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Precambrian Research 234 (2013) 322– 350
Contents lists available at SciVerse ScienceDirect
Precambrian Research
j o ur nal hom epa ge: www.elsev ier .com/ locate /precamres
ow long-lived is ultrahigh temperature (UHT) metamorphism? Constraintsrom zircon and monazite geochronology in the Eastern Ghats orogenic belt, India
.J. Korhonena,∗, C. Clarka, M. Brownb, S. Bhattacharyac, R. Taylora
Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, AustraliaLaboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, USAGeological Studies Unit, Indian Statistical Institute, Kolkata, India
r t i c l e i n f o
rticle history:eceived 2 March 2012eceived in revised form 4 December 2012ccepted 5 December 2012vailable online 20 December 2012
eywords:astern Ghatseochronology
ndiaonaziteHT metamorphismircon
a b s t r a c t
Along the coast of Peninsular India, the Eastern Ghats expose a deep crustal section through a com-posite Proterozoic orogenic belt. To quantify the late Mesoproterozoic–early Neoproterozoic pressure(P)–temperature (T)–time (t) evolution of the Eastern Ghats Province, new SHRIMP U–Pb zircon andmonazite age data from multiple localities are reported and integrated with the results of phase equilib-ria modelling. Samples of residual granulite, migmatite and enderbite yield a spread of weighted mean207Pb/206Pb zircon and monazite ages between ca 970 and ca 930 Ma. Based on ranges of spot agesfrom several samples, the late prograde to peak ultrahigh temperature (UHT) metamorphism (counter-clockwise evolution (CCW) to T >950 ◦C at P >8 kbar) and initial cooling is interpreted to have occurredbetween ca 1130 and ca 970 Ma. Regionally extensive enderbite and charnockite magmas were emplacedinto the hot, suprasolidus crust around the time of peak metamorphism. For the residual granulites andmigmatites the retrograde P–T–t path is characterized by close-to-isobaric cooling to the variable butelevated solidi for different samples. Weighted mean ages between ca 970 and ca 930 Ma in several sam-ples are interpreted to record the timing of crystallization of melt trapped by the percolation thresholdin each of these samples. Two additional weighted mean ages of ca 980 Ma (from Korhonen F.J., Saw,A.K., Clark, C., Brown, M., Bhattacharya, S., 2011. New constraints on UHT metamorphism in the EasternGhats Province through the application of phase equilibria modelling and in situ geochronology. Gond-
wana Research 20, 764–781) extend this range back in time by 10 My. The variability in the calculatedweighted mean ages across the region is interpreted to be due mainly to differences in the temperatureof the elevated solidus from sample to sample, suggesting a slow cooling rate of ∼1 ◦C/My during the ret-rograde stage of this long-lived UHT metamorphism. These results demonstrate that the Eastern GhatsProvince sustained UHT conditions (T >900 ◦C) for �50 My, and perhaps for as long as 200 Ma from ca1130 to 930 Ma, during a single CCW tectono-metamorphic event.
. Introduction
Increasingly ultrahigh temperature (UHT) metamorphism (T900 ◦C) is recognized from the exhumed hinterlands of orogensHarley, 1998; Brown, 2006, 2007; Kelsey, 2008). The recogni-ion that the crust can achieve and sustain temperatures >900 ◦Cas significant implications for crustal evolution, including growthnd differentiation, and for tectonics, due to the weakening andtrengthening effect of melting and melt loss, respectively, on
rustal rheology. However, there is little consensus on how andt what rate the heat for such extreme metamorphism is generatedr for how long temperatures >900 ◦C are sustained.
∗ Corresponding author at: Geological Survey of Western Australia, East Perth,A 6004, Australia. Tel.: +61 8 9222 3482; fax: +61 8 9222 3633.
The Eastern Ghats Province, a subdivision of the EasternGhats orogenic belt in India, is characterized by the widespreadoccurrence of late Mesoproterozoic–early Neoproterozoic UHTmetamorphism. However, conflicting petrologic and structuralinterpretations relating to the pressure (P)–temperature (T)–time(t) evolution of the Province (counter-clockwise (CCW) versusclockwise (CW)) and difficulties in deciphering the geologic sig-nificance of ages that span a range from the late Mesoproterozoicto the early Neoproterozoic (a single versus a polyphase meta-morphic evolution) have led to the development of contrastingtectono-metamorphic models for the Province. As a result of theseconflicting interpretations the Eastern Ghats Province providesan ideal location for an integrated investigation of the petrologic
and chronologic evolution of the UHT metamorphism, using state-of-the-art petrologic phase equilibria modelling combined withpetrographically constrained accessory mineral U–Pb geochronol-ogy, to resolve the controversy.
In this contribution, new Sensitive High Resolution Ion Micro-robe (SHRIMP) U–Th–Pb age data from zircon and monazite areeported from several localities in the Eastern Ghats Province. Thesege data are retrieved from zircon and monazite grains individu-lly chosen from thin sections so that the results can be integratedith the results of quantitative phase equilibria modelling to defineell-constrained P–T–t paths. These integrated results allow an
ssessment of the geologic significance of the ages in relation tohe apparently prolonged prograde and retrograde evolution atHT metamorphic conditions. The new ages provide limits on
he timing of UHT metamorphism and determine when crystal-ization of the last vestiges of melt trapped by the percolationhreshold occurred in residual UHT granulites as they crossed thelevated solidus during cooling. Such data from individual locali-ies show that the evolution for the Province can be characterizedy a broadly similar CCW P–T path and provides an explanation forhe spread of ages within the context of a single UHT metamor-hic event characterized by slow cooling after the metamorphiceak.
. Regional geology
The Eastern Ghats extends over 1000 km along the eastern coastf Peninsular India (Fig. 1a) and exposes a deep crustal sectionhrough a composite Proterozoic orogenic belt. Based on geologi-al and isotopic data the Eastern Ghats Belt has been separated intoour discrete crustal provinces with contrasting histories (Rickerst al., 2001a; Dobmeier and Raith, 2003), despite broad lithologicalimilarities.
The Eastern Ghats Province (Fig. 1a), as proposed by Dobmeiernd Raith (2003), is the largest crustal province located in theentral and northern parts of the belt. Previous petrologic studiesn the Eastern Ghats Province have estimated peak UHT condi-ions for the late Mesoproterozoic–early Neoproterozoic orogenicvent to be greater than 950 ◦C and 9 kbar, but both CW andCW P–T–t paths have been proposed from different localitiese.g. Lal et al., 1987; Kamineni and Rao, 1988; Sengupta et al.,990; Dasgupta et al., 1995; Sen et al., 1995; Mukhopadhyay andhattacharya, 1997; Mohan et al., 1997; Shaw and Arima, 1997;ose et al., 2000, 2006; Rickers et al., 2001b; Bhattacharya andar, 2002; Sarkar et al., 2003; Das et al., 2006, 2011; Bose andas, 2007; Nasipuri et al., 2008; Korhonen et al., 2011; Dharmaao et al., 2012a). In addition to contrasting P–T–t paths, therere disparities in the number of metamorphic events recognizednd in the timing and duration of the UHT metamorphism. As
result of the ambiguity, conflicting tectono-metamorphic mod-ls have been proposed for the evolution of the Eastern Ghatsrovince.
Evidence for granulite facies metamorphism and felsic magma-ism between ca 1000 and ca 950 Ma has been well documentedGrew and Manton, 1986; Shaw et al., 1997; Mezger and Cosca,999; Bhattacharya et al., 2003; Simmat and Raith, 2008; Dast al., 2011; Korhonen et al., 2011; Bose et al., 2011). However,lder ages ranging from ca 1400 Ma (U–Pb in zircon, Pb–Pb ineldspar–reviewed in Simmat and Raith, 2008) to ca 1250–1100 MaTh–U–Pb chemical ages from monazite – Simmat and Raith, 2008)ave been interpreted to date an early UHT metamorphic eventM1). In these studies, close-to-isobaric cooling to ∼750–800 ◦C isnferred to have followed the metamorphic peak for this early UHTvent. A later pervasive granulite facies event (M2) is argued toave occurred at ca 1000–950 Ma, reaching conditions of ∼850 ◦C
nd 8 kbar and characterized by near-isothermal decompressiono ∼5 kbar following the metamorphic peak (see Dasgupta andengupta, 2003; Simmat and Raith, 2008; Bose et al., 2011). Inontrast to this polyphase interpretation for the Eastern Ghats
esearch 234 (2013) 322– 350 323
Province, the UHT metamorphism has been interpreted as a singlelong-lived event in the late Mesoproterozoic to early Neoprotero-zoic (e.g. Korhonen et al., 2011; Gupta, 2011; Dharma Rao et al.,2012a).
The present study area is in the central part of the Eastern GhatsProvince (Fig. 1), in Domain II of Rickers et al. (2001a). The dom-inant rocks include migmatitic sillimanite–garnet-bearing gneiss(referred to locally as khondalite), orthopyroxene-free garnetif-erous quartzofeldspathic gneiss (referred to locally as leptynite),orthopyroxene-bearing charnockitic and enderbitic quartzofelds-pathic gneiss, and two-pyroxene mafic granulite. Small lensesof calc-silicate and high Mg–Al granulites also occur in thegneisses.
The P–T–t histories proposed in previous studies that specif-ically relate to the new results presented here are summarizedin more detail below. Unless otherwise indicated, these interpre-tations have relied on petrogenetic grids in simplified chemicalsystems and conventional thermobarometry to determine P–T con-ditions. The P–T–t evolution of the samples dated in this study hasbeen evaluated using state-of-the-art phase equilibria modellingin the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3(NCKFMASHTO) chemical system, and will be presented indetail in future contributions (Korhonen, Brown and Clark,unpublished data), although the results critical to a properinterpretation of the new ages are summarized later in thispaper.
3. Geochronological methods
3.1. SHRIMP U–Pb zircon
Zircon grains were separated from crushed rock samples byconventional magnetic and heavy liquid separation. In addition,selected zircon grains from two samples (EGB-09-01, -19) weredrilled from polished thin sections. Handpicked grains and thin sec-tion fragments were mounted in epoxy resin discs. Each mountwas imaged using a cathodoluminscence (CL) detector fitted toa Phillips XL30 scanning electron microscope at a working dis-tance of 15 mm and using an accelerating voltage of 12 kV, whichhighlights distortions in the crystal lattice that are related to trace-element distribution and/or radiation damage (e.g. Nasdala et al.,2003; Rubatto and Gebauer, 2000). The mounts were then ultrason-ically cleaned in ethanol, petroleum ether and detergent (Decon),rinsed in distilled and deionized water, and dried in an oven at60 ◦C. The polished surface of each mount was then coated with athin membrane of gold producing a resistivity of 10–20 � acrossthe disc.
