit; triangle: tonalite; brown circle: granite


- 21 -

4. Xietieshan terrane

The Xitieshan eclogite-bearing terrane is located in the middle part of the Qaidam UHP belt, ~130 km north to the Golmud city. The average altitude of this region is about 3100~3400m. Xitieshan is famous for its lead-zinc deposit, and this ore deposit occurs in the green-coloured volcanic rocks (Ordovician?) bounded by fault to the eclogite-bearing terrane (Fig. 14).

Fig.14. Geological map of Xitieshan terrane

The Xitieshan terrane mainly consists of amphibolites, pelitic gneiss, granitic

gneiss and marble of the Dakendaban metamorphic group and is bounded by thrust fault with the lower Paleozoic volcanic and sedimentary rocks of the Tanjianshan Group (Fig.14). The two types of gneisses mainly consist of garnet-sillimanite-biotite gneiss (pelitic gneiss) and two-mica plagioclase gneiss (granitic gneiss). These gneisses are folded by SSE–NNW trending open folds. Sub-horizontal stretching lineations marked by oriented sillimanite and biotite suggest high-temperature deformation, possibly related to exhumation (Zhang J. et al., 2005). The Dakendaban


- 22 -

Group is intruded by early Paleozoic granites (428Ma, Meng et al., 2003) in the east part of the Xitieshan area. Petrologic and geochronologic investigations show that the Dakendaban Group is a complex related to early Paleozoic subduction and collision (Zhang et al., 2001b; Wan et al., 2001). Eclogites occure as boudins and lenses within granitic and pelitic gneiss (Fig. 15).

Fig.15. Field realationship between eclogite and its country rock

4.1. Eclogite

On the basis of mineral assemblage, two types of eclogite can be recognized, i.e., amphibole (Amp)-eclogite and phengite (Phn)-eclogite. Amphibole eclogite is wildly distributed, and strongly retrograde to garnet-amphibolites and amphibolites on the outcrop. Only a few fresh eclogite can be preserved in the center of the large boudins. Phengite eclogite is minor distributed in this area, which is only preserved in the location of Hangyanggou, which is also extensively retrograde to amphibolites (Zhang C et al., 2009).

Well preserved amphibole eclogite consists of garnet (35-40%), omphacite (30-40%), amphibole (10-15%) and minor quartz and rutile (Fig. 16a). Idioblastic to xenoblastic garnets (0.1-0.3mm) are coexist with omphacite and rutile, and always surrounded by retrograde Amp (pargasite) + Pl coronas. In addition, garnet porphyroblastics contain abundant mineral inclusions, which are mainly composed of omphacite and sodic-calcic amphibole, with minor rutile and quartz. Omphacite in the matrix is partially replaced by fine-grained vermicular symplectite of Cpx+Pl, which is partially changed to Amphibole+Pl in the intensive retrograde samples. Omphacite as the inclusions in the garnet is relatively well preserved. Only rim of the omphacite inclusion turn into sodic-calcic amphibole, which may be related to the melt or fluid during the exhumation of the eclogite. Coesite pseudomorph was identified as the inclusion of the omphacite (Fig. 16b) (Zhang C et al., 2009), but no coesite relict was


- 23 -

found in the eclogite so far. Phengite eclogtie is a minor component in this area. The peak assemblage of the

phengite eclogite is Grt + Omp + Phn + Qtz + Rt, and show coarse-grained, granular texture. Most of them consist of 30-40% modal of content of garnet, 10-15% omphacit, 2-5% phengite, 30-40% symplectite formed in the retrograde process and minor accessory mineral such as quartz, rutile and zircon. Most of phengite eclogite are retrograde to garnet-amphibolite, only few fresh samples can be preserved in the inner core of the eclogite lence. Phengite are all as inclusion wrapped in the garnet with muscovite rim whose Si had lost during the retrograde process. In the matrix, all phengite has been changed into biotite or biotite + plagioclase.

Fig. 16. Photomicrographs of eclogite in the Xitieshan area

a. Mineral assemblage of amphibole eclogite in Xitieshan. b. The polycrystalline quartz

inclusion in omphacite from Xitieshan amphibolites eclogites. c. Kyanite porphyroblast shows

undulatory extinction. d. Oriented biotite, sillimanite, quartz and feldspar define the foliation.

The peak metamorphic condition of the eclogite was calculated using

Grt–Omp–Phn geothermobarometer of Ravna and Terry (2004). Intersection of the Grt–Cpx thermometer and Grt–Phn–Cpx barometer curves yeild P=2.71-3.17 GPa, T=751-791 °C for the eclogite (Zhang C et al., 2009), which is consistent with the coesite pseudomorph in the omphacite.


- 24 -

4.1.2. Gneiss Pelitic gneiss and granitic gneiss from Dakendaban group compose most of the

rocks in Xitieshan area. The field relationship between the pelitic gneiss and granitic gneiss is difficult to determine but some authors suggest the paragneiss is intruded by the orthogneiss (Wan et al., 2006). The major pelitic gneiss is Garnet–kyanite (sillimanite)–biotite gneiss with an assemblage of Grt + Bt + Ky + Pl + Kfs + Qtz + Rt. The slightly deformed matrix of quartz and feldspar contains porphyroblasts of garnet and kyanite (Fig. 16c, d). Garnet composition is homogenous in the core with MgO and CaO decreasing, and FeO increasing towards to rim. Biotite has straight grain boundaries with kyanite, garnet, plagioclase and quartz. Rutile inclusions occur within garnet and kyanite porphyroblasts. Oriented rutile needles can also be recognized in garnet grains. Some kyanite grains show undulatory extinction and shape-preferred orientation parallel to the foliation. Plagioclase contains an An content 20–30 mol %, and plagioclase rim is slightly more calcic than the core K-feldspar contains a minor albite component (5–13 mol %) (Zhang J et al., 2009). Using the compositions of garnet core, plagioclase core and kyanite, the GASP barometry (Grs + 2Ky + Qtz = 3An) yields pressures of 9.5–11 kbar at 800. The Grt–Bt thermometry yields temperatures between 719 and 835 °C at 10 kbar (Ferry and Spear, 1978) for the pelitic gneiss (Zhang J et al., 2009). Granitic gneiss which is the country rock of most eclogites consist consists of K-feldspar + plagioclase + quartz + biotite and lacks evidence of high-pressure metamorphism.

4.2. Geochronology of the Xitieshan HP-UHP rocks Xitieshan as part of the North Qaidam HP/UHP metamorphic belt is interpreted

as an Early Paleozoic continental collision zone. Zircon U–Pb TIMS and SHRIMP analysis from eclogites in Xitieshan indicates eclogite facies metamorphism at 480~486 Ma, and magmatic crystallization age of protolith at 750~800 Ma. Ar/Ar ages of amphibole in retrograde eclogites suggest that the eclogites had cooled and exhumed to mid-crustal depths by 407 Ma (Zhang et al., 2005). Zhang et al (2008) obtain the SHRIMP age of eclogite between 426 Ma ~ 467Ma, and further divided into two group. One group with rare rutile inclusions gives the weighted mean age of 452 ± 12 Ma, which is the time of metamorphic zircon growth during HP granulite facies metamorphism. The other group containing amphibole and clinozoisite inclusions yields 430 ± 4 Ma as the age of amphibolites facies overprint.


- 25 -

Zircon TIMS dating of an orthogneiss enclosing retrograded eclogite gave an upper intercept of 952 ± 13 Ma and a lower intercept of 478 ± 43 Ma, interpreted as a magmatic crystallization age of the granitic protolith and an early Paleozoic metamorphic age, respectively (Zhang J. et al., 2006). Zircon SHRIMP dating for garnet-kyanite gneiss records the age of HP metamorphism at 461 Ma, and garnet-sillimanite gneiss yield the age between 423 ~ 430 Ma (Zhang J. et al., 2009). Therefore, Zhang et al. (2009) interpreted that the high pressure or even ultra-high-pressure metamorphism in the Xitieshan occurs at about 480Ma and granulite facies overprinting at ca. 450–423 Ma.

5. Luliangshan garnet peridotite massif

Garnet peridotite is volumetrically small yet a common component in ultrahigh-pressure (UHP) metamorphic terranes in many continental-type subduction belts such as the well-studied West Gneiss Region of Norway (e.g., O'Hara and Mercy 1963; Carswell, 1983; Medaris and Carswell, 1990) and the Dabie-Sulu terrane of eastern China (e.g., Yang et al., 1993, 2000; Zhang et al., 2000). This type of peridotite has received much attention for their special textures, mineral assemblages and ultradeep origin (Dobrzhinetskaya et al., 1996; van Roermund et al., 1998, 2001; Song et al., 2004, 2005a,b; Liu et al., 2005; Spengler et al., 2006). The observation that the orogenic garnet peridotite is exclusively associated with zones of continental collision, but is absent in zones of oceanic lithosphere subduction, has previously suggested a genetic affinity with continental subduction and subsequent continental collision. In contrast, garnet-free, strongly serpentinized peridotite is commonly believed to be restricted to oceanic-type subduction zones and is believed to represent the fragment of oceanic lithospheric mantle (Song et al., 2009b).

The Luliangshan garnet peridotite massif is a large ultramafic body in the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt, a 400-km-long continental collision belt of the Early Palaeozoic at the northern edge of the Tibetan Plateau. It was first reported by Yang et al. (1994) and further studied by Song et al. (2004, 2005a,b, 2007) and Yang & Powell (2008). Mineral exsolution lamellae of rutile + two pyroxene + sodic amphibole in garnet and ilmenite + Al-chromite in olivine (Song et al., 2004, 2005a) as well as the presence of a diamond inclusion in a zircon, suggest that this peridotite body experienced ultrahigh-pressure metamorphism at depths in excess of 200 km.


- 26 -

5.1. Occurrence and petrography

The Lüliangshan garnet peridotite occurs as a large (~ 500×800 m in size) massif, located in Lüliangshan area, ~ 20 km south of Da Qaidam town, and is hosted within an eclogite-bearing quartzofeldspathic gneiss terrane (see Figure 1 for its locality). This garnet peridotite massif comprises a wide range of lithologies from rocks dominated by olivine to those dominated by pyroxene. On the basis of field and petrographic observations, Song et al. (2005a, 2007) grouped the rocks into four types: (1) mostly garnet lherzolite with minor amounts of (2) garnet-bearing harzburgite/dunite, (3) garnet-free dunite and (4) garnet pyroxenite dikes/dikelets (see Fig. 17 for field occurrence). Figure 19 shows the reconstructed low-pressure (e.g., stable in the spinel peridotite field) (Niu, 1997) modes of these four rock types from their bulk-rock compositions. The garnet-free dunite plots in the harzburgite field, the garnet-bearing dunite at the boundary between harzburgite and lherzolite fields, garnet lherzolite in the fields of lherzolite and olivine websterite, and garnet pyroxenite mainly in the websterite field.

Fig. 17. (a) Geological overview and location map of the Luliangshan garnet peridotite body. (b)

Geological map of the garnet peridotite (after Song et al., 2004).


- 27 -

Fig. 18. Photographs showing field occurrences of various rock types of the Luliangshan garnet

peridotite massif. (a) Garnet lherzolite with layered structure; (b) garnet peridotite interlayered

with garnet–olivine pyroxenites, note that the rhythmic variation of garnet content in the

peridotite layers; (c) garnet‐free dunite and garnet peridotite; (d) garnet lherzolite with

subsequent foliations along which olivines were serpentinized; (e) garnet lherzolite interlayered

with garnet dunite, olivine in dunite is strongly serpentinized and shows dark colored layers; (f)

garnet pyroxenite dyke cutting garnet peridotite.

5.1.1. Garnet lherzolite

The garnet lherzolite constitutes ~ 70–80 vol % of the garnet peridotite massif. It is massive and coarse-grained without obvious foliations. All peridotites in the field occur as layers with various thickness (from meters to centimeters) and sharp


- 28 -

boundaries (Figs. 18a-d). Pyroxenes vary in abundance on various scales, and so the lithology could be termed as either garnet lherzolite or olivine-websterite. The main constituent minerals are garnet, olivine, orthopyroxene (Opx), clinopyroxene (Cpx) and minor Cr-rich spinel (Spl). Garnets are mostly porphyroblastic and vary in size (3 to 10 mm) and abundance (~ 5 to 15 vol %). Olivine shows a wide range of Fo values (i.e., Mg#, Mg/[Mg+Fe2+] = 0.84-0.91) and constitutes 40–60 vol % of the rock. Opx and Cpx take ~ 10-30 and 5-15 vol % of the rock, respectively. Fine-grained Cr-rich spinel (Cr#, [Cr/Cr+Al] = 0.60–0.69) is scattered fairly uniformly both in the matrix and as inclusions of major silicate minerals. Al-in-Opx geobarometry (Brey and Köhler, 1990) and Grt-Ol geothermometry (O’Neill and Wood, 1979) yield P = 5.0–6.5 GPa and T = 960–1040°C for the garnet lherzolite.

Fig. 19. IUGS rock classification (Streckeisen, 1976) of samples from the Luliangshan

metamorphic garnet peridotite using the protolith mineralogy (Olivine, Opx, Cpx plus minor

spinel) calculated from the bulk‐rock major element compositions using the procedure

developed for ultramafic rocks by Niu (1997)

5.1.2. Garnet-bearing dunite

The garnet-bearing dunite occurs either as layers varying in thickness from 10’s cm to up to 2 meters within the garnet lherzolite or as rhythmic bands that vary gradually from garnet-bearing dunite to harzburgite to garnet lherzolite. Garnet content varies widely in different layers. The rock is medium-grained and has an equigranular texture dominated by olivine (Fo90.6-92.0) (> 90 vol %), plus variable amounts of garnet, orthopyroxene (Mg# = 0.90–0.92), and clinopyroxene (Mg# = 0.94–0.96). Garnets are porphyroblastic and Mg-rich (69–75 mol % pyrope, 11–18% almandine, 3–8% grossular, 0.8–2.0% spessartine and 3–6% uvarovite). Fine-grained


- 29 -

Cr-spinel (Cr# = 0.61–0.65) occurs as product of decompression-induced breakdown of previous high-Cr garnet and pyroxenes. Al-in-Opx geobarometry (Brey and Köhler, 1990) and Grt-Ol geothermometry (O’Neill et al., 1979) yield P-T conditions of P = 4.6-5.3 GPa and T = 980-1130°C.

5.1.3. Garnet-free Dunite

Garnet-free dunite occurs apparently as layers and/or lenses varying in thickness of up to 10 meters with brown-colored weathering surfaces distinguished from black-colored garnet lherzolite and garnet-bearing dunite (Fig. 18b). It is medium-grained, dominated by strongly serpentinized olivine (> 90 vol %) with minor orthopyroxene and Cr-rich spinel. Magnetites are precipitated at the boundaries of these serpentinized olivine crystals and show a triple-junction texture. Olivine contains the higher Fo contents ranging 0.924 to 0.937, while spinel has higher Cr# (0.66–0.73) than that in garnet lherzolite.

5.1.4. Garnet pyroxenite

The garnet pyroxenite is a minor component, occurring as interlayers with the garnet peridotite, or as dikes cross-cutting the apparent layering of the massif (Fig. 18f). Most samples are fresh with pink garnet and pale-green pyroxene conspicuous in the field. The constituent phases are garnet (20–30 vol %), orthopyroxene (5–10%), clinopyroxene (40–60%) and phlogopite (2–5%) with no olivine observed. It shows a fairly uniform medium-grained granular texture. The garnet is also Mg-rich (62–68 mol. % pyrope, 21–24% almandine, 9.5–11% grossular, < 1% spessartine, 0.8–1.5% uvarovite). It shows a fairly uniform medium-grained granular texture. Most garnets are rimmed with a kelyphitic Opx+Cpx+Spl assemblage and some break down to granular-textured high-Al Opx, Cpx and Al-Spl, which are characteristic decomposition features. These observations suggest that these rocks have also once equilibrated at high pressures. Their occurrences as dikelets, lack of olivine, low MgO (~ 18%), low Mg# (~ 0.81), and the presence of minor phlogopite are all consistent with the garnet pyroxenite being cumulate from more evolved mantle wedge melts (Song et al., 2007).

5.2. Mineral composition

5.2.1. Garnet Most garnet crystals in the Lüliangshan garnet-peridotite and garnet-bearing

dunite are porphyroblasts with varying size (3–10 mm across). Almost all the garnet


- 30 -

crystals exhibit kelyphitized rims of clinopyroxene, orthopyroxene and spinel aggregates interpreted as resulting from decompression (Fig. 20), some being completely replaced by the kelyphitic Opx + Cpx + Spl. Pargasitic amphiboles appear at the outer circle of the kelyphite, suggesting a late retrograde metamorphic event.

Fig. 20. Kelyphite texture of garnet from the Lüliangshan garnet peridotite.

Electron microprobe (EMP) analysis shows that garnets from garnet lherzolite

and dunite are Mg-rich with a wide range of pyrope (58–74 mol %), almandine (13–25%), grossular (4–10%), spessartine (0.9–1.8%) and uvarovite (2–5%) in different samples but have a quite homogeneous composition from core to rim for a given crystal (Fig. 21). As expected, the garnet pyrope component correlates positively whereas almandine correlates negatively with the whole-rock Mg#, suggests that garnet is in equilibrium with coexisting minerals in the rock.

Fig. 21. Compositional profiles of garnets from garnet lherzolite samples.


- 31 -

5.2.2. Olivine Olivine from the Lüliangshan garnet peridotite shows a large compositional

variation in terms of Fo content (i.e., Mg# of olivine), which correlates positively with modal percentage of olivine. Olivine in garnet-free dunite has the highest Fo content of 0.927~0.937, which is slightly higher than that of garnet-bearing dunite (0.906~0.926). Fo content of olivine in garnet lherzolite, on the other hand, varies from 0.830 to 0.906. NiO concentrations vary from 0.34 to 0.55 wt%.

Fig. 22. Compositional variations of olivines (a) and orthopyroxenes (b) from the two types of

peridotite in the North Qaidam UHP belt, in comparison with olivines from present‐day abyssal

peridotite (shadowed field) (Dick et al., 1989). Fo‐content of olivines from the Luliangshan

garnet‐peridotite massif varies in a wide range from 83.0 to 93.7 mol%, whereas relic olivines

(Ol1) from the Shaliuhe harzburgite in a narrow range from 88.3 to 91.5 mol%, in consistent with

abyssal peridotite and olivines (Ol2) metamorphosed from the former serpentine have high

Fo‐content from 94.3 to 96.6.

5.2.3. Orthopyroxene

Opx in the Lüliangshan garnet peridotite also shows a large compositional variation in terms of Mg# (Fig. 22b). Opx from garnet-bearing dunite has high mg# (0.94 – 0.96) whereas Opx from garnet lherzolite has significantly lower mg# (0.87-0.93). Opx Mg# values are positively correlated with Fo values of coexisting olivine. All Opx crystals in the Lüliangshan garnet peridotite (including garnet lherzolite and garnet-bearing dunite) are extremely depleted in Al2O3. Al2O3 in Opx from lherzolite is low (0.38–0.66 wt%), and is slightly higher in dunite (0.55–0.73 wt%). Opx crystals from garnet pyroxenite have lower Mg# (0.86-0.90) and higher Al2O3 (0.88–1.17 wt %) than those from garnet peridotite.

5.2.4. Clinopyroxene

Cpx crystals in the Lüliangshan garnet peridotite are rich in Cr2O3 (0.6 to 1.6


- 32 -

wt%). Cpx Mg# varies from 0.91 to 0.96 and is positively correlated with Mg# of olivine and Opx. Al2O3 varies from 1.13 to 3.19 wt%, and Na2O from 0.43 to 1.32 wt%.

5.2.5. Spinel

Spinel crystals in the Lüliangshan garnet peridotite are secondary and occurs as

fine-grained (5–100 μm) euhedral crystals scattered fairly uniformly both in-between and as inclusions of major silicate minerals in garnet lherzolite and garnet-bearing dunite, or occurs as exsolved lamellae in porphyroblastic olivine and Cpx (Song et al., 2004, 2007). Cr# (Cr/[Cr+Al]) of the spinel varies from 0.54 to 0.73 and correlates positively with whole-rock Mg# (Song et al., 2007).

4.6. Phlogopite

Phlogopite mainly occurs in garnet pyroxenite and coexists with garnet and the two pyroxenes (Fig. 3f). However, phlogopite is not a primary phase of the fresh garnet peridotite. Very minor phlogopite is also observed in some strongly serpentinized garnet peridotite. Mg# of the phlogopite varies from 0.92 to 0.95 in the serpentinized garnet peridotites and from 0.87 to 0.89 in the garnet pyroxenites. TiO2 concentrations in all phlogopite crystals are < 1 wt %, and Al2O3 ranges from 17.1 to 18.2 wt %.

5.2.7. Amphibole

Amphiboles are the retrograde phase that overprints the primary mineral assemblage Grt + Opx + Cpx (+ Ol) in garnet peridotites and garnet pyroxenite. They are pargasitic in composition with low TiO2 (< 0.8 wt %), high Al2O3 (11.8–16.7 wt %) and MgO (16.8–18.7 wt %).