Zircon U–Th–Pb isotope data were collected using the SHRIMPII based in the John de Laeter Centre of Mass Spectrometry, Perth,Western Australia. The sensitivity for Pb isotopes in zircon usingSHRIMP II was 21 cps/ppm/nA, the primary beam current was2.5–3.0 nA and mass resolution was ∼5000. Correction of mea-sured isotope ratios for common Pb was based on the measured204Pb in each sample and routinely represented a <1% correctionto the 206Pb counts. Corrections for common Pb were estimatedfrom 204Pb counts and the Stacey and Kramers (1975) commonPb model for the approximate U–Pb age of each analysis. Pb/Uisotope ratios were corrected for instrumental interelement dis-crimination using the observed covariation between Pb+/U+ andUO+/U+ (Compston et al., 1984; Hinthorne et al., 1979) deter-
mined from interspersed analyses of the standard zircon BR266.BR266 is a single zircon megacryst from Sri Lanka with an age of559 ± 0.3 Ma, 206Pb/238U = 0.09059, U and Th contents of 909 and201 ppm, respectively (Stern, 2001).
324 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Fig. 1. (a) Simplified geological map of the Eastern Ghats Province; modified after Ramakrishnan et al. (1998) and Rickers et al. (2001a). Stars indicate study areas. (b) GoogleEarth image showing sample locations in the northern sector. ** Sunki samples described in Korhonen et al. (2011). (c) Google Earth image showing sample locations in thec ctor.
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entral sector. (d) Google Earth image showing sample locations in the southern se
.2. SHRIMP U–Pb monazite
Thin section fragments with selected monazite grains from sam-les EGB-10-82 and -84 were cast with chips of the India monazitetandard Ind-1 (509 Ma, 206Pb/238U = 0.082133; Korhonen et al.,011), and were prepared in a similar way to the zircon mounts.onazite elemental mapping for Ce, Y, Th, U and Pb was under-
aken using the JEOL JXA-8530F Electron Probe Microanalyzer at
he Centre for Microscopy, Characterisation and Analysis, Univer-ity of Western Australia. An accelerating voltage of 15 kV and
current of 200 nA with a step size of 0.5 �m in both the xnd y directions were used for elemental mapping. Maps were
processed in NIH Image using the Fire lookup table, and all mapswere processed to the same degree so colour intensities are directlycomparable between grains. Monazite grains from samples EGB-09-04, -10-72, and -74 were imaged using a backscatter electron(BSE) detector fitted to a Phillips XL30 scanning electron micro-scope at a working distance of 5 mm and using an acceleratingvoltage of 20 kV. U–Pb isotope measurements were carried outusing the SHRIMP II, and analysed with ∼0.5 nA O2
− primary beam
focused on to ∼10 �m spots, a 5-scan duty cycle, and a massresolution of ∼5000 (a more detailed description of the CurtinSHRIMP procedure for monazite analysis is provided by Foster et al.(2000)).
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. Sample descriptions
.1. Samples from the northern sector of the study areaSalur–Pachipenta–Sunki)
The localities comprising the northern sector of the study areare shown on Fig. 1a as point ‘N’. Three samples were collectedrom two localities (Fig. 1b). Samples EGB-10-71 and EGB-10-72ere collected ∼9 km to the west of Salur. Sample EGB-10-74 was
ollected ∼6 km to the south, near Pachipenta. This area compriseselitic migmatites and granulite facies metapelitic to semipeliteneisses with minor enderbite–charnockite, calc-silicate gneissesnd metaquartzites. A garnetiferous gneiss intrudes the metased-mentary gneisses and is exposed east of the sample localitiesMukhopadhyay and Bhattacharya, 1997).
Mukhopadhyay and Bhattacharya (1997) documented threehases of deformation in this area. The earliest two phases repre-ent a progressive deformation associated with prograde heatingnd crustal melting along a CCW P–T–t path. Weakly foliated patchnd vein leucosomes in some outcrops indicate ongoing deforma-ion at the time of melt crystallization. The progressive deformations expressed as isoclinal folds (D1a) with strongly attenuatedimbs forming an axial planar gneissosity (D1b) and reclined foldsD2) that refolded the early structures to produce the map-scalenterference patterns (Mukhopadhyay and Bhattacharya, 1997).
esoscopic dome and basin structures in calc-silicate gneissesre due to interference between the early folds and later openpright folds (D3). Peak metamorphic conditions were achievedyn- to post-D2 and were estimated to be 8 kbar and 850 ◦C byukhopadhyay and Bhattacharya (1997) based on inferences fromicrostructures and the stability of univariant mineral reactions in
etrogenetic grids calculated for simplified chemical systems.A more robust approach was used by Korhonen et al. (2011)
n a study from a locality ∼10 km southwest of Sunki (Fig. 1b), tohe west-southwest of Salur in the metasedimentary gneisses of
ukhopadhyay and Bhattacharya (1997). Korhonen et al. (2011)inked microstructures from two high Mg–Al granulite samples
ith phase assemblage fields in P–T pseudosections (isochemi-al phase diagrams) calculated for individually constrained H2Ond Fe2O3 contents as appropriate to each sample. For one sam-le, the peak metamorphic phase assemblage was interpreted toe garnet + orthopyroxene + quartz + ilmenite + melt and the post-eak evolution involved recrystallization of garnet and the growthf orthopyroxene, sillimanite, cordierite and biotite. In the sec-nd sample, the peak metamorphic phase assemblage comprisedbundant elongate orthopyroxene porphyroblasts, rare grains ofpinel and sparse isolated coarse-grained sillimanite, inferred toave been in equilibrium with melt. The post-peak evolution
nvolved growth of sillimanite with biotite, likely at the expensef orthopyroxene, followed by growth of cordierite and a fine-rained cordierite–K-feldspar intergrowth. Using phase equilibriaodelling, Korhonen et al. (2011) constrain peak conditions to
ave been in excess of 950 ◦C and 9.5 kbar, followed by decom-ression and minor cooling to the elevated solidus at ∼900 ◦C and.5 kbar. Monazite analysed in situ with the SHRIMP yielded aeighted mean 207Pb/235U age of ca 980 Ma (Table 1), which was
nterpreted to broadly constrain the timing of high-temperatureonazite growth during decompression and crystallization of melt
rapped by the percolation threshold during a single UHT metamor-hic event. These authors interpreted the spread of 207Pb/235U spotges that extended from ca 1043 to ca 922 Ma as evidence for pro-racted monazite growth during the high-temperature retrograde
volution, implying very slow cooling at a rate of <1 ◦C/Ma afterecompression and possibly diffusive lead loss.
The samples considered in the present study were collectedrom the metasedimentary gneisses described in Mukhopadhyay
esearch 234 (2013) 322– 350 325
and Bhattacharya (1997). These authors interpreted the massif ofintrusive gneiss exposed east of the sample localities to have beenderived by melting of a deeper crustal source early during the pro-grade metamorphic history.
4.1.1. EGB-10-71 (N18◦31′33.1′′, E083◦7′34.5′′)Sample EGB-10-71 is an enderbite comprising
orthopyroxene–quartz–magnetite–plagioclase–K-feldspar, withaccessory zircon and ilmenite. The enderbite has intrusive anddiffuse contacts with the host migmatitic gneiss (Fig. 2a), althoughleucocratic material derived from the host gneiss also back-veinsinto the enderbite (Fig. 2b). The enderbite contains small leu-cocratic patches (±orthopyroxene) that suggest in situ melting(Fig. 2a and c). Orthopyroxene commonly occurs where thesepatches coalesce into a larger network (Fig. 2c).
Zircon mainly occurs along grain boundaries in the leucocraticlayers and along grain boundaries with orthopyroxene. Inspectionof separated zircon grains from this sample by CL imaging revealedboth zoned and homogeneous grains (Fig. 3a). Some grains containdiscrete weakly luminescent structural cores, commonly less than15 �m in size (Fig. 3a and b). Larger cores may preserve oscillatoryzoning (Fig. 3c). The dark cores are surrounded by more lumines-cent rims, which may be overgrown or truncated by thicker, lessluminescent outer rims (Fig. 3a–c). These outer rims have similarCL characteristics to the homogenous grains (Fig. 3a).
4.1.2. EGB-10-72 (N18◦31′33.7′′, E083◦7′35.3′′)Sample EGB-10-72 is a garnet–cordierite migmatitic gneiss
closely associated with intrusive enderbite similar to sam-ple EGB-10-71. The mineral assemblage in this sample isgarnet–sillimanite–cordierite–biotite–quartz–plagioclase–biotite–titanohematite, with accessory magnetite, zircon and monazite. Agarnet-rich selvedge ∼5 cm in thickness occurs at the contact withenderbite (Fig. 2d).
Zircon is distributed throughout the sample, but most com-monly occurs along grain boundaries and as inclusions incoarse-grained quartz in leucosomes, and as inclusions in cordieriteand garnet. Monazite also occurs throughout the sample, and maybe more concentrated in monazite-rich layers parallel to the folia-tion. Monazite grains generally occur along grain boundaries withquartz, cordierite and titanohematite, and along fractures withingarnet porphyroblasts.
CL imaging of zircon separates from this sample show weaklyluminescent and resorbed cores, which are overgrown by morebrightly luminescent rims (Fig. 3d). In some grains, these over-growths may display oscillatory zoning (Fig. 3e). A less luminescentovergrowth is also present in some grains (Fig. 3d–f). Grains of mod-erately luminescent zircon with only relics of brighter cores werealso observed (Fig. 3f). Monazite separates do not show obviouscompositional zoning based on BSE imaging.
4.1.3. EGB-10-74 (N18◦28′45.5′′, E083◦6′19.4′′)Sample EGB-10-74 was collected from a quarry comprising stro-
matic metatexite migmatite with a vertical E–W-trending foliation.Layers in the outcrop preserve isoclinal folds with a steep axial pla-nar gneissosity parallel to the foliation, consistent with a compositeplanar fabric (Fig. 2e), similar to the S1a and S1b fabrics describedby Mukhopadhyay and Bhattacharya (1997). In some places atoutcrop, an anastomosing network of leucosome in patch or veinmigmatite has a foliation defined by fine-grained biotite, althoughcoarse-grained biotite aggregates in the centre of the leucosomes
may be oriented at any angle (Fig. 2f). These leucosomes are inter-preted to post-date the deformation associated with the stronglyattenuated D1 fold limbs and the composite S1a/S1b fabric. Thus, themelting recorded by the patch and vein migmatite is interpreted to
326 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Table 1Summary of U–Pb zircon and monazite ages from this study.
Sample Location Mineral Age (Ma)* Comments
Northern sector of the study area (Salur–Pachipenta–Sunki)EGB-10-71 (enderbite) N18◦31′33.1′′
E083◦7′34.5′′Zircon 954 ± 14b Approximate age of crystallization of low-volume in situ melt
will be slightly younger than weighted mean age** defined by31 concordant (<10% discordance) analyses from overgrowthdomains. Two concordant core analyses yield spot ages of ca1009 and ca 973 Ma; broadly interpreted as the timing ofenderbite emplacement.