5.3. Exsolution lamellae in rock-forming minerals

Most high-P-T minerals are complex solid solutions. They formed originally as compositionally uniform homogeneous single crystals at high temperature and pressure. During decompression and cooling non-ideal solid solutions may become compositionally unstable. Hence, spinodal exsolution may separate an initially uniform single crystal in situ into physically and chemically distinctive crystals. In the Luliangshan garnet peridotite, decompression-induced exsolution textures are very common in all peak metamorphic minerals including garnet, olivine and two pyroxenes in garnet-bearing dunite/harzburgite, garnet lherzolite and pyroxenite (Fig. 23).


- 33 -

5.3.1. Exsolusions in Olivine

Exsolutions in olivine from the Luliangshan garnet peridotite are rods or needles of ilmenite and chromium spinel (Song et al., 2004). The ilmenite [(Fe,Mg)TiO3] rods (~20–100 µm long and 0.3–3 µm wide/thick; Fig. 23a) are densely packed and well oriented, parallel to [010] of the olivine host. The abundance of the ilmenite rods corresponds to a maximum of about 0.7 wt.% ilmenite in olivine. The exsolution of ilmenite and Al-chromite needles (Fig. 23b) from the olivine is consistent with the peridotite once being equilibrated at very high pressures (> 300 km) (Dobrzhinetskaya et al., 1996). Experimental data also indicate that dissolution of such high TiO2 content in olivines requires high temperature (>> 700°C) (Dobrzhinetskaya et al., 1999).

Fig. 23. Exsolution lamellae from major minerals from garnet peridotite and pyroxenite. (a)

Ilmenite rods in olivine; (b) Cr‐spinel needles in olivine; (c) pyroxene lamellae in garnet; (d) rutile

+ sodic amphibole lamellae in garnet; (e) Cr‐spinel needles in opx (BSE image); (f) Opx and

Cr‐magnetite lamellae in Cpx; (g) Cr‐spinel + Amp (+ quartz) lamellae in Cpx; (h) Opx + Amp

lamellae in Cpx (BSE image); (i) Amp + quartz + Cr‐magnetite lamellae in Cpx.


- 34 -

5.3.2. Exsolution in garnet Exsolution lamellae in garnets include densely packed rods of rutile, Opx, Cpx

and sodic amphibole (Song et al., 2004, 2005a). The pyroxene exsolutions suggest that their parental garnet host crystals originally contained excess silicon (Fig. 23c), i.e., they were majoritic garnet that is only stable at depths well in excess of 150 km (Ringwood & Major, 1971; Irifune, 1987). Similar textures were also described in garnet from the Norwegian peridotite (van Roermund & Drury, 1998). The exsolution of rutile and sodic amphiboles (Fig. 23d) further suggest that the inferred majoritic garnet also contains excess Ti, Na and hydroxyl that is only soluble at very high pressures of ≥ 7 GPa.

5.3.3. Exsolution in orthopyroxen

Decompression-induced exsolution needles are abundant in some porphyroblastic Opx (Fig. 8e). Energy-dispersive X-ray spectrometer (EDS) scanning reveals that those needles are Cr-rich spinel, suggesting that the original host Opx was rich in Cr.

5.3.4. Exsolution in clinopyroxene Clinopyroxene has complex exsolution products in various rock types.

Exsolutions in Cpx from garnet dunite and lherzolite include three lamella assemblages: 1) Opx + Cr-magnetite (Cmt), 2) amphibole + Cr-spinel and 3) Opx + amphibole rods/lamellae (Fig. 23f-h). Exsolutions in Cpx from garnet pyroxenite mainly consist of densely packed lamellae of amphibole, quartz, Cr-magnetite (Fig. 23i) and minor phlogopite. These exsolutions are interpreted as resulting from decompression (Song et al., 2004, 2005b), which points to originally high-Si, Cr and hydroxyl in Cpx at peak metamorphic conditions. BSE images show that 8-10 vol.% amphibole lamellae and up to 4 vol.% quartz rods are present in some Cpx crystals. This means that the parental Cpx was supersilicic and also contained a significant amount of hydroxyl (~ 2000-3000 ppm). Similar observations have been reported in the ultrahigh-pressure (> 6 GPa) Cpx of eclogites from the Kokchetav UHP terrane, Kazakhstan (Katayama & Nakajima, 2002) and of garnet peridotites from the Sulu terrane, eastern China (Chen & Xu, 2005).

5.4. Metamorphic ages

Garnet lherzolite, garnet-bearing harzburgite and garnet pyroxenite were selected for zircon SHRIMP dating (Song et al., 2005). Most zircons from the garnet lherzolite show rather complex zoning. One diamond and a few graphite inclusions are


- 35 -

identified in some zircons by Raman spectroscopy. SHRIMP dating on these zircons show four major age groups: (a) 484–444 Ma (weighted mean age, 457±22 Ma) for cores of most crystals, whose morphology and rare earth element (REE) systematics (i.e., very high [Lu /Sm]CN=88–230) suggest a magmatic origin, consistent with the protolith being magmatic cumulate; (b) 435–414 Ma with a mean of 423±5 Ma, which, given by mantle portions of zircon crystals, is interpreted to record the event of ultrahigh-pressure metamorphism (UHPM) at depths greater than 200 km in an Andean-type subduction zone (Fig. 24a); (c) 402–384 Ma (mean age 397±6 Ma) for near-rim portions of zircon crystals. Inherited cores in two zircon crystals were identified using CL and found to be Proterozoic. Morphology and CL images show that zircons from dunite and garnet pyroxenite are of metamorphic origin. The mean age of dunite zircons is 420±5 Ma, which overlaps the mantle age of the garnet lherzolite zircon (Fig. 24b). The mean age of garnet pyroxenite zircons is 399±8 Ma, which overlaps ages of near-rim domains in garnet lherzolite zircons. Some garnet pyroxenite zircons also recorded a retrograde event at 358±7 Ma.

Fig. 24. Histogram of zircon apparent 206Pb/238U age of garnet peridotite from Luliangshan, North

Qaidam UHP belt (data from Song et al., 2005).

Garnet lherzolite

Garnet dunite


- 36 -

5.5. Origin of protoliths and tectonic evolution

The peridotites show compositional layering that is largely defined by modal variations of major minerals (garnet, olivine, orthopyroxene and clinopyroxene). Rhythmic crystallization bands of the protoliths can be convincingly inferred in some outcrops (Fig. 2c). These textural observations, together with relic accumulate textures in garnet lherzolite, the documented major and trace element systematics, as well as magmatic zircon cores, strongly suggest that the Luliangshan garnet peridotite ultramafic cumulate complex crystallized from high-Mg melts in an arc environment before UHP metamorphism. Normative igneous assemblages calculated from the bulk-rock major element compositions (Niu, 1997) vary from harzburgite to lherzolite, and to olivine websterite consistent with increasing bulk-rock Al2O3 (also modal garnet) and decreasing bulk-rock MgO (also modal olivine) during crystallization (see fig. 12 of Song et al., 2007).

The bulk-rock trace element systematics is consistent with the protoliths being in equilibrium with subduction-zone melts (see fig. 11 of Song et al., 2007) or being contaminated by continental crust material in their petrogenesis. Their enriched isotopic compositions (e.g., Nd(t) < 　 - 0.5, - 6.8 ~ -0.5) and high enrichment in incompatible elements (Song et al., 2007) indeed suggest that a continental crust component has been incorporated in their parental melts either in the form of (subducted) terrigeneous sediments or from crustal contamination. Furthermore, the 700 Myr age of a zircon core (Song et al., 2005b) favours subduction zone magmatism at an active continental margin. The accumulated and documented large volume of complementary data and observations is overwhelming evidence that the Luliangshan garnet peridotite originates from a late Precambrian Alaskan-type ultramafic intrusion.

The field observations and petrologic and geochemical data discussed above suggest that the Luliangshan garnet peridotite massif is of cumulate origin crystallized from Mg-rich melts generated in a mantle wedge overlaying a paleo-subduction zone. We further propose a three-stage tectonic evolution model for the garnet peridotite massif in the North Qaidam UHP Belt as illustrated in Fig. 25. Stage I (~ 460 Ma): Dehydration of subducted oceanic lithosphere (eclogite-facies metamorphic stage, Song et al., 2006) caused mantle wedge partial melting and high-Mg melt generation in a sub-arc environment; cooling-induced crystallization of these melts led to the formation of the cumulate assemblage in the subarc lithospheric mantle in the spinel peridotite stability field at about 460Ma. Stage II: Subducting slab induced mantle


- 37 -

wedge asthenospheric corner flow transported the cumulate peridotite body deep into the mantle in the subduction zone. Stage III: The subducted continental crust seized the peridotite body and carried it to depths in excess of 200 km at about 423 Ma before exhumed with the supracrustal felsic gneisses to the middle-crust level at about 400 Ma.

Fig. 25. A tectonic modal that illustrates the forming process and environment for Luliangshan

garnet peridotite. (A) Subduction of North Qilian Ocean and sub‐arc cumulate chamber at about

460 Ma. (B) Corner flow of the mantle wedge during continental subduction has dragged the

cumulate down to depths about 150‐200 km at about 420 Ma and exhumed during subsequent

continental collision (~ 400 Ma) (Song et al., 2007)

6. Yuka eclogite terrane

The Yuka eclogite terrane is located in the northwest end of the North Qaidam UHPM belt. It extends in NW-SE from Yuka River to the Luliangshan in the southwest. This terrane mainly consists of various-sized eclogite blocks within para- and ortho-gneisses. Low-grade metamorphosed volcanic rocks (Tangjieshan group, Ordovician?) are accompanied the Yuka HP-UHP terrane and bounded by faults. Eclogite was first discovered by Yang et al. (1998) and summarized later by Yang et al.


- 38 -

2001 and Mattinson et al. (2007). More detailed work has been carried out by Chen et al. (2009).

6.1. Eclogite

Eclogite occurs as lens- or layer-shaped blocks within granitic and pelitic gneisses. Most samples are fresh with mineral assemblage of Grt + Omp + Phn + Rt + Amp (retrograde?) and show granoblastic texture without shape preferred orientation. Rutile and phengite contents are generally higher than eclogites from Dulan and Xitieshan but the modal content of minerals varies in different blocks. P-T conditions estimated by Grt-Omp-Phn(-Ky) geothermobarometry of Ravna & Terry (2004) are T = 650-750 °C, P = 2.6-3.2 GPa. Rare coesite or pseudomorphs were found (Zhang G. et al., 2009).

6.2. Granitic gneiss The granitic gneiss is the major component and occupied volume content of ~

80% of the UHP terrane. The rocks are off-white coloured with mineral assemblage of Pl + Qtz + Mus + minor Kfs. Grt can be seen in some samples. Foliation and isoclinal folds are well-developed in the granitic geniss. Zircon separates yielded in situ SHRIMP ages ranging from 900-960 Ma.

6.3. Pelitic gneiss Pelitic gneiss usually occurs closed with eclogite blocks. Mineral assemblage in

the peltic gneiss is Grt + Phn (Mus) + Qtz ± Pl ± Ky with variable modal contents. No UP or UHP index minerals have been found so far in either metrix or zircon inclusions.

6.4. Ages of protoliths and HP-UHP metamorphism for continental subduction Zircon in situ LA-ICPMS U–Th–Pb gave metamorphic ages of 431 ± 4 to 436 ±

3 and 431 ± 3 to 432 ± 19 Ma for eclogites and gneisses, respectively. These nearly identical metamorphic ages suggest that the eclogites and host gneisses underwent coeval early Paleozoic HP/UHP metamorphism. A few inherited zircon cores of eclogites have magmatic characters of high REE and HREE abundance, HREE-enriched REE pattern, and yield protolith ages of >750 Ma (Chen et al., 2009).

Geochemical data show that the Yukahe eclogite has alkali basaltic protoliths with high TiO2 and K2O + Na2O contents and Nb/Y ratios, REE patterns and trace element contents similar to those of within plate basalts (WPB). Nd isotope analyzes


- 39 -

give εNd ranges from -5.34 to +4.47, suggesting an enriched mantle source. These characteristics together with the geochronological results conclude that the Yukahe eclogite protoliths likely formed in a Neoproterozoic continental rift setting, and have subjected to HP/UHP metamorphism during early Paleozoic continental subduction (Chen et al., 2009).

Fig. 26. Histogram of apparent 206Pb/238U age of all zircon SHRIMP analyses from Yuka eclogite,

North Qaidam UHP belt (data from Chen et al., 2009).


- 40 -

Field trip itinerary

DAY 1: Monday, August 31, 2009

Leave Xining early in the morning (~ 8: 00 am), via the 100-km-long Qinghai Lake (altitude ~ 3100-3200 m), passing across the 3810m Xiangpishan, to Dulan for ca. 420 km along the Qinghai-Tibet highway (G109). The trip will examine the UHP metamorphosed ophiolite sequence including harzburgite, ultramafic cumulate, banded kyanite eclogite, and epidote-eclogite and phengite-eclogite and their country rocks in the Shaliuhe cross-section (Stop 1). Night in Dulan.

Fig. 27. Geological map showing locations of Stops 1 and 2.

Stop 1. Shaliuhe ophioite sequence and gneisses Leader: Guibin Zhang & Shuguang Song (Peking University, Beijing, China)

The Shaliuhe eclogite-gneiss terrane located at the eastern part of the North


- 41 -

Qaidam UHP belt. Three types of eclogite (i.e., kyanite (Ky) -eclogite, epidote (Ep) -eclogite and phengite (Phn) -eclogite) and associated serpentinized harzburgite represent a snapshot of subduction-zone metamorphism of oceanic lithosphere before the closure of the Paleo-Qilian ocean basin.

Coesite inclusion, mineral data and metamorphic P-T paths indicate that the eclogites may have once been subducted to a depth under UHP conditions. The serpentinized harzburgite is typical mantle peridotite. It may have also been metamorphosed and subducted along the subduction zone. Geochemical and isotopic data suggest that eclogites have oceanic affinities. The protoliths of these lithologies, altogether, constitute an ophiolitic stratigraphy. Zircon U-Pb SHRIMP dating shows that their protolith age is 516±8Ma and metamorphic age is 440-445 Ma. These data suggest that a Paleo-Qilian Ocean was developed between Qaidam and Qilian blocks before the early Ordovician. As relict of consumed oceanic crust during Ordovician-Silurian subduction and continental collision, the ophiolitic lithology may witness the completely dynamic process of oceanic subduction, continental collision and induced exhumation of these oceanic lithologies.

Fig. 28. Geological sketch map and a representative cross section (The dashed segment means

the abbreviatory part) (After Zhang G. et al., 2008).


- 42 -

Stop 1A: Kyanite-eclogite

Fig. 29. Field view of kyanite‐eclogite blocks and country rocks in the Shaliuhe terrane, North

Qaidam UHPM belt, China. (a) and (b) showing banded structure of the kyaniteeclogite. (c)

Serpentinized harzburgite enclosed within the kyanite‐eclogite. (d) The granitic gneiss that host

the eclogite.

The kyanite-eclogite is coesite-bearing meta-gabbro (Zhang et al., 2009a), which located at the noth end of the cross-section. It is a large lensoid block, about 800 m wide and 1700 m long (Fig. 28), enclosing the serpentinized harzburgite and was intruded by a 280 Ma granite. It shows a banded structure (Fig. 29a & b) defined by compositional bands of pink garnet-rich and green omphacite-rich layers. The layering may be inherited from the protolith (i.e., cumulate gabbro; Zhang et al., 2008). The garnet-rich layers contain more kyanite and finer-grained garnets (~3 mm) than the omphacite-rich layers. Garnets from the omphacite-rich layers are coarse porphyroblasts (about 5 mm and >10mm for the largest grain), surrounded by omphacite and less kyanite (Fig. 30a).


- 43 -

Fig. 30. Xenoblastic garnet associated with omphacite and kyanite in Ky‐eclogite (a). Exsolution

quartz needles within omphacite in Ky‐eclogite (b). The coesite inclusion in Omp (c & d).

Detailed petrography indicates that the kyanite-eclogite records complex metamorphic histories that can be broadly described in terms of three stages (Fig. 31): (1) a pre-peak metamorphic stage characterized by the mineral assemblage of Grt + Amp + Pl + Qtz at P = 0.49-0.67 GPa and T = 410-490 °C; (2) a peak eclogite-facies stage with a mineral assemblage of Grt + Omp + Ky + Phn + Rt ± Qtz/Coe at P = 2.7-3.4 GPa and T = 610-700 °C; (3) a retrograde “stage” represented by phenomena that may reflect several events during exhumation. These phenomena include the garnet reaction rims, the breakdown of omphacite, the symplectitic breakdown of kyanite, the retrograded garnet amphibolite assemblage and the latest greenschist-facies overprint. For the peak stage, Zr-in-rutile thermometry also give similar but precise results (Zhang et al.,

Fig. 31. P‐T path for Ky ‐ and Phn‐ eclogites from

Shaliuhe terrane.


- 44 -

2009b in review). Geochemically,

kyanite-eclogite and epidote-eclogite have cumulate gabbroic protoliths with distinct positive Eu anomalies (Fig. 32a, c), and low TiO2 high Al2O3 and MgO contents, and the kyanite-eclogite shows relic cumulate-layering structure. Sr-Nd isotope analysis indicates that kyanite-eclogite and epidote-eclogite have consistent (87Sr/86Sr) i ratios (0.703-0.704) and εNd (T) values (5.9-8.0) (Fig. 33).

Fig. 33. εNd(T) vs.

147Sm/144Nd plots for eclogite from Shaliuhe terrane (T=445Ma).

For zircon U-Pb dating, the older magmatic core mean age of 516±8 Ma represents the protolith age of kyanite-eclogite. CL patterns, inclusions and low Th/U values of the rims indicate that the younger rim mean age (445±7 Ma) represents the UHP metamorphic age (Fig. 34).

Fig. 32. REE patterns and spidergrams of eclogites


- 45 -

Stop 1B: Peridotite

As shown in Fig. 28, the harzburgite occurs as blocks with varying size in ortho- and para-gneisses. The largest is a lensoid block (~500 m wide and 1400 m long) within the banded kyanite-eclogite (Figs. 28). The harzburgite blocks are in gradational contact with eclogites. The harzburgite is strongly serpentinized with black and dark green colors (Fig. 29c).

It mainly consists of serpentine, olivine, relict orthopyroxene and minor chromites. Two generations of olivine can be distinguished based on composition and inner texture. The first generation (Ol1) occurs as relict of serpentinized olivine (Fig. 35a), showing some kink bands (Fig. 35b). And the Ol1 displays similar compositions to present-day abyssal peridotites. In contrast, the second generation (Ol2) has much higher Fo (94–97) and contains a great number of small fluid inclusions (Fig. 35c). Olivine with so high Fo content is obviously of metamorphic origin resulting from recrystallization of precursive serpentine (Bruce et al., 2000). Compositionally, the Opx plots in the modern abyssal peridotite area and exhibit an entire distinction with the garnet

Fig. 34. CL images of zircons from the Ky‐eclogite

Fig. 35. Photomicrographs of harzburgite.


- 46 -

peridotites of Sulu and North Qaidam (Fig. 36).

Fig. 36. (a) Mg# vs. wt% NiO in olivine in harzburgite. (b) Cation ratio 100Mg/ (Mg+Fetotal) vs. wt.%

Al2O3 in orthopyroxene in harzburgite (modified from Zhang et al. 2005). The field of abyssal

peridotite is taken from Dick (1989), Niu et al. (1997a,b); field of garnet peridotites in the North

Qaidam is from Song et al. (2007) and in the Sulu UHP terrane is from Zhang et al. (2000) and

Yang et al. (2000).

Stop 1C: Granitic gneiss The country rock of ky-eclogite is granitic gneiss (Fig. 29d). It shows a

coarse-grained granoblastic texture with vary degrees of mylonitization. It consists of K-feldspar, plagioclase, quartz, muscovite, and minor tourmaline. Zircon U-Pb dating gives a protolith age of 929 ± 26Ma, and no large enough metamorphic rim can be used to determine the metamorphic age.

Stop 1D: Phn-eclogite and country rocks Phn-eclogite forms small lenses within pelitic gneisses (Fig. 37a), or interlayers

and lenses within marbles (Fig. 37b) located towards the south of the cross section. The peak assemblage of well-preserved Phn-eclogites is garnet + omphacite + phengite + rutile, and show fine-grained, granular texture. The phengite-eclogite records a peak eclogite-facies stage at P = 2.7-2.9 GPa and T = 645-725 °C, and a amphibolite-facies retrogression stage at P=1.2-1.4 Gpa, T=610-670 °C (Fig. 31). Geochemically, phengite-eclogite has higher TiO2 and lower Al2O3, MgO contents, and REE and trace elements resemble E-MORB and OIB affinities (Fig. 32e, g); and higher 87Sr/86Sr ratios (0.705-0.716) and lower εNd (T) values (1.4-4.1) (Fig. 33).

Pelitic gneisses mainly consist of garnet-bearing muscovite-biotite schist with mineral assemblage of garnet-muscovite-biotite-plagioclase-k-feldspar-quartz and garnet-free muscovite-biotite schist with scovite-biotite-plagioclase-k-feldspar-quartz.

For the zircon U-Pb dating, Phn-eclogite and surrounding marble record similar


- 47 -

maetamorphic age with the Ky-eclogite.