EGB-10-72 (migmatitic gneiss) N18◦31′33.7′′
E083◦7′35.3′′Zircon 950 ± 17b
1516 ± 34c
613 ± 93c
Approximate age of melt crystallization at conditions ofelevated solidus during high-temperature metamorphismdefined by weighted mean age** of nine concordant rimanalyses (MSWD = 1.4). Anomalously old spot ages (ca 1195, ca1146, and ca 1025 Ma) not included in the calculation of theweighted mean age imply older zircon growth, possibly duringa protracted high-temperature event. An older population(1514–1220 Ma) plots in a discordant array with poorlydefined intercepts: the upper intercept age corresponding toinherited grains and the lower intercept age implyingNeoproterozic Pb-loss.
Monazite 966 ± 6b Approximate age of melt crystallization at conditions ofelevated solidus during high-temperature metamorphismdefined by weighted mean age** of 24 concordant analyses(MSWD = 1.0).
EGB-10-74 (migmatitic gneiss) N18◦28′45.5′′
E083◦6′19.4′′Zircon 1005 ± 22 to
874 ± 20aProtracted zircon growth during the high-temperatureretrograde evolution observed in zircon cores andovergrowths.
Monazite 980 ± 15 to921 ± 15a
633 ± 17b
533 ± 12b
Range of spot ages reflects protracted monazite growth duringthe high-temperature retrograde evolution to conditions of theelevated solidus from ca 980 to 920 Ma. The youngest agepopulation from 11 grains (n = 12; 533 ± 12 Ma) impliesmonazite growth during the younger Pan-Africantectonothermal event. The intermediate age population of ca633 Ma either corresponds to localized monazite growthduring a Neoproterozoic metamorphic event or mixingbetween the older and younger age populations.
SK2-6-05d ∼N18◦30′1.3′′
∼E83◦2′53.6Monazite 979 ± 8b Approximate age of melt crystallization at conditions of
elevated solidus during high-temperature metamorphismdefined by weighted mean age** of 23 concordant analyses(MSWD = 0.41).
D1-3-S3d ∼N18◦30′1.3′′
∼E83◦2′53.6Monazite 1042 ± 41b Weighted mean age based on analyses (n = 3) from a single
monazite grain included in an orthopyroxene porphyroblast;interpreted to date the late prograde growth of peakmetamorphic minerals.
977 ± 13b Approximate age of melt crystallization at conditions ofelevated solidus during high-temperature metamorphismdefined by weighted mean age** of 23 concordant analyses(MSWD = 1.11).
Central sector of the study area (Anantagiri–Sunkarametta)EGB-09-38 (sapphirine-bearing high
Mg–Al granulite)N18◦14′8.0′′
E083◦0′41.6′′Zircon 1120 ± 41 to
794 ± 119aRange of concordant spot ages from metamorphicovergrowths reflects protracted zircon growth during thehigh-temperature retrograde evolution. All analyses fromovergrowth domains (n = 23) yield a poorly constrained andshallow discordia with an upper intercept age of 1007 ± 97,broadly consistent with the timing of high-temperaturemetamorphism in other samples, and a lower intercept of566 ± 320 Ma, suggesting possible overprinting associatedwith Pan-African tectonism. However, the errors of thediscordia line do not permit a meaningful interpretation.
E083◦0′48.2′′Zircon 970 ± 28b Approximate age of melt crystallization at conditions of
elevated solidus during high-temperature metamorphismdefined by weighted mean age** of 10 concordant rim analyses(MSWD = 1.1). Two older concordant spot ages (ca 1059, ca1055) from metamorphic rims imply protracted zircon growth.
EGB-10-82 (high Mg–Al granulite) N18◦16′22.6′′
E082◦57′21.9′′Monazite 953 ± 7b Approximate age of melt crystallization at conditions of
elevated solidus during high-temperature metamorphismdefined by weighted mean age of 37 concordant analyses(MSWD = 1.7).
EGB-10-84 (high Mg–Al granulite) N18◦16′22.6′′
E082◦57′21.9′′Monazite 948 ± 5b Approximate age of melt crystallization at conditions of
elevated solidus during high-temperature metamorphismdefined by weighted mean age of 33 concordant analyses(MSWD = 1.6).
F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350 327
Table 1 (Continued)
Sample Location Mineral Age (Ma)* Comments
Southern sector of the study area (Paderu–Gangaraja Madugula)EGB-09-01 (migmatitic gneiss) N18◦2′10.4′′
E082◦33′38.7′′Zircon 929 ± 17b Approximate age of melt crystallization at conditions of
elevated solidus during high-temperature metamorphismdefined by weighted mean age of 22 concordant rim analyses(MSWD = 1.0).
EGB-09-04 (migmatitic gneiss) N18◦3′15.8′′
E082◦33′23.5′′Monazite 968 ± 20b
822 ± 31bWeighted mean age of ca 968 Ma approximates age of meltcrystallization at conditions of elevated solidus duringhigh-temperature metamorphism (13 concordant rim analysesfrom four grains; MSWD = 2.0). Weighted mean age of ca822 Ma from one grain may be related monazite growth duringlater fluid infiltration.
EGB-09-19 (granite with garnetschlieren)
N18◦3′11.8′′
E082◦33′10.7′′Zircon 1129 ± 46 to
885 ± 15a
1620 + 44/−45c
855 + 14/−15c
Range of concordant spot ages from metamorphicovergrowths (n = 8) reflects protracted zircon growth duringthe high-temperature retrograde evolution. Approximate ageof melt crystallization at conditions of elevated solidus isapproximated by three concordant rim analyses yielding agesof 965–960 Ma. Four younger ages (909–885 Ma) from threegrains occurring along fractures in garnet are interpreted asthe final release of fluids associated with the waning stages ofmetamorphism at greater depth (also lower intercept age ofdiscordia line; MSWD = 1.2). Upper intercept age of discordialine is age of inherited grains.
* Age = 207Pb*/206Pb* age:a Age range reflecting protracted growth.b Weighted mean age.
ulated
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c Regression age.** Approximate age of melt crystallization at the solidus will be younger than calcd From Korhonen et al. (2011).
e syn-D2 with the fabric developing as the melt crystallized lateuring D2.
Sample EGB-10-74 is a garnet-bearing stromatic metatex-te migmatite. The mineral assemblage in this sample is garnet–illimanite–biotite–quartz–plagioclase–titanohematite–magnetiteinor cordierite and orthopyroxene with accessory zircon andonazite are also present. Zircon is most common in leuco-
omes, occurring along grain boundaries and as inclusions inoarse-grained quartz. Trace monazite occurs throughout theample, CL imaging shows that zircon separates from this sampleave discrete structural cores, which are typically overgrown byore brightly luminescent rims (Fig. 3g and h). However, some
rains show a less luminescent overgrowth (Fig. 3i). BSE imagingf monazite separates does not reveal obvious compositionaloning.
.2. Samples from the central sector of the study areaAnantagiri–Sunkarametta)
The localities in the central sector of the study area are shownn Fig. 1a as point ‘C’. Samples EGB-09-38 and EGB-09-39 were col-ected just east of Anantagiri (Fig. 1c). This area is characterized bylongate NE–SW-trending bodies of pelitic migmatite, charnockite,nd garnetiferous quartzofeldspathic gneiss with gradational con-acts. Mafic granulites occur as lenses within the gneisses. Minorapphirine-bearing, high Mg–Al granulites in the Anantagiri areaypically occur as enclaves within charnockite (e.g. Fig. 4a). The out-rops generally have moderate to steeply dipping foliation, parallelo the rock contacts.
Petrologic interpretations from sapphirine-bearing granulitesave been used by Sengupta et al. (1990) to propose a CCW P–T–tvolution for this area, with a high-T/low-P prograde path reach-ng peak P–T conditions of 8.3 kbar above 950 ◦C, followed by near
sobaric cooling to ∼675 ◦C and 7.5 kbar, and a final decompressionegment from 7.5 to 4.5 kbar. Dasgputa et al. (1991) proposed aroadly similar retrograde P–T–t path for mafic granulites in therea.
weighted mean age; see text for further discussion.
The interpretation of a CCW P–T–t evolution by Senguptaet al. (1990) relies on the inference that tiny inclu-sions of spinel in sapphirine record a reaction such asrutile + spinel + quartz → sapphirine + ilmenite, which they calcu-lated to occur at pressures <3 kbar for temperatures of 700–900 ◦C.Given the involvement of rutile, this calculated stability is veryunlikely to be correct, and the occurrence of spinel inclusions insapphirine in aluminous granulites that have followed a clockwiseP–T–t evolution from pressures >10 kbar is well documented (e.g.Galli et al., 2011). To address this potential ambiguity, the P–T–tevolution of the samples dated in this study from these localitieshas been evaluated using phase equilibria modelling. Although theresults of this modelling will be presented in detail elsewhere, theresults indicate peak conditions >950 ◦C at between 7 and 9 kbarfollowed by near isobaric cooling to the elevated solidus at ∼950 ◦C(Korhonen, Brown and Clark, unpublished data).
Samples EGB-10-82 and EGB-10-84 were collected approx-imately 7 km northwest of Anantagiri and 1.5 km west ofSunkarametta (Fig. 1c), separated by ∼2 m in the same out-crop. This area exposes migmatitic gneiss, mafic granulite,and charnockite–enderbite. High Mg–Al granulite (±sapphirine)occurs as small elongate bodies within the dominant migmatiticgneiss near the margins with mafic and enderbitic gran-ulites (Bose et al., 2000). Bose et al. (2000) have interpretedorthopyroxene–cordierite and orthopyroxene–spinel associationsin massive and migmatitic high Mg–Al granulites from this localityin terms of prograde melting of biotite–plagioclase–quartz-bearingprotoliths at ∼6–8 kbar and temperatures in excess of 850 ◦C, withpeak metamorphic conditions reaching 9 kbar and 950 ◦C, followedby slight decompression and cooling to ∼700–750 ◦C (see also Boseet al., 2006). Similar results have been obtained from the samplesdated in this study using phase equilibria modelling, with estimatedpeak conditions near 950 ◦C and 8 kbar followed by near isobariccooling (Korhonen, Brown and Clark, unpublished data).
Geochronological data from this general area have been recentlyreported by Das et al. (2011) and Bose et al. (2011; Domain2). Oscillatory-zoned zircon cores that yield near-concordant207Pb/206Pb ages ranging from ca 1780 to ca 1700 Ma are
328 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Fig. 2. Field photographs from the northern sector of the study area. (a) Enderbite with intrusive and diffuse contacts with the host gneiss. Arrows denote small leucocraticpatches (±orthopyroxene) consistent with in situ partial melting. (b) Leucocratic material derived from the host migmatitic gneiss back-veins into the enderbite. (c) Leucocraticpatches in enderbite. Orthopyroxene (opx) commonly occurs where these patches coalesce into a larger network. (d) Contact between migmatitic gneiss and enderbite ismarked by a garnet-rich selvedge. (e) Layering in stromatic metatexite migmatites (S1a) preserves isoclinal folds with a steep axial planar gneissosity (S1b) parallel to thefoliation, consistent with a composite planar fabric (S1a/S1b). (f) Patch migmatite with S1a foliation is crosscut by leucosomes with S1b foliation defined by fine-grained biotite.Both the S1a foliation in the host and the S1b foliation in the leucosomes are crosscut by the composite S1a/S1b fabric.