Fig. 37. Field views showing occurrence of phn‐eclogite within pelitic gneiss (a) and marble (b).

DAY 2: Tuesday, September 1, 2009 Leave Dulan at ~ 8: 00 am in the morning; go first to Stop 2 to examine the

post-collision granite-granodiorite between the North and South Dulan belts. Go furth west along the Qinghai-Tibet highway to Golmud for about 400 km. Night in Golmud.

Stop 2. Post-collision granite-granodiorite in the Dulan UHP terrane

Trip leader: Shuguang Song, Guibin Zhang

Because it is difficult to approach the outcrops of eclogite blocks in the Yematan area of the North Dulan belt, we have chosen the special post-collision granite sequence for the one examining stop. As described above, the diorite-granodiorite-granite plutonic zone occurs between the north and south Dulan belts. These plutons show homogeneous medium to coarse-grained granitic texture without any deformation. Rock assemblage includes amphibole diorite, tonalite, grano-diorite and monzonitic granite, similar to magmatic assemblage in the active continental margin. Fig. 38 shows distribution of various rock types of the plutons and their forming ages by zircon LA-ICP-MS dating (Song et al., unpublished data).

Granodiorite contains large quantity of intermediate to basic zenoliths and diabase dykes (Fig. 39). Zircon dating reveals that xenoliths have the same age as the host pluton. In this stop, participants will examine (1) the granodiorite with mafic


- 48 -

zenoliths and dykes and (2) monzonitic granite.

Fig. 38. Geological map showing various post‐collision granitic plutons and their forming ages.

Fig. 39. Photographs showing granodiorite with zenoliths and dyke (A‐C)

and monzonitic granite (D)


- 49 -

DAY 3: Wednesday, September 2, 2009

Leave Golmud at ~ 8: 00 am in the morning northwards to Xietieshan along the road G215 for ~130 km. Examine the eclogite blocks and gneisses in the Xitieshan terrane (Stop 3), and then further north to Luliangshan examine the Luliangshan garnet peridotite (Stop 4). Go back to Golmud after the examination, and night in Golmud.

Stop 3. Eclogites and associated rocks in the Xitieshan terrane

Leader: Jianxin Zhang (Institute of Geology, CAGS, Beijing, China)

The Xitieshan eclogite-gneiss unit lies in the middle segment of the North

Qaidam HP/UHP metamorphic terrane. It mainly consists of garnet - sillimanite (±kyanite) - biotite paragneiss, granitic orthogneiss and metabasite (eclogite and retrograded eclogite) lenses. It was overthrust by rocks of the Tanjieshan Group and was intruded by granite plutons dated at 428 ± 1 Ma (Figure.1, Meng et al., 2005). Predominant deformation fabrics are SSE - NNW trending foliations and sub-horizontal stretching lineations defined by oriented sillimanite and biotite in paragneisses, suggesting high-temperature deformation. The foliation is deformed by SSE - NNW trending folds. Some paragneisses exhibit migmatitic characteristics. Leucosomes related to partial melting occur parallel to the foliations. Eclogite boudins and lenses in the Xitieshan gneisses are more extensively retrogressed than the eclogites in other units of the North Qaidam. Well-preserved eclogites are only found in a few outcrops, preserved in the centers of large boudins (thicknesses up to 30 metres). Zhang et al. (2005) determined minimum eclogite-facies conditions of >14 kbar, 730 - 830 °C for eclogite, followed by HP granulite facies conditions of 10 - 14 kbar, 750 – 865 °C. Zircon U-Pb geochronology from eclogites indicates eclogite-facies metamorphism at 480 - 486 Ma. Zircon TIMS dating of an orthogneiss enclosing retrograded eclogite gave an upper intercept of 952 ± 13 Ma and a lower intercept of 478 ± 43 Ma, interpreted as a magmatic crystallization age of the granitic protolith and an early Paleozoic metamorphic age, respectively (Zhang et al., 2006). Recently, zircon SHRIMP dating of paragneisses obtained ages of 427-461 Ma (Zhang et al., 2008, 2009), and was interpreted as time of high-pressure granulite –upper amphibolite facies metamorphic overprint, suggesting about 30 Ma residence time in medium-lower crustal levels after an earlier eclogite facies metamorphism (ca.


- 50 -

480 Ma). The field trip will stop two localities and visit two outcrops in the Xitieshan

area (Fig. 40). The purpose of these two stops is to observe eclogite, retrograded eclogite and hosting gneiss, and their relationships each other.

Fig.40. Geological map showing the geological setting of the Xitieshan area and Stop locations.

Stop 3A

The outcrop observed at stop A is the first true eclogite outcrop described in the Xitieshan area (Zhang et al., 2000). In this outcrop, well-preserved eclogites occur only in the central part of a large metabasite block, up to 30 metres across (Fig. 41). The well-preserved eclogite consists mainly of garnet+omphacite+rutile+quartz with small amounts of clinopyroxene, amphibole and plagioclase formed during decompression (Figure 3). The metabasite block shows a successive transition from the core outward of eclogite, garnet granulite (Grt+Cpx+Pl), garnet amphibolite (Grt+Amp+Pl), and finally amphibolite (Amp+Pl). Zircon cores from the well-preserved eclogite in this outcrop yielded ages of 750-850 Ma, which are believed to represent the magmatic crystallization age of the protlith (Figure 4). Zircon rims gave a mean age of 480±16 Ma, representing the age of eclogite facies metamorphism (Zhang J. et al., 2009)


- 51 -

Stop 3B Stop B is located at about 3 km north of Stop A. In this stop, we will observe

retrograded eclogite lens and associated orthogneiss and paragneiss. The lens, about 10 m wide and 30 m long, is enclosed in granitic gneiss. In the retrograded eclogite, the omphacite is completely replaced by symplectite of Cpx+Pl with a sieve-like texture. Garnet is rimmed by small amphibole grains (Fig. 42). Rutile is partly replaced by ilmenite. U-Pb dating of zircons from retrograded eclogite from this outcrop gave a mean age of 486 ± 2 Ma. In garnet-sillimanite-biotite gneiss, we will see SSE - NNW trending foliations, and in granitic gneiss, L>S fabrics will be observed. These have been interpreted as high temperature dedormation after eclogite facies metamorphism.

Fig. 41. Eclogite in Stop A, fresh samples occur only in the central part of the block

Fig.42. mircophotos of the well‐preserved eclogite in Stop A and retrograded eclogite in Stop B


- 52 -

Fig. 43. CL images and U‐Pb concodia diagram for zircons from well‐preserved eclogite in Stop A


- 53 -

Stop 4: Luliangshan garnet peridotite

Leaders: Shuguang Song (Peking University) Jianjun Yang (Institute of Geology and Geophysics, CAS)

The Luliangshan garnet peridotite massif is located between the Xitieshan terrane

in the southeast and the Yuka terrane in the northwest. As described above (Pages 26 to 38), it occurs as a large (~ 500×800 m in size) massif, ~ 20 km south of Da Qaidam town, and is hosted within an eclogite-bearing quartzofeldspathic gneiss terrane. This garnet peridotite massif comprises a wide range of lithologies from rocks dominated by olivine to those dominated by pyroxene. On the basis of field and petrographic observations, Song et al. (2005a, 2007) grouped the rocks into four types: (1) mostly garnet lherzolite with minor amounts of (2) garnet-bearing harzburgite/dunite, (3) garnet-free dunite and (4) garnet pyroxenite dikes/dikelets (see Fig. 17 for field occurrence).

Two locations are planed to examine: (1) garnet pyroxenite and it occurrence with peridotite, (2) garnet peridotite and garnet-free dunite.

Stop 4A: Garnet pyroxenite As shown in Figure 44, garnet pyroxenite occurs as interlayers or dyke/dykelet

cross-cutting the apparent layering of the massif. Most samples are fresh with pink garnet and pale-green pyroxene conspicuous in the field (Fig. 44 b,c). The constituent phases are garnet (20–30 vol %), orthopyroxene (5–10%), clinopyroxene (40–60%) and phlogopite (2–5%) with no olivine observed. It shows a fairly uniform medium-grained granular texture. The garnet is also Mg-rich (62–68 mol. % pyrope, 21–24% almandine, 9.5–11% grossular, < 1% spessartine, 0.8–1.5% uvarovite). It shows a fairly uniform medium-grained granular texture. Most garnets are rimmed with a kelyphitic Opx+Cpx+Spl assemblage (Fig. 44d). Densely packed lamellae of amphibole, quartz, Cr-magnetite and minor phlogopite are found in Cpx. These exsolutions are interpreted as resulting from decompression (Song et al., 2004, 2005b), which points to originally high-Si, Cr and hydroxyl in Cpx at peak metamorphic conditions.

Twenty-five zircon grains from the two samples were studied using CL and SHRIMP (Fig. 45). The morphology and trace element systematics of zircons in the garnet pyroxenite suggest that they may be of metamorphic origin (Song et al., 2005). The mean age of garnet pyroxenite zircons is 399 ± 8 Ma, which younger than peak


- 54 -

metamorphic ages of garnet lherzolite and dunite, but overlaps retrograde ages of near-rim domains in garnet lherzolite zircons. Some garnet pyroxenite zircons also recorded a retrograde event at 358 ± 7 Ma.

Fig. 44. Photographs showing outcrops and micro‐textures of garnet pyroxeninte. (a) garnet

pyroxenite occurs as a 3‐5‐meter‐wide dyke cross‐cut the peridotite. (b) Fresh sample with pink

garnet and green Cpx. (c) Granoblastic texture with Grt + Cpx + Opx + Phl. (d) Opx lamellae in Cpx

and kylephite around garnet. (e) Amp + Qtz +Cmt lamellae in Cpx. (d) BSE image showing Amp +

Qtz +Cmt lamellae in Cpx (perpendicular to the lamellae).


- 55 -

Fig. 45. CL images and TW diagram for all SHRIMP analyses of zircon from garnet pyroxenites with

mean ages of 358 and 400 Ma


- 56 -

Stop 4B. Garnet-free dunite and garnet peridotite As shown in Fig. 46 a, b, garnet-free dunite occurs as a ca. 10-meter-thick layer

interbedded with the garnet peridotite. It is strongly serpentinized, shows brown color on the weathering surface and dark to light green in fresh samples, and then it is mining for serpentine jade now. The dunite is medium-grained, dominated by strongly serpentinized olivine (> 90 vol %) with minor orthopyroxene and Cr-rich spinel (Fig. 3e). Magnetites are precipitated at the boundaries of these serpentinized olivine crystals and show a triple-junction texture. The olivine is Mg rich (Fo92.4-93.7) and the spinel is Cr rich (i.e., Cr# = 0.66–0.73) relative to those in the garnet lherzolite.

Fig. 46. Photographs showing occurrence of garnet‐free dunite (a, b), garnet peridotite (lherzolite

and harzburgite) (c‐d). Most peridotites are strongly serpentinized (e).

Garnet peridotite is in black color on the weathering surface and most are


- 57 -

strongly serpentinized. Fresh sample, unfortunately, is difficult to get in the outcrops. As pyroxenes vary in abundance on various scales, the garnet peridotite could be termed as garnet harzburgite, garnet lherzolite or olivine-websterite. Garnet lherzolite, harzburgite and dunite display apparent inter-layering on different scales determined by modal variation of olivine and pyroxene. Such layering could well be inherited from primary igneous layering such as seen in layered cumulates but could also be of metamorphic origin (Figs. 46 & 47). Fo content of olivine in garnet lherzolite varies from 0.830 to 0.926. Opx also has wide compositional variations (mg# 0.96 – 0.87) from garnet- dunite to garnet lherzolite. The ages of the garnet peridotite are described in pages 35-36 and Fig. 24.

Fig. 47. Photomicrographs showing garnet‐free dunite (a, b), Grt‐dunite (c), Grt‐harzburgite (d)

and Grt‐lherzolite (e,f)


- 58 -

DAY 4: Wednesday, September 2, 2009

Leave Golmud at ~ 8: 00 am in the morning northwards again on the same route to Yuka via Xietieshan, Da Qaidam along the road G215. Examine the eclogite blocks and gneisses along the north bank of the Yuka River (Stop 5). Go further north and get across the Dangjinshan pass (3650 m) into the Altyn Tagh Fault, and arrive in Dunhuang City in the late afternoon. Night in Dunhuang.

Stop 5. The Yuka eclogite terrane Leader: Liang LIU & Danling CHEN (Northwest University, Xi’an, China)

The Yuka eclogite section locates at the western segment of the North Qaidam UHP belt, about 40km northwest of the Greater Qaidam town (Fig. 1). Lithological characteristics, chronology and their structural relationship of eclogites, host gneisses and associated volcanic rocks in this section will be the main topic of the day.

Fig. 48. Geological map of the Yuka area (The size of eclogite bodies is exaggerated for clarity)

The Yuka eclogite section is spread well along the north bank of Yuka river; it

thrusts onto a Jurassic coal bed in the east, and is covered by Tertiary sediments in the west (Figs 48, 49). Eclogites occurs as lenses or interlayers within the gneisses of the Mesopreterozoic Dakendaban Group; their elongation is parallel to the gneissosity of the country rocks. The gneisses consist mainly of Grt-bearing muscovite granitic gneiss and minor paragneisses (schist) including Grt-Ky-bearing muscovite gneiss


- 59 -

and Amp-bearing mica quartz gneiss. This trip will start from the Ordovician volcanics (Fig. 49), and then go

westwards along the Yuka River, the end locality is up to the time permit.

Fig. 49. Cross section of the Yuka eclogite terrane

Stop 5A. Ordovician volcanics

Ordovican volcanics in the North Qaidam orogen belong to Tanjianshan group, is a suit of intermediate to basic volcanics and commonly associated in space with the HP-UHP rocks. These volcanics were considered forming during Ordovican just because of the associated sedimentary stratum. Few geochemical analyses suggest that, some of these rocks have characters of island arc volcanics and some have characters both IAB and MORB, formed at island arc or back-arc oceanic basin (Wang et al., 2005). Zircon U-Pb TIMS age of 495 Ma was yielded for the Tanjianshan group grabbro in Luliangshan area. Details of its genesis, geological setting, the accurate formation age and the relationship with the North Qaidam UHP belt await further study.

Fig. 50. Chondrite‐normalized REE patterns (a) and MORB‐normalized spider diagram of the

Ordovican volcanics from Yuka eclogite section

As shown in Fig. 49, Ordovican volcanics in this section is in fault contact with


- 60 -

the eclogite-gneisses terrane. Geological analyses show that this rock belong to Andesite/Basalt series with SiO2 range from 52.46 to 56.48 wt%, and has characteristics of enriched in MgO, Ni and Cr, depleted in HFSE (Ti, P, Zr, Y, Yb and Nb) and REE; has high Al2O3／TiO2 ratios and low Ti／Zr and high Zr／Y ratios, and exhibits U-shape REE pattern (Fig. 50) . These characters are similar to those of boninite, indicating that this volcanics is formed in the fore-arc setting.

Stop 5B. Felsic gneiss

Felsic gneiss is fault contact with Ordovican volcanics (Fig. 48), mainly consists of quartz (50-70%), muscovite (5-20%), plagioclase (5-10%), amphibole (1-15%), biotite (~5%) and apatite (5%). Geochemical analyses suggest that the protolith of this gneiss is lithic sandstone formed in the basins related to active continental margins or back-arc. LA-ICP-MS zircon U-Pb dating display multiple peaks (Fig. 51), no Paleozoic metamorphic age have been obtained so far. No eclogite body has been fund in it.

Fig. 51. Concordia diagram (a) and probability density plot (b) of zircons from Yuka felsic gneiss

Stop 5C. Eclogite and its wall rock gneisses

Stop 5C-1. Eclogite Petrology: Eclogite in this area mostly hosted in the granitic gneiss (80%), minor

in pelitic gneiss (20%). The typical mineral assemblage of the Yuka eclogite is garnet, omphacite, phengite and rutile (Fig. 52a). Coarse-grained garnet porphyroblast generally displays obvious prograde compositional zonation, and the inclusions within the garnet also display obviously systematic zonal distribution in mineral categories, composition and granularity, can be divided from core to rim into four rings, core


- 61 -

(GrtI), mantle (GrtIIA), rim (GrtIIB) and outermost rim (GrtIII) (Fig. 53). Omp and Phn can be seen both in the mantle inclusion of the Grt and consistent with Grt rim in the matrix, Omp and Phn in the matrix have higher Jd concent and Si value and decrease from core to rim, respectively. Three types of amphibole are recognized: the first type (AmpI) is magnesiohornblende, and occurs as inclusions associated with epidote and plagioclase within garnet cores; the second type (AmpII) is Mg-Kat, which replaced Omp (Fig. 52b) or occurs associated with Omp rim in the matrix, indicating that it formed after the matrix Omp and in equilibrium with the rim of Grt, Phn and Omp; the third type (AmpIII) is edenitic to pargasitic, and occurs in kelyphitic rims around Grt , Mg-Kat, Cpx+ Pl (Fig. 52c,d).

Fig. 52. Microphotographs of the Yukahe eclogite showing (a): mineral assemblage of Grt, Omp

and Phn (plane polarized light); (b): Omp been replaced by Mg‐Kat (plane polarized light); (c):

Omp rimmed by thin Cpx–Pl coronas; (d): Mg‐Kat been replaced by amphibole and Pl.

The textural relations observed and mineral chemistry suggest three successive stages of pre-eclogite, eclogite and post-eclogite of the Yuka eclogite: the pre-eclogite stage (I) Grt (core) and the core inclusions of AmI + Zoi + PlI + Q. Eclogite facies stage can be further divided into three sub-stages of early-, peak- and retrograde-eclogites according to the content of Jd in omphacite, mineral zoning, assemblage and occurrence. Early-eclogite stage (IIA) Grt (mantle) and mantle inclusions of Omp (lower Jd content) +Phn + Ru. Peak-eclogite stage (IIB) Grt (rim) , the core part of Omp and Phn in the matrix (GrtIIB+OmpIIB+PheIIB+Ru),


- 62 -

Retrograde-eclogite stage (IIC) Grt (outmost) +Mg-Kat +Omp(rim) and Phn (rim) in the matrix. Post-eclogite stage: Cpx +Ab+ Amp + Pl. Grt–Cpx geothermometer of Power (1985) and Ravna (2000), Grt-Phn geothermometer of Green et al., (1982), and Grt–Cpx–Phn barometer of Waters and Martin (1993), suggest a P–T condition of

630–680 °C and 3.0–3.4 GPa (Chen et al., 2005) for peak metamorphism, it lie in the coesite stability field. No coesite or coesite pseudomorphs have been found in eclogite or pelitic gneiss so far.

Fig. 53. Microphotography (a) and composition profile (b) of garnet of the Yuka eclogite

Dating：CL investigation and LA-ICP-MS in situ analyses were carried out for zircons from the Yuka eclogites (Chen et al., 2009). Zircons from the Yukahe eclogites show obvious metamorphic recrystallization or overgrowth internal texture in CL images, inherited core could be seen in few large grains (Fig. 54a). LA-ICP-MS in situ trace element and U-Th-Pb analyses revealed that zircon rims show metamorphic characters of lower REE and HREE abundance, HREE-depleted REE pattern (Fig. 54b)

and Th/U<<0.1, and yield the metamorphic age of 431 ± 4Ma and 436 ± 3 Ma for the two eclogites respectively (Fig. 55). Inherited zircon cores have magmatic characters of high REE and HREE abundance, HREE-enriched REE pattern (Fig. 54b) and Th/U > 0.4, and yield protolith ages of >750 Ma (Fig. 55).

Fig. 54. CL images (a) and REE patterns (b) of zircon grains from Yuka eclogites


- 63 -

Fig. 55. U‐Pb concordant plots (a) and weighted mean age (b) of Yuka eclogite

Geochemistry: Geochemical analyses suggest that the protolith of the Yuka eclogites are mainly alkali basalt with high TiO2 and K2O + Na2O contents and Nb/Y ratios. They contain high REE abundances and exhibit REE patterns of LREE strongly enrichment without any pronounced Eu anomalies (Fig. 56a), and obviously enriched in K, Rb, Ba, Th, Ta and Nb, slightly enriched in Ce, P, Zr, Hf, Sm and Ti on the MORB-normalized spidergrams(Fig. 56b), which is similar to that of the present-day OIB. And they also show distinct positive Pb anomalies and slight negative Nb anomalies on the primitive mantle normalized spidergrams (Fig. 56c).

Fig. 56. REE patterns (a) and spidergrams (b, c) of the Yuka eclogite

The εNd value of the Yukahe eclogites at 800 Ma ranges from –5.34 to + 4.47, have both oceanic and continental affinities. And their 143Nd/144Nd values (range from 0.51217 to 0.51265) is lower than the MORB value of > 0.5130, and their 147Sm/144Nd values lower than the chondritic values of 0.1967, similar to the LREE-enriched characteristics of continental basalts (Jahn et al., 2003). All these characters suggest that the protolith of Yukahe eclogites are derived from enriched mantle source and might have formed at an intraplate rift setting.


- 64 -

Stop 5C-2 Gneisses Granitic gneiss: it is the main component of this section, even in the whole belt。It consists of Qtz+Pl+Kfs+Ms (3.1–3.2 Si p.f.u.)±Grt. Geochemical studies suggest that the protolith of the granitic gneiss has characteristics of peraluminous syncollision granite (Fig. 57).