F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350 329
Fig. 3. Representative cathodoluminescence (CL) images of zircons from all samples and SHRIMP analytical spots (ellipses). Sample name is displayed and grain numbersrefer to the analyses presented in tables. 207Pb/206Pb ages are quoted, those marked with asterisks (*) are >10% discordant.
330 F.J. Korhonen et al. / Precambrian R
Fig. 4. Field photographs from the central and southern sectors of the study area.(a) Minor sapphirine-bearing, high Mg–Al granulites in the Anantagiri area typicallyoccur as enclaves within charnockite. (b) Orthopyroxene-rich granulite from theSunkarametta area, occurring as a layer within migmatitic gneiss. (c) Compositionallayering in residual granulite from the Sunkarametta area, with a distinct cordierite-rich domain on the left of the photograph and a cordierite-poor, garnet-rich domainon the right.
esearch 234 (2013) 322– 350
interpreted to record magmatic events in the source of these detri-tal grains. Discordant ages of 1600–1100 Ma are inferred to bemixing ages resulting from UHT metamorphism at 1030–990 Ma(M1), based on chemical dates of monazite inclusions in orthopy-roxene. A second phase of granulite metamorphism (M2) wascharacterized by significant zircon growth at 980–900 Ma, pre-ceded and/or contemporaneous with emplacement of voluminousgranite at 990–980 Ma.
4.2.1. EGB-09-38 (N18◦14′8.0′′, E083◦0′41.6′′)Sample EGB-09-38 is a sapphirine-bearing gran-
ulite. The mineral assemblage in this sample comprisessapphirine–garnet–orthopyroxene–sillimanite–cordierite–spinel–magnetite–rutile–quartz–plagioclase–K-feldspar. Zircon generallyoccurs along grain boundaries and as inclusions in quartz andcordierite.
CL imaging of zircon separates from this sample show bothzoned and homogeneous grains (Fig. 3j). Many grains containweakly luminescent and resorbed cores; some with well developedoscillatory zoning. These dark cores are commonly surrounded bybrightly luminescent overgrowth domains, which have similar CLcharacteristics to the homogenous grains (Fig. 3j).
4.2.2. EGB-09-39 (N18◦14′12.3′′, E083◦0′48.2′′)This sample is similar to sample EGB-09-38. Zircon sepa-
rates typically display weakly luminescent, oscillatory-zoned cores,which are overgrown by more brightly luminescent rims (Fig. 3k).
4.2.3. EGB-10-82 (N18◦16′22.6′′, E082◦57′21.9′′)Sample EGB-10-82 is an orthopyroxene-rich granulite,
occurring as a layer within migmatitic gneiss (Fig. 4b).The mineral assemblage in this sample is composed oforthopyroxene–sillimanite–cordierite–rutile–quartz–plagioclase–K-feldspar–biotite–ilmenite.
Monazite grains were analysed in situ to preserve the petrologiccontext. Monazite grains are closely associated with cordierite ororthopyroxene, occurring either along grain boundaries (Fig. 5a,c and d) or as inclusions (Fig. 5b). Some grains are chemicallyhomogeneous (e.g. Fig. 6e and l), whereas other grains show com-positional zoning in Y with relative enrichment in the core regionsand depletion in overgrowth domains (e.g. Fig. 6c and f). Thinouter rims with Y enrichment may also be preserved. The Y-poordomains are typically associated with a slight increase in Th con-tent, whereas variations in Ce, Pb, and U are less pronounced.Monazite grains do not show a correlation between petrographicsetting and chemical zoning.
4.2.4. EGB-10-84 (N18◦16′22.6′′, E082◦57′21.9′′)This sample is similar to sample EGB-10-82, collected
from the same outcrop. The mineral assemblage comprisesorthopyroxene–sillimanite–cordierite–rutile–quartz–plagioclase–K-feldspar–biotite. Minor garnet and sapphirine are also presentin discrete layers.
Monazite grains selected for in situ analysis occur as inclu-sions in garnet (Fig. 5e), sillimanite (Fig. 5h), orthopyroxene andcordierite, and within and closely associated with cordierite + K-feldspar intergrowths (Fig. 5f, g, and i). Elemental mapping ofmonazite in this sample reveals that most of the analysed monazitegrains are chemically homogeneous (e.g. Fig. 6m–p), although thinY-rich overgrowths are present on some grains. One grain showsevidence for oscillatory zoning in the core, which is truncated by
a thin Y-depleted overgrowth. Two grains hosted in garnet (Fig. 6rand s) have Y-enriched core domains with cuspate embaymentssurrounded by Y-depleted overgrowths. These features suggestfluid-assisted dissolution and reprecipitation.
F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350 331
F amplem
4(
sasw0miecgpMdcB
Pf
ig. 5. Photomicrographs of representative monazite grains analysed in situ from sonazite grains listed in Tables 8 and 9.
.3. Samples from the southern sector of the study areaPaderu–Gangaraja Madugula)
The localities in the southern sector of the study area arehown on Fig. 1a as point ‘S’. Sample EGB-09-01 was collectedbout 4 km northeast of Gangaraja Madugula, approximately 12 kmouthwest of Paderu (Fig. 1d). Samples EGB-09-04 and EGB-09-19ere collected about 2 km north–northwest of sample EGB-09-
1, separated by about 400 m (Fig. 1d). This area exposes peliticigmatite (e.g. Fig. 4c), mafic granulite, charnockite–enderbite, and
ntrusive granites. Rare high Mg–Al granulites (±sapphirine) gen-rally occur as small blocks and folded layers within massif-typeharnockite–enderbite bodies and granites. Orthopyroxene-freeranites in this area have been linked to the pelitic granulites byrograde high-temperature hydrate-breakdown melting in high-g pelitic and greywacke protoliths, followed by high-temperature
ecompression from ∼10 to ∼8 kbar at 1000 ◦C and subsequentooling from >900 ◦C to ∼600 ◦C (Bhattacharya and Kar, 2002;
hattacharya et al., 2003).
Various P–T–t paths have been proposed for the Paderu area.reliminary phase equilibria modelling predicts peak conditionsor sample EGB-09-01 to be >860 ◦C at 6–11 kbar, and a retrograde
s EGB-10-82 and -84. Samples numbers and grains are labelled, and correspond to
trajectory of near-isobaric cooling just prior to conditions of finalcrystallization of melt trapped by the percolation threshold. SampleEGB-09-04 records higher peak metamorphic conditions of ∼945 ◦Cand 8.3 kbar. The preserved assemblage in this sample provides atight constraint on the solidus conditions where the last melt crys-tallized, estimated at ∼900 ◦C and 8.3 kbar (Korhonen, Brown andClark, unpublished data).
A study of sapphirine-bearing granulites near GangarajaMadugula by Mohan et al. (1997) constrained peak metamorphicP–T conditions to have been >900 ◦C at 8.4 kbar. Following peakmetamorphism, these authors proposed a retrograde path charac-terized by a decrease in pressure of up to 3 kbar with cooling of150–200 ◦C. A history of high-temperature decompression follow-ing peak UHT metamorphism has also been proposed by DharmaRao et al. (2012a) for samples collected ∼2 km northeast of Paderu.These authors use the results of phase equilibria modelling inNCKFMASH and thermobarometry to propose peak metamorphicconditions >1000 ◦C and >10 kbar, followed by decompression to
pressures <10 kbar, and subsequent near-isobaric cooling to tem-peratures <900 ◦C. In contrast to these studies, Sengupta et al.(2004) reinterpreted the data of Bhattacharya and Kar (2002) andBhattacharya et al. (2003) to suggest decompression from a lower
332 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Fig. 6. Yttrium X-ray compositional maps and SHRIMP analytical spots (ellipses) of monazite grains from samples EGB-10-82 and -84. Samples numbers are abbreviated to‘82 mXX’ and ‘84 mXX’, respectively, where ‘XX’ refers to monazite grains listed in Tables 8 and 9. Maps were processed in NIH Image using the Fire lookup table, and allmaps were processed to the same degree so colour intensities are directly comparable between grains. Scale bar is 20 �m. (For interpretation of the references to colour inthis figure legend, the reader is referred to the web version of this article.)
rian R
ppB∼a
4
afsbA(tmcNao
4
afsr((i
4
afisn
acinais
5
i6cCvot2
‘
5
faft
F.J. Korhonen et al. / Precamb
ressure of ∼7–8 kbar at 850 ◦C, inferred to postdate cooling fromeak UHT conditions along a general CCW P–T–t path. Pal andose (1997) also proposed a CCW path, reaching peak conditions of1000 ◦C and 9.5 kbar, followed by near-isobaric cooling to 900 ◦Cnd 9 kbar.
.3.1. EGB-09-01 (N18◦2′10.4′′, E082◦33′38.7′′)Sample EGB-09-01 is a migmatitic gneiss with a mineral
ssemblage comprising garnet–orthopyroxene–magnetite–K-eldspar–biotite–quartz–plagioclase–ilmenite. Zircon grainselected for in situ analysis occur as inclusions and along grainoundaries of quartz and plagioclase in leucosomes (Fig. 7a–c).
single grain (A-6) occurs as an inclusion at the edge of garnetFig. 7c). CL imaging of the zircon grains from this sample revealedhe presence of weakly luminescent, oscillatory-zoned cores in
any grains (Fig. 3l). These cores were commonly truncated andompletely overgrown by more brightly luminescent rims (Fig. 3l).early homogenous grains of moderately luminescent zircon werelso observed, although in some grains there is a slightly darkervergrowth (Fig. 3m).
.3.2. EGB-09-04 (N18◦3′15.8′′, E082◦33′23.5′′)This sample is a migmatitic gneiss (Fig. 4c) with a mineral
ssemblage of garnet–orthopyroxene–cordierite–sillimanite–K-eldspar–biotite–plagioclase–quartz–rutile. Monazite grainselected for in situ analysis are closely associated with orthopy-oxene, occurring along grain boundaries (Fig. 7d), as inclusionsFig. 7e), or within a fine-grained intergrowth of sillimanite + quartz±biotite) replacing the orthopyroxene (Fig. 7f). BSE imaging ofn situ monazite grains reveals that the monazite is unzoned.
.3.3. EGB-09-19 (N18◦3′11.8′′, E082◦33′10.7′′)Sample EGB-09-19 is a foliated garnet-bearing granite, collected
bout 2 km northwest of sample EGB-09-01 (Fig. 1d). Schlieren ofne-grained garnet are abundant and define the foliation in thisample. Quartz, plagioclase, K-feldspar, biotite, ilmenite and mag-etite are also present.