Fig.57. Chondrite‐normalized REE patterns (a) and ORG normalized spidergrams (b) of the granitic

gneiss in the Yuka area

Zircon LA-ICP-MS U-Pb dating yield the detrital zircon age rang from 1200 to

2200 Ma (an oldest age of 3062±18Ma), magmatic age of around 952 Ma and metamorphic age from 420 to 480Ma (Fig. 58).

Fig. 58. REE patterns (a) and U‐Pb dating results of zircons from the granitic gneiss

Pelitic gneiss Pelitic gneiss is another main country rock of the Yuka eclogite and is a little far from


- 65 -

the jumping-up-point. It is composed of Grt + Ky + Phn + Qtz + Cld + Pl (Fig. 59). Phn and Ky were discovered in the matrix or as inclusions in the rim of garnet, and Phn has Si- content of 3.45–3.46 p.f.u for inclusions and 3.20–3.37 for matrix ones; Pyr component in Grt increases from core to rim and matrix chloritoid contains 2.38 wt.%

MgO. It gives the peak metamorphic conditions of T > 650 °C and P > 2.5 GPa, calculated by Grt–Phe geothermobarometer of Green et al., (1982) and the Grt–Ky–Phe–Coe/Quartz of Ravna et al., (2004).

Metamorphic overgrowth of zircon in the pelitic gneiss is under development, three grain rims have lower REE and HREE abundance, HREE-depleted REE pattern

(Fig. 59) and Th/U<<0.1, give 432 ± 19 Ma as their metamorphic age.

Fig.59. Microphotography of the pelitic gneiss and CL images and Chondrite normalized REE

patterns of zircons from the pelitic gneiss

Grt-bearing plagiogneiss: it is also a little far from the jumping-up-point, we can go there if time is permit. It is a country rock interlayered with eclogite and both form a large lensoid block contained in granitic gneiss (Fig. 60). The gneiss consists mainly of Grt + Phn + Amp + Pl + Rt. Garnet has high almandine content, and the pyrope and grossular components decrease from core to rim. Phn occurs both in matrix and as inclusion in Grt and Amp, and has Si value of about 3.4 p.f.u, which is similar to those


- 66 -

in adjacent Phn eclogite (Chen et al., 2005). Feldspar is nearly Ab (Ab98Or0.5An1.5). Compositional zone of garnet together with the presence of abundant albite and amphibole suggest that the mineral assemblage represent retrograde overprint. However, the high Si value of 3.4 p.f.u of phengite and its interbedded occurrence with the UHP eclogite indicate that the country rocks have been subjected to coeval UHP metamorphism.

Zircons from the Grt-bearing plagiogneiss show sector and fir tree internal texture (Fig. 15a), HREE-depleted REE patterns (Fig. 61) and Th/U<<0.1, typical of

metamorphic zircons; and LA-ICP-MS analyses yield the metamorphic age of 431 ± 3 Ma (Fig. 62).

Fig. 60. Photographs showing field occurrences of plagiogneiss interbeds with the Phn eclogite

Fig. 61. Zircon CL images and Chondrite normalized REE patterns for plagiogneiss


- 67 -

Fig. 62. Concordia diagram for LA‐ICP‐MS analyses of the eclogite 04QH15. (b) Weighted mean

age diagram of the metamorphic domains.

Reference list Brueckner, H.K. and van Roermund, H.L.M., 2004. Dunk tectonics: A multiple

subduction/eduction model for the evolution of the Scandinavian Caledonides. Tectonics, 23(2): 20.

Chen D.L., Liu L., Sun Y and Liou J.G., 2009. Geochemistry and zircon U–Pb dating and its implications of the Yukahe HP/UHP terrane, the North Qaidam, NW China. Journal of Asian Earth Sciences, 2009, 35(3-4): 259–272

Chen DL, Sun Y, Liu L, Luo JH, Wang Y, Zhang AD. 2005. The metamorphic evolution of the Yuka eclogite in the NorthQaidam, NW China: evidences from the compositional zonation of garnet and reaction texture in the rock. Acta Petrologica Sinica, 21: 1039–1048.

Chen DL, Sun Y, Liu L. 2007a. In situ LA-ICP-MS zircon U-Pb age of ultrahigh-pressure eclogites in the Yukahe area, Northern margin of the Qaidam basin. Science in China, Series D, 37 (S1): 279-287.

Chen DL, Sun Y, Liu L. 2007b. The metamorphic ages of the country rock of the Yukahe eclogites in the North Qaidam and its geological significance. Earth Science Frontiers, 14: 108-116.

Chen N.S., Gong S., Sun M. et al., 2009. Precambrian evolution of the Quanji Block, northeastern margin of Tibet: Insights from zircon U–Pb and Lu–Hf isotope compositions. Journal of Asian Earth Sciences, 35, 367–376.

Davies JH, von Blanckenburg F. 1995. Slab breakoff: a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens. Earth and Planetary Science Letters, 129: 85–102.

Ernst WG. 1972. Occurrence and mineralogical evolution of bluschist belts with time. American Journal of Science, 272: 657–668.

Ernst WG. 2001. Subduction, ultrahigh-pressure metamorphism, and regurgitation of buoyant crustal slices — implications for arcs and continental growth. Physics of the Earth and Planetary Interiors, 127: 253–275.

Feng YM, He SP. 1995. Research for geology and geochemistry of several ophiolites in the North Qilian Mountains, China. Geological Review, 40: 252–264. (in Chinese with English abstract)


- 68 -

Ferry, J.M. and Spear, F.S., 1978. Experimental Calibration of Partitioning of Fe and Mg between Biotite and Garnet. Contributions to Mineralogy and Petrology, 66(2): 113-117.

Green, T.H., Hellman, P.L., 1982. Fe–Mg partitioning between coexisting garnet and phengite at high pressure, and comments on a garnet-phengite geothermometer. Lithos 15, 253–266.

Hacker, B.R., Calvert, A., Zhang, R.Y., Ernst, W.G. and Liou, J.G., 2003. Ultrarapid exhumation of ultrahigh-pressure diamond-bearing metasedimentary rocks of the Kokchetav Massif, Kazakhstan? Lithos, 70(3-4): 61-75.

Jahn, B.M., Rumble, D., Lion, J.G., 2003. Geochemistry and isotope tracer study of UHP metamorphic rocks. In: Carswell, D.A., Compagnoni, R., Rolfo, F. (Eds.), Ultrahigh Pressure Metamorphism. European Mineralogical Union Notes in Minera1ogy, Vol. 5. 365–414. Budapest: Eötvös University Press.

Liati, A. and Gebauer, D., 1999. Constraining the prograde and retrograde P-T-t path of Eocene HP rocks by SHRIMP dating of different zircon domains: inferred rates of heating, burial, cooling and exhumation for central Rhodope, northern Greece. Contributions to Mineralogy and Petrology, 135(4): 340-354.

Liou JG, Wang X, Coleman RG. 1989. Blueschists in major suture zones of China. Tectonics, 8: 609– 619.

Liou J.G., Ernst W.G., Song S.G., Jahn B.M., 2009. Tectonics and HP–UHP metamorphism of northern Tibet – Preface. Journal of Asian Earth Sciences, 35, 191–198.

Liu L, Che Z C, Luo J H. 1997. Recognition and implicaion of eclogite in the western Altun Mountains, Xinjiang. Chinese Science Bulletin, 42: 931-934.

Liu L, Sun Y, Xiao PX. 2002. Discovery of ultrahigh-pressure magnesite-bearing garnet lherzolite (>3.8 GPa) in the Altyn Tagh, Northwest China, Chinese Science Bulletin, 47: 881-886.

Liu, L., Che, Z.C., Wang, Y., Luo, J.H., Chen, D.L., 1999. The petrological characters and geotectonic setting of high-pressure metamorphic rock belts in Altun Mountains. Acta Petrologica Sinica 15 (01), 57–64. in Chinese with English abstract.

Ma XD, Chen DL. 2006. LA-ICP-MS zircon U-Pb dating of quartzo-feldspathic gneisses-the country rocks of ultrahigh-pressure metamorphic rocks on the northern margin of the Qaidam basin, Northwest China. Geological Bulletin of China, 25: 99-103.

Maruyama S, Liou J G, Terabayashi M. 1996. Blueschicts and eclogites of the world and their exhumation. International Geology Review, 1996, 38: 485-594.

Mattinson CG, Wooden JL, Liou JG, Bird DK, Wu CL. 2006. Age and duration of eclogite-facies metamorphism, North Qaidam HP/UHP terrane, western China. American Journal of Science, 306: 683-711.

Mattinson, C.G., Menold, C.A., Zang, J.X. and Bird, D.K., 2007. High- and ultrahigh-pressure metamorphism in the North Qaidam and South Altyn Terranes, western China. International Geology Review, 49(11): 969-995.

Mattinson CG, Wooden JL, Zhang JX, Bird DK. 2009. Paragneiss zircon geochronology and trace element geochemistry, North Qaidam HP/UHP terrane, western China. Journal of Asian Earth Sciences, 35, 298–309.

Meng, F.C., Zhang, J.X., Yang, J.S. and Xu, Z.Q., 2003. Geochemical characteristics of eclogites in Xitieshan area, North Qaidam of northwestern China. Acta Petrologica Sinica, 19(3):


- 69 -

435-442. Meng, F.C., Zhang, J.X., Yang, J.S., 2005. Tectono-thermal event of post-HP/UHP metamorphism

in the Xitieshan area of the North Qaidam Mountains, Western China: isotopic and geochemical evidence of granite and gneiss. Acta Petrologica Sinica, 21, 45-56 (in Chinese with English abstract).

Menold C.A., Manning C.E., Yin A., Tropper P., Chen X.-H., Wang X.-F., 2009. Metamorphic evolution, mineral chemistry and thermobarometry of orthogneiss hosting ultrahigh-pressure eclogites in the North Qaidam metamorphic belt, Western China. Journal of Asian Earth Sciences, 35, 273-284.

Powell R. 1985. Regression diagnostics and robust regression in geothermometer /geobarometer calibration: the clinopyroxene geothermometer revisited. J Metamorphic Geol., 3: 231-243.

Ravna E K. 2000. The garnet-clinopyroxene Fe2+-Mg geothermometer: an updated calibration. Journal of Metamorphic Geology, 18: 211~219.

Ravnae, J. K., and Terry, M. P., 2004. Geothermobarometry of UHP and HP eclogites and schists –an evaluation of equilibria among garnet–clinopyroxene–kyanite–phengite–coesite/quartz. Journal of Metamorphic Geology, 22, 579–592.