The petrographic settings for the zircon grains selected for in situnalyses are either along grain boundaries of quartz and plagio-lase or included in garnet along cracks or along edges of magnetitenclusions. CL imaging shows that the grains display brightly lumi-escent cores, some with oscillatory zoning (Fig. 3n). These coresre commonly overgrown by less luminescent rims. Zircon grainsn this sample are more fractured than grains in other samples. Inome grains, the fractures occur only in the core regions (Fig. 3o).
. Results
U–Pb isotope ages for zircon and monazite are summarizedn Table 1, together with the monazite ages from samples SK2--05 and D1-3-S3 reported in Korhonen et al. (2011), and theomplete data for the new samples are reported in Tables 2–13.oncordia regressions, concordia age and weighted average agealues were calculated with ISOPLOT v3 (Ludwig, 2003) and plottedn Tera-Wasserberg concordia diagrams (Fig. 8a–q) and conven-ional concordia diagrams (Fig. 8r). Unless otherwise indicated,07Pb/206Pb spot ages (±1�) are reported. Analyses described asconcordant’ refer to <10% discordance.
.1. EGB-10-71
Thirty-seven cores and rims from 29 zircon grains were analysed
rom sample EGB-10-71 (Fig. 8a, Table 2). Thirty-three analysesre <10% discordant, which yield a range of 207Pb/206Pb spot agesrom 1042 ± 28 to 842 ± 30 Ma. The weakly luminescent struc-ural cores were typically too small to analyse, but two concordant
esearch 234 (2013) 322– 350 333
analyses from dark, oscillatory-zoned cores yield 207Pb/206Pb spotages of 1009 ± 22 and 973 ± 24 Ma. These ages are interpreted asapproximating the timing of enderbite crystallization. The concord-ant analyses from overgrowth domains (n = 31) yield a weightedmean 207Pb/206Pb age of 954 ± 14 Ma (95% confidence, MSWD = 1.5;Fig. 8b), corresponding to new zircon growth during UHT metamor-phism. There is no systematic age difference between the brightlyluminescent domains and the less luminescent domains character-istic of the outer rims and homogenous grains (Table 2).
5.2. EGB-10-72
Twenty-six cores and rims from twenty-three zircon grainswere analysed from sample EGB-10-72 (Fig. 8c, Table 3). Theanalyses reveal two distinct age populations. An older popu-lation with 207Pb/206Pb spot ages ranging from 1514 ± 15 to1220 ± 17 Ma plots in a discordant array with poorly defined upperand lower intercepts of 1516 ± 34 and 613 ± 93 Ma, respectively(±2�, MSWD = 5.0; Fig. 8d). The array is broadly consistent withthe inheritance of late Mesoproterozoic detrital zircons affectedby a possible Pan-African thermal disturbance (ca 550–500 Ma;e.g. Mezger and Cosca, 1999; Gupta, 2011). Nine analyses from ayounger, concordant population yield a weighted mean 207Pb/206Pbage of 950 ± 17 Ma (95% confidence, MSWD = 1.4; Fig. 8e). Five con-cordant analyses, three of which correspond to anomalously old207Pb/206Pb spot ages of 1195 ± 17, 1146 ± 16, and 1025 ± 19 Ma,and two which yield younger 207Pb/206Pb spot ages of 851 ± 48 and488 ± 30 Ma, were not included in the calculation of the weightedmean age because they are outliers based on 1� uncertainties. Theobservation that the group of nine concordant analyses are fromrims is consistent with an interpretation that these ages date newzircon growth during UHT metamorphism.
Twenty-five analyses from multiple monazite grains yield aspread of 207Pb/206Pb spot ages ranging between 1001 ± 17 and915 ± 16 Ma (Fig. 8f, Table 4). All of the analyses are <10% dis-cordant. The youngest spot age of 915 ± 16 Ma is outside the1� uncertainties of the other analyses, and is therefore consid-ered to be an outlier. The remaining analyses define a weightedmean 207Pb/206Pb age of 966 ± 6 Ma, MSWD = 1.0 (95% confidence,MSWD = 1.0; Fig. 8f).
5.3. EGB-10-74
Thirty-three analyses from twenty-four zircon grains wereobtained from sample EGB-10-74 (Fig. 8g, Table 5). All of the anal-yses are <10% discordant. The three youngest spot ages (816 ± 19,803 ± 30 and 753 ± 55 Ma) are outliers with respect to the otheranalyses based on 1� uncertainties. Excluding these three ages,twenty-three analyses from overgrowth domains have a range of207Pb/206Pb spot ages from 1001 ± 28 to 874 ± 20 Ma, and sevenanalyses from the core domains have a similar spread of ages from1005 ± 22 to 893 ± 26 Ma (Fig. 8g).
Twenty-five analyses from sixteen monazite grains yielda spread of 207Pb/206Pb spot ages ranging from 980 ± 15 to519 ± 20 Ma (Fig. 8h, Table 6). All of the analyses are <10% dis-cordant. The analyses define four 207Pb/206Pb age populations of980–941 Ma (n = 3; Fig. 8i), 934–921 Ma (n = 4; Fig. 8i), 656–614 Ma(n = 6), and 551–519 Ma (n = 12; Fig. 8h). The oldest populationcorresponds to spot ages from a single grain, whereas the four207Pb/206Pb spot ages from 934 ± 11 to 921 ± 15 Ma are fromthree different grains. These two age populations are inter-
preted as evidence for protracted monazite growth during thehigh-temperature retrograde evolution to the solidus from ca980 to ca 920 Ma. The youngest age population from 11 grains(n = 12) has a weighted mean 207Pb/206Pb age of 533 ± 12 Ma (95%
334 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Fig. 7. Photomicrographs of selected zircon and monazite grains analysed in situ from samples EGB-09-01 and 04, respectively.
Table 2Ion microprobe results for zircons from sample EGB-10-71.
Ion microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(238U/206Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).All errors are at the 1� level; f204% is the percentage of common 206Pb, estimated from the measured 204Pb; grain spot is the individual grain and analysis identification.C, core analysis; M, metamorphic growth; D, >10% discordant; luminescence: m, moderate; b, bright; d, dark.
* Analyses with >10% discordance. Data arranged according to increasing 207Pb/206Pb age within domain groups.
F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350 335
Table 3Ion microprobe results for zircons from sample EGB-10-72.
Ion microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(238U/206Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).All errors are at the 1� level; f204% is the percentage of common 206Pb, estimated from the measured 204Pb; grain spot is the individual grain and analysis identification.M, metamorphic growth; D, >10% discordant.
* Analyses with >10% discordance. Data arranged according to increasing 207Pb/206Pb age within domain groups.
Table 4Ion microprobe results for monazite from sample EGB-10-72.
Age not included in calculation of weighted mean age72-2.1 1194 77,833 67.3 −0.526 6.621 0.788 0.06956 0.79737 907 7 915 16 0.7
Ion microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(238U/206Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).All errors are at the 1� level; f204% is the percentage of common 206Pb, estimated from the measured 204Pb; grain spot is the individual grain and analysis identification.Data arranged according to increasing 207Pb/206Pb age within domain groups.
336 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Fig. 8. U–Pb concordia plots of zircon and monazite analyses.
F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350 337
Fig. 8. ( Continued ).
338 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Fig. 8. ( Continued ).
F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350 339
Table 5Ion microprobe results for zircons from sample EGB-10-74.
on microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(2
ll errors are at the 1� level; f204% is the percentage of common 206Pb, estimated f, core analysis; M, metamorphic growth; data arranged according to increasing 207
onfidence, MSWD = 0.25; Fig. 8h), indicating monazite growthuring the younger Pan-African tectonothermal event. Six analy-es from five grains define a weighted mean 207Pb/206Pb age of33 ± 17 Ma (95% confidence, MSWD = 0.6; Table 1). This inter-ediate age population either corresponds to localized monazite
rowth during a Neoproterozoic metamorphic event or mixingetween the older and younger age populations.
.4. EGB-09-38
Forty-two cores and rims from thirty-eight zircon grains werenalysed from this sample. Twenty analyses are <10% discord-nt, which yield a range of 207Pb/206Pb spot ages from 2039 ± 95o 794 ± 119 Ma (Fig. 8j, Table 7). Older spot ages are typicallyssociated with core domains (ca 2039–1153 Ma; n = 9), whereasges obtained from brightly luminescent overgrowth domainsre younger, ranging from 1120 ± 41 to 794 ± 119 Ma (n = 10;ig. 8k). A single age from the core domain of a brightly lumi-escent grain yields an age of 1517 ± 151 Ma. The dataset fromhe overgrowth domain yields a poorly constrained and shal-ow discordia with an upper intercept age of 1007 ± 97 and aower intercept of 566 ± 320 Ma (not shown). This discordia lineoes not permit a meaningful estimate of the intercept ages,lthough the upper intercept age is broadly consistent with meta-orphic ages from the other samples. The lower intercept age
ay reflect overprinting associated with the younger Pan-African
ectonothermal event, as suggested by the rutile 206Pb/238U agef 451 ± 5 Ma for this sample (sample EB-38 of Taylor et al.,012).
6Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).e measured 204Pb; grain spot is the individual grain and analysis identification.
6Pb age within domain groups.
5.5. EGB-09-39
Forty-one analyses from thirty-nine zircon grains were obtainedfrom sample EGB-09-39 (Fig. 8l, Table 8). Seven concordant anal-yses from core domains yield 207Pb/206Pb spot ages ranging from1813 ± 180 to 954 ± 27 Ma. Twelve concordant analyses from theovergrowth domains correspond to 207Pb/206Pb spot ages between1059 ± 67 and 756 ± 47 Ma. Excluding the youngest spot age of954 ± 27 Ma from a core domain, the ages from the metamorphicovergrowths are younger than the cores. Ten of these analysesdefine a weighted mean 207Pb/206Pb age of 970 ± 28 Ma (95% con-fidence, MSWD = 1.1; Fig. 8m). The two youngest spot ages of772 ± 163 and 756 ± 47 Ma were not included in the calculation ofthe weighted mean age. In contrast to sample EGB-09-38, the zir-con age data in this sample provide little evidence for Pan-Africanoverprinting, despite a rutile 206Pb/238U age of 449 ± 6 Ma for thissample (sample EB-39 of Taylor et al., 2012).
5.6. EGB-10-82
Thirty-nine analyses from ten monazite grains were performedin situ to preserve their petrographic context (Table 9). Thirty-seven concordant analyses yield a spread of 207Pb/206Pb spot agesranging between 996 ± 14 and 902 ± 13 Ma (Fig. 8n, Table 9). Thespread of ages does not correlate with petrographic setting or
systematic differences in Y or Th zoning (Table 9 and Fig. 6a–iand l). These analyses define a weighted mean 207Pb/206Pb ageof 953 ± 7 Ma (95% confidence, MSWD = 1.7; Fig. 8n). The twoyoungest spot ages of 874 ± 20 and 854 ± 22 Ma are −9.7% and
340 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Table 6Ion microprobe results for monazite from sample EGB-10-74.
on microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(2
ll errors are at the 1� level; f204% is the percentage of common 206Pb, estimated fata arranged according to increasing 207Pb/206Pb age within domain groups.
17.9% discordant, respectively, and were not included in the cal-ulation of the weighted mean age.
.7. EGB-10-84
Thirty-four analyses performed in situ from nine monaziterains yield similar results to sample EGB-10-82, with a spread of07Pb/206Pb spot ages ranging between 990 ± 14 and 883 ± 12 MaFig. 8o, Table 10). There is no detectable correlation between age,etrographic setting, or chemical zoning. All of the analyses are10% discordant. Thirty-three of the analyses define a weightedean 207Pb/206Pb age of 948 ± 5 Ma (95% confidence, MSWD = 1.6;
ig. 8o). The youngest spot age of 883 ± 12 Ma is just within the 1�ncertainties of the other analyses, but has the highest discordance−7.7%). Therefore this spot age is considered to be anomalouslyoung and was not included in the calculation of the weighted meange. Two monazite grains included in garnet show cuspate embay-ents in core domains (Fig. 6r and s). These features are consistentith fluid-assisted dissolution and reprecipitation in the presence
f melt or fluid derived from crystallizing rocks at depth.
.8. EGB-09-01
Twenty-six analyses from eight zircon grains were analysed initu. Twenty-two analyses from metamorphic overgrowths yield
range of 207Pb/206Pb ages 990 ± 42 to 832 ± 51 Ma (Fig. 8p,able 11), which define a weighted mean 207Pb/206Pb age of29 ± 17 Ma (95% confidence, MSWD = 1.0; Fig. 8p).
.9. EGB-09-04
Nineteen monazite analyses from five grains were analysed initu. 207Pb/206Pb spot ages range from 1014 ± 27 to 804 ± 37 Ma
6Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).e measured 204Pb; grain spot is the individual grain and analysis identification.
(Fig. 8q, Table 12). All of the analyses are <10% discordant. Thirteenanalyses from four grains define a weighted mean 207Pb/206Pb ageof 968 ± 20 Ma (95% confidence, MSWD = 2.0; Fig. 8q). One grainalong the grain boundary between orthopyroxene and quartz yieldssignificantly younger spot ages (836–804 Ma; Fig. 8q), defining aweighted mean 207Pb/206Pb age of 822 ± 31 Ma (95% confidence,MSWD = 0.12; Table 1).
5.10. EGB-09-19
Twenty-two analyses from eleven zircon grains were per-formed in situ. The 207Pb/206Pb spot ages range from 1754 ± 44 to885 ± 15 Ma (Fig. 8r, Table 13), and eighteen of the analyses plot in adiscordant array with upper and lower intercepts of 1620 + 44/−45and 855 + 14/−15 Ma, respectively (±2�, MSWD = 1.2; Fig. 8r). Eightanalyses from weakly luminescent overgrowths in five grains are<10% discordant, yielding ages from 1129 ± 46 to 885 ± 15 Ma(Fig. 8r). Four of these analyses from three grains occurringalong fractures in garnet correspond to the youngest spot ages(909–885 Ma). The other four analyses yield distinctly older agesfrom 1129 ± 46 to 960 ± 17 Ma, although three of the grains yieldspot ages of 965–960 Ma. These three grains occur along a fracturein garnet, as an inclusion in quartz, and along a grain boundarybetween garnet and quartz.
6. Discussion
6.1. Age and duration of UHT metamorphism
Each of the samples dated in this study yields data that pro-vides constraints on the age and duration of UHT metamorphismin the Eastern Ghats Province. The data are of four types: (1) dis-crete, concordant populations of zircon overgrowths on older cores;
F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350 341
Table 7Ion microprobe results for zircons from sample EGB-09-38.
Ion microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(238U/206Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).All errors are at the 1� level; f204% is the percentage of common 206Pb, estimated from the measured 204Pb; grain spot is the individual grain and analysis identification.C
06Pb a
(aicaaiPNa
6(
sa(st
, core analysis; M, metamorphic growth; D, >10% discordant.* Analyses with >10% discordance. Data arranged according to increasing 207Pb/2
2) ranges of concordant zircon spot ages with weighted meanges from discrete, concordant populations; (3) upper and lowerntercept ages of discordant arrays of partially reset inherited zir-on cores and metamorphic rims that approximate concordantnalyses; and, (4) discrete, concordant populations from monazitenalysed in situ and as grain separates. These results have directmplications for models of the UHT evolution of the Eastern Ghatsrovince. Distinctly younger age populations associated with lateeoproterozoic overprinting do not bear on the UHT evolution andre not further considered.
.1.1. Northern sector of the study areaSalur–Pachipenta–Sunki)
Zircon and monazite from the three samples in the northernector of the study area (EGB-10-71, -72, -74) yield broadly similar
ge results. The range in zircon spot ages from sample EGB-10-711042 ± 28 to 842 ± 30 Ma) is within error of the spot ages fromample EGB-10-74 (1005 ± 22 to 874 ± 20 Ma), and implies pro-racted growth during a high-temperature history. Zircon from
ge within domain groups.
EGB-10-72 yields a weighted mean age of 950 ± 17 Ma. Monazitespot ages are less scattered as compared to zircon. Monazite fromsample EGB-10-74 yields spot ages from 980 ± 15 to 921 ± 15 Ma,although monazite from sample EGB-10-72 defines a weightedmean age of 964 ± 7 Ma. These age populations are interpretedas evidence for protracted monazite growth during the high-temperature retrograde evolution to the solidus from ca 980 to ca920 Ma.
Monazite analyses from two high Mg–Al granulites near Sunki,reported in Korhonen et al. (2011), show slight reverse discordance,indicating excess 206Pb. For this reason, 207Pb/235U ages were con-sidered to be more reliable, as excess 206Pb does not affect the207Pb/235U ratio (e.g. Kirkland et al., 2009). Monazite from bothsamples yielded weighted mean ages of ca 980 Ma, with a spread of207Pb/235U spot ages from ca 1043 to ca 922 Ma. A monazite inclu-
sion in an orthopyroxene porphyroblast was interpreted to date thelate prograde growth of the peak minerals at ca 1042 Ma. Althoughzircon and monazite from the samples in the present study yieldyounger weighted mean ages as compared to the data from the
342 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Table 8Ion microprobe results for zircons from sample EGB-09-39.
Ion microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(238U/206Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).A rom thC oderat
06Pb a
SS((mspfs
grcitbctt
ll errors are at the 1� level; f204% is the percentage of common 206Pb, estimated f, core analysis; M, metamorphic growth; D, >10% discordant; luminescence: m, m
* Analyses with >10% discordance. Data arranged according to increasing 207Pb/2
unki high Mg–Al granulites, the spread in monazite spot ages fromunki is similar to the range of zircon spot ages in the enderbitesample EGB-10-71; Fig. 8a, Table 2) and in the host migmatitesamples EGB-10-72, -74; Fig. 8c and g, Tables 3 and 5). Further-
ore, the weighted mean monazite ages of ca 980 Ma are broadlyimilar to two 207Pb/206Pb spot ages from zircon cores in sam-le EGB-10-71 (Table 2). A similar age of ca 979 Ma was reportedor emplacement of porphyritic charnockite collected about 60 kmouth of this area by Grew and Manton (1986).
The breakdown of LREE- and Zr-bearing phases during the pro-rade history may result in the growth of monazite and zircon,espectively, prior to peak metamorphism. Therefore the spread ofoncordant spot ages in each sample is likely to have geologic signif-cance, and may record protracted accessory mineral growth duringhe early high-to-ultrahigh temperature history. Nevertheless, the
ulk of zircon and monazite growth is predicted to occur duringrystallization of residual melt (cf. Kelsey et al., 2008) retained athe melt percolation threshold (Cheadle et al., 2004). Therefore,he weighted mean ages are interpreted as statistically valid; they
e measured 204Pb; grain spot is the individual grain and analysis identification.e; b, bright; d, dark.ge within domain groups.
will typically record the timing of melt crystallization close to thesolidus, possibly by diffusive loss of H2O (cf. White and Powell,2010), even though accessory mineral growth will have occurredover a range of temperature (cf. Kelsey et al., 2008) and, therefore,time according to the cooling rate.
Although the monazite and zircon weighted mean ages of ca966 and 950 Ma in sample EGB-10-72 overlap at 2� uncertainty,and the range of spot ages for each is similar, within the concord-ant age population the oldest of the monazite spot ages is older thanthe oldest of the zircon spot ages in the same sample (Fig. 8e and f).The same is broadly true for the range of spot ages for monazite andzircon in sample EGB-10-74, although in this case the zircon agesextend to <900 Ma. These observations contrast with the postu-late that zircon growth in residual pelites typically begins at highertemperatures than monazite (cf. Kelsey et al., 2008). In addition, the
Sunki samples yield weighted mean ages from monazite that areolder than the monazite weighted mean age from sample EGB-10-72. Thus, residual melt in the Sunki rocks may have been trappedby the percolation threshold earlier than in the migmatite sample.
F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350 343
Table 9Ion microprobe results for monazite from sample EGB-10-82.
Ion microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(238U/206Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).A 206 m the 204
aa
ghted
AbEe
iz
ll errors are at the 1� level; f204% is the percentage of common Pb, estimated frorranged by analysis in order to preserve petrographic setting informationdj., ajdacent to; cd, cordierite; opx, orthopyroxene; sill, sillimanite.
* Analyses with >10% discordance, which were not included in calculation of wei
s a result, during cooling the residual melt in these rocks may haveecome saturated in LREE at a higher temperature than in sampleGB-10-72, which, in turn, may have led to growth of monazite
arlier.
The timing of enderbite emplacement (sample EGB-10-71) isnferred from two concordant analyses from dark, oscillatory-oned cores that yield ages of 1009 ± 22 and 973 ± 24 Ma. These
measured Pb; grain spot is the individual grain and analysis identification. Data
mean age.
ages are consistent with the timing of the dominant charnockite-forming event in the Eastern Ghats Province at ca 1000–900 Ma(reviewed in Rajesh, 2012). In this interpretation, emplacement
of enderbite into the surrounding pelitic migmatite gneisses (asrepresented by sample EGB-10-72) was contemporaneous withthe immediate post-peak P–T conditions recorded at ca 980 Ma inhigh Mg–Al granulites at the Sunki locality to the west-southwest
344 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Table 10Ion microprobe results for monazite from sample EGB-10-84.