Rubatto, D. and Hermann, J., 2001. Exhumation as fast as subduction? Geology, 29(1): 3-6. Rubatto, D. and Scambelluri, M., 2003. U-Pb dating of magmatic zircon and metamorphic

baddeleyite in the Ligurian eclogites (Voltri Massif, Western Alps). Contributions to Mineralogy and Petrology, 146(3): 341-355.

Shi RD, Yang JS, Wu CL, Wooden J. 2004. First SHRIMP Dating for the Formation of the Late Sinian Yushigou Ophiolite, North Qilian Mountains. Acta Geologica Sinica, 78, 649-657.

Song S G, Niu Y, Zhang L F, Wei C J, Liou J G, Su L. 2009a. Tectonic evolution of Early Paleozoic HP metamorphic rocks in the North Qilian Mountains, NW China: New perspectives. Journal of Asian Earth Science, 35, 334–353.

Song S G, Su L, Niu Y, Zhang G B, Zhang L F. 2009b. Two types of peridotite in North Qaidam UHPM belt and their tectonic implications for oceanic and continental subduction: a review. Journal of Asian Earth Science, 35, 285–297.

Song SG, Su L, Niu Y, Zhang LF. 2007a. Petrological and Geochemical Constraints on the Origin of Garnet Peridotite in the North Qaidam Ultrahigh-pressure Metamorphic Belt, Northwestern China. Lithos, 96: 243-265.

Song S G, Zhang L F, Niu Y L, Wei C J, Liou J G, Shu G M. 2007b. Eclogite and carpholite-bearing meta-pelite in the North Qilian suture zone, NW China: implications for Paleozoic cold oceanic subduction and water transport into mantle. Journal of Metamorphic Geology, 25: 547-563.

Song SG, Yang JS, Liou JG, Wu CL, Shi RD, Xu ZQ. 2003a. Petrology, Geochemistry and isotopic ages of eclogites in the Dulan UHPM terrane, the North Qaidam, NW China. Lithos, 70: 195–211.

Song SG, Yang JS, Xu ZQ, Liou JG, Shi RD. 2003b. Metamorphic Evolution of the Coesite-bearing Ultrahigh-pressure Terrane in the North Qaidam, northern Tibet, NW China. Journal of Metamorphic Geology, 21: 631–644.

Song SG, Zhang LF, Niu YL. 2004. Ultra-deep origin of garnet peridotite from the North Qaidam ultrahigh-pressure belt, Northern Tibetan Plateau, NW China. American Mineralogist, 89:


- 70 -

1330–1336. Song SG, Zhang LF, Chen J, Liou JG, Niu Y. 2005a. Sodic amphibole exsolutions in garnet from

garnet-peridotite, North Qaidam UHPM belt, NW China: implications for ultradeep-origin and hydroxyl defects in mantle garnets. American Mineralogist, 90: 814-820.

Song SG, Zhang LF, Niu YL, Su L, Jian P, Liu DY. 2005b. Geochronology of diamond-bearing zircons from garnet-peridotite in the North Qaidam UHPM belt, North Tibetan Plateau: A record of complex histories associated with continental collision. Earth Planetary Science Letters, 234: 99−118.

Song SG, Zhang LF, Niu YL, Su L, Song B, Liu DY. 2006. Evolution from oceanic subduction to continental collision: A case study of the Northern Tibetan Plateau inferred from geochemical and geochronological data. Journal of Petrology, 47: 435−455.

Walsh, E.O. and Hacker, B.R., 2004. The fate of subducted continental margins: Two-stage exhumation of the high-pressure to ultrahigh-pressure Western Gneiss Region, Norway. Journal of Metamorphic Geology, 22(7): 671-687.

Wan, Y.S., Xu, Z.Q., Yang, J.S. and Zhang, J.X., 2001. Ages and compositions of the Precambrian high-grade basement of the Qilian terrane and its adjacent areas. Acta Geologica Sinica-English Edition, 75(4): 375-384.

Wan, Y.S., Zhang, J.X., Yang, J.S. and Xu, Z.Q., 2006. Geochemistry of high-grade metamorphic rocks of the North Qaidam mountains and their geological significance. Journal of Asian Earth Sciences, 28(2-3): 174-184. Acta Petrologica Sinica, in press.

Wang, H.C., Lu, S.N., et al. 2005. An Early Paleozoic collisional orogen on the northern margin of the Qaidam basin, northwestern China．Geological Bulletin of China 24(7), 603–612 (in Chinese with English abstract).

Waters, D.J., Martin, H.N., 1993. Geobarometry of phengitebearing eclogites. Terra Abstract 5, 410–411 (abs).

Yang J J, Powell R. 2008. Ultrahigh-pressure garnet peridotites from the devolatilization of sea-floor hydrated ultramafic rocks. Journal of Metamorphic Geology, 26: 695-716.

Yang J J, Zhu H, Deng J F, Zhou T Z, Lai S C. 1994. Discovery of garnet-peridotite at the northern margin of the Qaidam Basin and its significance. Acta Petrologica et Mineralogica, 13: 97–105.

Yang J S, Xu Z Q, Li H B, Wu C L, Cui J W, Zhang J X, Chen W. 1998. Discovery of eclogite at northern margin of Qaidam basin, NW China. Chinese Science Bulletin, 43: 1755–1760.

Yang J S, Xu Z Q, Song S G, Wu, C L, Shi R D, Zhang J X, Wan Y S, Li H, Jin X, Jolivet M. 2000. Discovery of eclogite in Dulan, Qinghai Province and its significance for studying the HP-UHP metamorphic belt along the Central Orogenic Belt of China. Acta Geologica Sinica, 74: 156–168.

Yang J S, Xu Z Q, Song S G, Zhang J, Wu C, Shi R, Li H, Brunel M. 2001. Discovery of coesite in the North Qaidam Early Palaeozoic ultrahigh pressure (UHP) metamorphic belt, NW China. Comptes Rendus De L Academie Des Sciences Serie II Fascicule A-Sciences De La Terre et Des Planetes, 333 (11): 719-724.

Yang J S, Xu Z Q, Song S G, Zhang J X, Wu C L, Shi R D, Li H B, Brunel M, Tapponnier P. 2002a. Subduction of continental crust in the early Paleozoic North Qaidam ultrahigh-pressure


- 71 -

metamorphism belt, NW China: Evidence from the discovery of coesite in the belt. Acta Geologica Sinica, 76: 63–68.

Yang J S, Xu Z Q, Zhang J X, Song S G, Wu C L, Shi R D, Li H B, Brunel M. 2002b. Early Palaeozoic North Qaidam UHP metamorphic belt on the north-eastern Tibetan plateau and a paired subduction model. Terra Nova, 14 (5): 397-404.

Yang J S, Wu CL, Zhang JX, Shi RD, Meng FC, Wooden J, Yang HY. 2006. Protolith of eclogites in the north Qaidam and Altun UHP terrane, NW China: Earlier oceanic crust? Journal of Asian Earth Sciences, 28: 185-204.

Zhang, C., Zhang, L.F., Zhang. G.B., 2009. Petrology and calculation of retrograde PT path of eclogites from Xitieshan, north Qaidam, China.

Zhang GB, Song SG, Zhang LF, Niu Y, Shu GM. 2005. Ophiolite-type mantle peridotite from Shaliuhe, North Qaidam UHPM belt, NW China and its tectonic implications. Acta Petrologica Sinica, 21: 1049-1058.

Zhang GB, Song SG, Zhang LF, Niu Y. 2008. The subducted oceanic crust within continental-type UHP metamorphic belt in the North Qaidam, NW China: evidence from petrology, geochemistry and geochronology. Lithos, 104: 99-108.

Zhang GB, Zhang LF, Song SG, Niu YL. 2009. UHP metamorphic evolution and SHRIMP dating of meta-ophiolitic gabbro in the North Qaidam, NWChina, Journal of Asian Earth Sciences, doi: 10.1016/j.jseaes.2008.11.013.

Zhang J X, Xu Z Q, Chen W, Xu H F. 1997. A Tentative Discussion on the Ages of the Subduction – Accretionary Complex/Volcanic Arcs in the Middle Sector of North Qilian Mountain. Acta Petrologica et Mineralogica 16: 112–119. (in Chinese with English abstract)

Zhang J.X, Xu Z.Q., Yang J.S., Li H.B., Wu C.L. 2000.The Altun-northern Qaidam eclogite belt in western China---another HP-UHP metamorphic belt truncated by large scale strike-slip fault in China. Earth Science Frontiers (China University of Geosciences, Beijing ),7 Suppl. 254-255.

Zhang, J.X., Wan, Y.S., Xu, Z.Q., Yang, J.S. and Meng, F.C., 2001. Discovery of basic granulite and its formation age in Delingha area, North Qaidam Monutains. Acta Petrologica Sinica, 17(3): 453-458.

Zhang J X, Yang J S, Mattinson C G, Xu Z Q, Meng F C, Shi R D. 2005. Two contrasting eclogite cooling histories, North Qaidam HP/UHP terrane, western China: Petrological and isotopic constraints. Lithos, 84: 51–76.

Zhang, J.X., Yang, J.S., Meng, F.C., Wan, Y.S., Li, H .B., Wu, C.L., 2006. U-Pb isotopic studies of eclogites and their host gneisses in the Xitieshan area of the North Qaidam Mountains, western China: New evidence for an early Paleozoic HP-UHP metamorphic belt. Journal of Asian Earth Sciences 28, 143-150.

Zhang J X, Meng F C, Yu S Y, Qi X X. 2007. Metamorphic history recorded in high pressure mafic granulites in the Luliangshan Mountains to the north of Qaidam Basin, northwest China: evidence from petrology and zircon SHRIMP geochronology. Earth Science Frontiers, 14: 85-97.

Zhang, J X, Meng F C, Wan Y S. 2007. A cold Early Palaeozoic subduction zone in the North Qilian Mountains, NW China: Petrological and U-Pb geochronological constraints. Journal of Metamorphic Geology, 25: 285-304.


- 72 -

Zhang, J., Mattinson, C.G., Meng, F., Wan, Y. and Tung, K., 2008. Polyphase tectonothermal history recorded in granulitized gneisses from the north Qaidam HP/UHP metamorphic terrane, western China: Evidence from zircon U-Pb geochronology. Geological Society of America Bulletin, 120(5-6): 732-749.

Zhang, J. X., Mattinson, C. G., Meng, F, C., Yang H, J. and Wan, Y. S. 2009. U-Pb geochronology of paragneisses and metabasite in the Xitieshan area, north Qaidam Mountains, western China: constraints on the exhumation of HP/UHP metamorphic rocks. Journal of Asian Earth Sciences, 35:245-258.

Guidebook for Post IEC

Documents

plane polarized

conference

paleozoic

trace element

imbricate

altyn tagh

active continental

qaidam uhp