Ion microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(238U/206Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).All errors are at the 1� level; f204% is the percentage of common 206Pb, estimated from the measured 204Pb; grain spot is the individual grain and analysis identification. Dataarranged by analysis in order to preserve petrographic setting information.a artz; s
((iBPfmp1tuistca
dj., ajdacent to; cd, cordierite; g, garnet; kfs, K-feldspar;opx, orthopyroxene; q, qu** Not included in calculation of weighted mean age.
Fig. 1b). At Sunki, the peak P–T conditions of T >950 ◦C at P >9.5 kbarKorhonen et al., 2011) were followed by close-to-isobaric cool-ng to the solidus (Fig. 9a, P–T–t paths ND1 and NSK; Korhonen,rown and Clark, unpublished data). In the absence of quantitative–T data from the migmatites, by analogy with the P–T–t pathor the high Mg–Al granulites at Sunki we interpret the younger
onazite and zircon weighted mean ages from migmatite sam-les EGB-10-72 and -74 to be consistent with a period of ca5–30 My of close-to-isobaric cooling from the peak P–T conditionso a lower temperature solidus than that for the highly resid-al granulites. Further, we infer that the enderbite was emplaced
nto suprasolidus crust at a temperature that must have been the
ame as or more likely hotter than the ambient temperature ofhe host rocks as suggested by field observations showing diffuseontacts between the enderbite and leucosome from migmatitend back-veining of leucosome into enderbite (Fig. 2a and b).
ill, sillimanite.
Subsequent low-volume melting of the crystallized enderbite in situ(Fig. 2a (arrows) and c) may have been facilitated by fluids releasedduring progressive crystallization of melt trapped by the percola-tion threshold in the host migmatites during around ca 950 Ma,consistent with growth of zircon in samples EGB-10-71 and -72.
Alternatively, the sparse number of zircon cores analysed inthe enderbite sample (EGB-10-71) leaves open the possibility thatthe enderbite may have been emplaced significantly before theUHT metamorphism, in which case all the spot ages obtained fromthis sample would be interpreted as related to the UHT metamor-phic event. Elsewhere in the Eastern Ghats orogenic belt an oldercharnockite-forming event has been identified at ca 1700–1600 Ma
(Dharma Rao et al., 2012b; Rajesh, 2012). However, the field obser-vations at this locality (Fig. 2) and the available zircon data fromthis sample are most simply interpreted as emplacement of theenderbite magma at ca 980 Ma during the UHT metamorphic event
F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350 345
Table 11Ion microprobe results for zircons from sample EGB-09-01.
Ion microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(238U/206Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).All errors are at the 1� level; f204% is the percentage of common 206Pb, estimated from the measured 204Pb; grain spot is the individual grain and analysis identification.adj., ajdacent to; cd, cordierite; opx, orthopyroxene; sill, sillimanite.M pl, p2
f9
7bptctos(ttcdwttTo
, metamorphic growth; D, >10% discordant; bi, biotite; g, garnet; ilm, ilmenite;07Pb/206Pb age within grains.
* Analyses with >10% discordance.
ollowed by limited in situ anatexis and new zircon growth at ca50 Ma.
Field observations of the outcrop from which sample EGB-10-4 was collected constrain the timing of partial melting recordedy folded stromatic migmatite and patch migmatite to be syn-D2,robably corresponding to a late stage in the crystallization of meltrapped by the percolation threshold (Fig. 2e and f). The age of meltrystallization is interpreted at ca 950 Ma broadly constrains theiming of D2. The timing of syn- to post-D1a melting may haveccurred as early as ca 980 Ma, but the sparse occurrence of olderpot ages from zircon overgrowths in samples EGB-10-71 and -72ca 1195 to ca 1025 Ma) implies protracted zircon growth duringhe prograde history and/or during ongoing partial melting at closeo peak metamorphic conditions. The absence of ages older thana 1005 Ma in sample EGB-10-74 likely reflects enhanced zirconissolution in this sample, such that most inherited zircon grainsere dissolved into the melt during prograde metamorphism to
he peak UHT conditions, as indicated by fewer inherited cores inhis sample as compared to other samples (e.g. EGB-10-72; Fig. 3).he ca 1005 Ma age may provide an upper constraint on the timingf peak UHT metamorphic conditions in this sample.
lagioclase; q, quartz. Data arranged by (1) domain, (2) grains, and (3) increasing
6.1.2. Central sector of the study area (Anantagiri–Sunkarametta)Older (ca 970 Ma) and younger (ca 950 Ma) age populations are
also present in the central sector. The two sapphirine-bearing, highMg–Al granulite samples from Anantagiri (samples EGB-09-38, -39) show a broadly similar range of zircon spot ages (Fig. 8j andl). Zircon rim analyses from sample EGB-09-38 yielded spot agesranging from 1120 ± 41 to 794 ± 119 Ma. Broadly similar resultswere obtained from sample EGB-09-39, with spot ages between1059 ± 67 and 756 ± 47 Ma and a weighted mean 207Pb/206Pb ageof 970 ± 28 Ma; the weighted mean age is interpreted to recordthe age of final crystallization of residual melt following close-to-isobaric cooling to the solidus (Fig. 9a, P–T–t path C39; Korhonen,Brown and Clark, unpublished data).
Monazite from the high Mg–Al granulites from Sunkaram-etta yielded slightly younger ages, with weighted mean ages of953 ± 7 Ma for sample EGB-10-82 and 948 ± 5 Ma for sample EGB-10-84 (Table 1). The probabilities of fit for these calculated ages are
very low (=0.004 and 0.015, respectively), implying that the uncer-tainties on the individual data do not account for the protracted agespread. Although only one discrete age population may be resolved,the range of protracted ages is interpreted to have geologic
346 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Table 12Ion microprobe results for monazite from sample EGB-09-04.
Ion microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(238U/206Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).All errors are at the 1� level; f204% is the percentage of common 206Pb, estimated from the measured 204Pb; grain spot is the individual grain and analysis identification.oD
snatlKmaat
dTrafds1h
6M
0ssSwttis
px, orthopyroxene; q, quartz; sill, sillimanite.ata arranged according to increasing 207Pb/206Pb age within grains.
ignificance. The majority of monazite is inferred to have grownear the conditions of the solidus, therefore the weighted meanges are interpreted to be statistically valid ages that record theiming of final crystallization of residual melt following a CCW evo-ution and cooling to the solidus (Fig. 9a, P–T–t paths C82 and C82;orhonen, Brown and Clark, unpublished data). Cuspate embay-ents in core domains of two monazite inclusions in garnet (Fig. 6r
nd s) indicate that fluid-assisted dissolution of pre-existing grainsnd reprecipitation in the presence of melt also occurred duringhe high-temperature retrograde evolution.
Additional age information for the Sunkarametta locality iserived from sample EG-San3A from the study of Bose et al. (2011).his orthopyroxene-bearing quartzofeldspathic orthogneiss has aeported location about 120 m southwest of samples EGB-10-82nd EGB-10-84. It yields a range of discordant 207Pb/206Pb spot agesrom recrystallized domains in zircon from ca 1600 to ca 1100 Ma,efining a lower intercept age of 1008 ± 49 Ma. Near-concordantpot ages from undisturbed oscillatory-zoned grains range from ca025 to ca 950 Ma. A single spot age from an overgrowth domainas an age of ca 980 Ma.
.1.3. Southern sector of the study area (Paderu–Gangarajaadugula)
The monazite weighted mean age of 968 ± 20 Ma in sample EGB-9-04 is older than the zircon weighted mean age of 929 ± 17 Ma inample EGB-09-01 (Table 1), although these samples yield a broadlyimilar range of concordant spot ages. Similar to the samples fromunkarametta in the central sector, the probability of fit for theeighted mean age in sample EG-09-04 is low (=0.02). However,
he weighted mean age is interpreted to be statistically valid ando record the timing of final crystallization of residual melt, assum-ng that the majority of accessory phase growth occurred near theolidus. The weighted mean ages from samples EGB-09-01 and -04
are within error of the ca 980–970 Ma and ca 950 Ma ages recordedin samples from the northern and central sectors of the study area.
Sample EGB-09-04 records peak metamorphic conditions of∼945 ◦C and 8.3 kbar followed by close-to-isobaric cooling to thesolidus at 900 ◦C (Fig. 9a, P–T–t path S; Korhonen, Brown andClark, unpublished data). Sample EGB-09-01 preserves evidencefor close-to-isobaric cooling just above the solidus, although esti-mates for peak conditions are more poorly constrained at >860 ◦C at6–11 kbar, with the minimum temperature defined by the solidus(Fig. 9a). Although the temperature of the solidus will be sensi-tive to the H2O content used in the phase equilibria modelling, themineral assemblages in these samples provide tight constraints onthe appropriate H2O content to be used in the modelling. Sincethe majority of accessory phase growth is predicted to occur closeto the solidus, the ∼40 ◦C difference in the predicted temperatureof the solidi for these two samples would have resulted in a sig-nificant age difference if the cooling rate was sufficiently slow.Mezger and Cosca (1999) proposed a cooling rate of 3 ◦C/My basedon monazite and titanite ages from the northern part of the EasternGhats Province, which would result in an age difference of 13 Mafor this scenario. A lower cooling rate of 1 ◦C/My could account forthe 40 My difference between the monazite weighted mean age of968 ± 20 Ma in sample EGB-09-04 and the zircon weighted meanage of 929 ± 17 Ma in sample EGB-09-01 (Fig. 9b).
The zircon age results from sample EGB-09-19 are more ambigu-ous. Three grains yield spot ages of ca 965 to ca 960 Ma (Table 13),and are within uncertainty of the ca 950 Ma ages recorded in othersamples. However, there is also evidence for a younger history, asindicated by the lower intercept age of 855 + 14/−15 Ma (Fig. 8p)
and the younger population from the concordant analyses (spotages from ca 909 to ca 885 Ma, Table 13). Sparse evidence for ayounger overprinting event is also recorded by a monazite grainalong the grain boundary between orthopyroxene and quartz in
F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350 347
Table 13Ion microprobe results for zircons from sample EGB-09-19.
Ion microprobe U–Pb data. Disc (%) is the age discordance as defined = 100 × (1 − [(238U/206Pb date) − (207Pb/206Pb date)]/(207Pb/206Pb date)).All errors are at the 1� level; f204% is the percentage of common 206Pb, estimated from the measured 204Pb; grain spot is the individual grain and analysis identification.C ite; pl
3) incr
sanz(aAaodtt
6
UPolssptwti
, core analysis; M, metamorphic growth; D, >10% discordant; g, garnet; ilm, ilmen* Analyses with >10% discordance. Data arranged by (1) domain, (2) grains, and (
ample EGB-09-04 (weighted mean age of 822 ± 31 Ma; Table 1),nd spot ages younger than ca 900 Ma recorded in samples from theorthern and central sectors of the study area. Similar ages fromircon and monazite have been interpreted by Simmat and Raith2008) to date local- to regional-scale ductile to brittle deformationnd associated fluid infiltration during the interval ca 900–750 Ma.lternatively, these ages could be related to the release of fluidsssociated with the waning stages of cooling and crystallizationf residual melt trapped by the percolation threshold at greaterepth in the orogeny, since the decay in the thermal structure ofhe orogeny is expected to young downwards to greater depth withime.
.2. Implications for the Eastern Ghats Province
Each of the sample localities in this study preserves evidence forHT metamorphism. Conditions of peak metamorphism from theaderu locality (samples EGB-09-01, -04) in the southern sectorf the study area are estimated to be ∼945 ◦C and 8.3 kbar, fol-owed by close-to-isobaric cooling to conditions of the elevatedolidus (Fig. 9a). Modelling results from samples in the centralector yield similar constraints (Fig. 9a). Peak conditions for sam-les from the Sunki locality in the northern sector were estimated
o be in excess of 950 ◦C and 9.5 kbar, although pressure was notell constrained (Korhonen et al., 2011). In many samples from all
hree sectors of the Eastern Ghats Province, cordierite–K-feldsparntergrowths occur. These are interpreted to represent peak to
, plagioclase; q, quartz.easing 207Pb/206Pb age within grains.
immediate post-peak retrograde reaction between osumilite andmelt and together with the early development of cordierite in somesamples requires that the prograde evolution was one of coevalincreasing temperature and increasing pressure (Korhonen, Brownand Clark, unpublished data). A reinvestigation of the Sunki sam-ples suggests that the pressure estimated by Korhonen et al. (2011)may have been high. New results indicate that the peak and retro-grade P–T–t conditions recorded in the Sunki samples are similar tothose retrieved from samples from the central and southern sectorsof the study area, as summarized in Fig. 9a. In summary, all samplesexhibit close-to-isobaric cooling to the solidus from broadly similarpeak P–T conditions and several define CCW P–T–t paths (Korho-nen, Brown and Clark, unpublished data). Thus, the results fromthis study demonstrate that widely spaced localities in the east-ern Ghats Province share a broadly similar tectono-metamorphichistory.
The spread of concordant 207Pb/206Pb spot ages between ca 1130and ca 980 Ma is interpreted as the approximate time span forreaching the peak of UHT metamorphism and cooling to tempera-tures close to the elevated solidi for the samples where it is inferredthat most new accessory mineral growth occurred; this implies along period of heating to and slow cooling from peak UHT con-ditions. Following the peak of UHT metamorphism, most samples
record close-to-isobaric cooling to an elevated solidus between ∼8and ∼7.5 kbar (Fig. 10). These residual granulites yield a spreadof weighted mean 207Pb/206Pb ages, ranging from ca 980 to ca930 Ma; they are interpreted as the best estimate for the timing of
348 F.J. Korhonen et al. / Precambrian Research 234 (2013) 322– 350
Fig. 9. P–T–t evolution of studied samples. (a) P–T paths and pos-itions of the solidi constrained by phase equilibria modelling in theNa2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO)chemical system. Methodology for modelling similar to that reported in Korhonenet al. (2011); the detailed results from this study will be presented elsewhere(Korhonen, Brown and Clark, unpublished data). Patterns for the solidi are shownin the legend and labelled with italic text. P–T–t paths are colour-coded arrows,correlated with the colour of the solidus. Results from the northern, central andsouthern sectors denoted by ‘Nxx’, ‘Cxx’ and ‘Sxx’, respectively, where xx refers tolast two digits of sample number. (b) Weighted mean 207Pb/206Pb age (Ma) versusestimated temperature of solidus (◦C) along P–T paths shown in Fig. 10a. Error barsfor age data are 95% confidence intervals and solidus temperature are ∼1�. (Forit
cosfa
a(nma
800 850 900 950 1000 1050
6
7
8
9
10
P (k
ba
r)
T (°C)
S01 C39S04
C84
C82
ND1NSK
Peak conditions
>>950°C, ~8 kbar
ca 930 ....... 980 Ma
929 ± 17 Ma
Fig. 10. Summary P–T–t path for the Eastern Ghats Province as a whole. The vari-
nterpretation of the references to colour in this figure legend, the reader is referredo the web version of this article.)
rystallization of residual melt trapped by the percolation thresh-ld and since the temperature of the solidus varies for each sample,o there is a range of ages (Fig. 9b). Thus, the temperature intervalrom the peak of UHT metamorphism to the solidus for each samplelso varies (Fig. 10).
There are similarities between the results presented herend the ages reported by Bose et al. (2011) and Das et al.
2011). These two studies interpreted oscillatory-zoned zircon withear-concordant ages ranging from ca 1780 to ca 1700 Ma to recordagmatic events in the source of the detrital grains, and discordant
ges from ca 1600 to ca 1100 Ma to represent mixing ages resulting
ability in the calculated weighted mean ages between the samples results mainlyfrom differences in the temperature of the solidi. See text for additional details.
from M1 UHT metamorphism and these detrital cores. The timing ofM1 UHT metamorphism was constrained to be ca 1030 to ca 990 Ma,based on chemical dates of monazite inclusions in orthopyrox-ene. Near concordant spot ages from zircon overgrowths betweenca 1000 and ca 900 Ma were interpreted by Bose et al. (2011) torecord a second granulite metamorphism (M2), which was pre-ceded and/or accompanied by emplacement of voluminous graniteat ca 990 Ma. However, it is likely that these two ‘events’ are partof one continuous P–T–t evolution through peak UHT metamorphicconditions. Das et al. (2011) estimated the timing of the M2 event tobe 953 ± 6 Ma based on seven concordant analyses from neoblasticzircon grains and overgrowths on pre-existing zircon, and explicitlyinfer that M2 was a separate tectonothermal event superimposedon UHT granulites that had already cooled from the M1 thermalpeak. Zircon growth at ca 900 Ma was interpreted by them to beassociated with later fluid-induced retrogression associated withmelt crystallization, whereas Bose et al. (2011) left open the possi-bility of younger ages being due to later tectonothermal reworkingor a Pan-African overprint.
The new results presented in this study allow for a more com-plete interpretation of the evolution of the Eastern Ghats Province.An important outcome is the inference that the age populationsbetween ca 1130 Ma and ca 930 Ma correspond to a single pro-grade, peak and retrograde metamorphic evolution, characterizedby close-to-isobaric cooling from peak UHT conditions to differentsolidi for individual samples (Fig. 10). The timing of final melt crys-tallization in each sample is broadly constrained by the monaziteand zircon weighted mean ages. However, the range of spot agesdoes suggest protracted accessory mineral growth during a long-lived high-temperature evolution involving an extended period ofheating followed by slow cooling and the possibility of ingress offluids from deeper in the crust. The variability in the calculatedweighted mean ages between the samples results mainly from dif-
ferences in the temperature of the solidi (e.g. Fig. 9b). In a slowlycooled terrane, these differences may result in distinct age popula-tions as proposed by Reno et al. (2012) and confirmed in this study.The dominant age population of ca 960 to ca 950 Ma that occurs
rian R
tBMfibbtii
EtitcbpCstmgEd2icFca
7
g9oeBaempttepyeArte
A
chaAaott
F.J. Korhonen et al. / Precamb
hroughout the Eastern Ghats Province (this study; Das et al., 2011;ose et al., 2011; Simmat and Raith, 2008; Bhattacharya et al., 2003;ezger and Cosca, 1999) might reflect broadly similar conditions of
nal melt crystallization in many parts of the Province, as suggestedy the cluster of solidi between 930–960 ◦C at 7.5–8 kbar predictedy the results of the phase equilibria modelling for the samples inhis study (Fig. 9). The emplacement of voluminous pophyritic gran-tes across the region between ca 985 and ca 955 Ma (summarizedn Dobmeier and Raith, 2003) is consistent with this interpretation.
The results from the Salur and Pachipenta localities (samplesGB-10-71, -72, -74) provide insight into the timing and condi-ions of enderbite emplacement. The charnockites and enderbitesn the Eastern Ghats Province are interpreted to be derived by par-ial melting of a hornblende-rich mafic source, with massif-typeharnockite representing the segregated partial melt and ender-ites representing a mixture composed of melt and entrainederitectic ± residual solids (Kar et al., 2003; Bhattacharya andhaudhary, 2010). Hornblende-breakdown melting of a maficource implies temperatures >900 ◦C, and the geochemistry of Pro-erozoic charnockites from Paderu and Sunki is consistent with
elting at relatively shallow depths in the stability field of pla-ioclase, as compared to Archaean charnockites elsewhere in theastern Ghats Province which were derived by melting at greaterepth in the stability field of garnet (Bhattacharya and Chaudhary,010). Thus, the petrogenesis of the charnockites and enderbites
s consistent with emplacement of magmas into hot, suprasolidusrust at ca 980 Ma during a single UHT tectono-metamorphic event.inal melt crystallization in the host rocks at ca 950 Ma further indi-ates that suprasolidus conditions in the crust were maintained forn additional 30 My.
. Conclusions
The Eastern Ghats Province records a single long-lived high-rade metamorphic evolution in the interval ca 1130 Ma to ca30 Ma. The timing of peak UHT metamorphism is estimated to belder than 980 Ma and possibly as old as 1042 ± 41 Ma (Korhonent al., 2011), consistent with the estimate of 1030–990 Ma fromose et al. (2011), with P–T conditions estimated to be >950 ◦Cnd ∼8 kbar. These peak metamorphic conditions preceded themplacement of regionally extensive charnockite–enderbite mag-as at ca 980 Ma. The predominant zircon and monazite age
opulations across the region in the interval ca 980–930 Ma recordhe immediate post-peak evolution, characterized by slow close-o-isobaric cooling from peak UHT conditions to ∼7.5 kbar at thelevated solidus for each sample (Fig. 10). Differences in the tem-erature of the solidi are responsible for the variability in theseounger age populations across the region (Fig. 9b), and the differ-nces in age are consistent with slow cooling at a rate of ∼1 ◦C/My.ges younger than ca 930 Ma are more enigmatic, but may beelated to the release of fluids associated with final melt crystalliza-ion at depth during the waning stages of this tectono-metamorphicvent.
cknowledgements
We would like to thank S. Dasgupta, B. Reno and S. Gupta forritical comments on an earlier version of this paper, and editorialandling by M. Satish-Kumar. SHRIMP analyses were undertakent the John de Laeter Centre for Mass Spectrometry, a Westernustralian state government supported Centre of Excellence. We
lso acknowledge the facilities, scientific and technical assistancef the Centre for Materials Research at Curtin University, andhe Australian Microscopy & Microanalysis Research Facility athe Centre for Microscopy, Characterisation & Analysis at the
esearch 234 (2013) 322– 350 349
University of Western Australia. We thank A. Josephs for assistancewith the SHRIMP at Curtin University. This project was fundedthrough the DIISR Australia–India Strategic Fund project ST030046.C. Clark acknowledges salary and research support from DECRA(DE120103067).
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