The building of south China: collision of Yangzi and Cathaysia blocks, problems and tentative answers

Journal of Southeast Asian Earth Sciences, Vol. 13, Nos 3-5, pp. 223-235, 1996 Copyright 0 19% Elsevier Science Ltd Pergamon

PII: SO743-%47(%)00029-3 Printed in Great Britain. All rights reserved

0743-9547/96 $15.00 + 0.00

The building of south China: collision of Yangzi and Cathaysia blocks, problems and tentative answers

Jacques Charvet, * Liangshu Shu,? Yangshen Shi,f Lingzhi Guoi and Michel Faure*

* DOpartement des Sciences de la Terre, Universitt d’orlians; and URA CNRS 1366, FR 009, BP 6759,45067 Orltans cedex, France; and 7 Department of Earth Sciences, Nanjing University, 210008

Nanjing, People’s Republic of China

Abstract-This paper aims to give answers to the questions of timing, tectonic style and geodynamic interpretation of south China geologic development. In the middle Jiangnan segment, Yangzi plate and Cathaysia got clearly welded during a Late Proterozoic orogeny of collisional type, marked by HP/LT metamorphism, ophiolite melange abduction, thrusting of greenschist nappes, emplacement of collisional S-type granites. This collision, which built the initial Jiangnan belt, began around 950 + 40 Ma and was completed at about 770-800 Ma ago. Kinematic study indicates that the Cathaysia plate was underthrust beneath the Yangzi plate. A likely earlier collisional event occurred around 1500 Ma, but is poorly preserved in the studied area. The proposed geodynamic model implies two successive suturings of oceanic domains during the Middle-Late Proterozoic: one about 1500 Ma and one about 950 Ma. A strong remobilization occurred during the Early Paleozoic ‘Caledonian’ orogeny, which induced transpressive ductile deformation. Thin-skinned folding and thrusting took place during the Mesozoic; this intracontinental shortening could be due to collision between the China-Indochina and west Philippines blocks. South China is a composite block, comprising the relics of at least three, maybe four sutures from Jiangnan to the coast. Copyright 0 1996 Elsevier Science Ltd

Introduction

The Asian continent was formed by amalgamation of major blocks (e.g. Chen et al., 1993). The south China block is classically separated into two tectonic regions (e.g. Huang, 1978; Huang et al., 1987; Wang, 1986): the Yangzi platform or paraplatform to the north and the south China fold system to the south, also called south China fold belt, south China orogen, or Cathaysian fold belt, or Cathaysia. Objecting to the view that these areas were not stable since the Early Paleozoic ‘Caledonian’ phase, as currently admitted (Huang et al., 1987; Yang et al., 1986), but were instead deformed in younger times, Hsii et al. (1988, 1990) proposed the terms of Yangzi folded belt and Huanan folded or deformed belt, respectively.

The transition between the Yangzi block and Cathaysia is occupied, in southern Anhui and northern Jiangxi, by the Jiangnan belt, or Jiangnan uplift (Wang, 1986), or Jiangnan platform uplift (Huang et al., 1987). The Jiangnan belt is usually regarded as the present southern edge of the Yangzi block, as it is bounded to the south by the Pingxiang-Jiangshan deep fault zone (Fig. l), a segment of the so-called Yichun-Shaoxing fault (Wang, 1986), which separates the two blocks.

The tectonic interpretation of the south China block is still a matter of debate. Some authors (e.g. Ren, 1991) argue for a unique block since the Precambrian, without any suture zone in it. In this view, the Jiangnan belt is a simple transitional polycyclic anticlinorium. For many others, the Jiangnan belt is the result of the amalgamation of the Yangzi and Cathaysia plates (Guo et al., 1980, 1985; Zhang et al., 1984) but has been

interpreted to have formed during collisional events in the:

-Middle-Late Proterozoic (Guo et al., 1980, 1985; Rowley et al., 1989; Chen et al., 1991; Xing et al., 1992);

-Ordovician-Silurian (Haynes, 1988); and -Late Paleozoic-Mesozoic (Hsii et al., 1988, 1990).

Lastly, admitting an initial Middle-Late Proterozoic collision, some geologists emphasize the later reworking of the belt due to younger collisional events (e.g. Shu et al., 1991; Shu, 1991; Xu et al., 1992; Shi et al., 1994).

Besides the Jiangnan belt itself, and to the south and east of it, the presence of several younger suture zones (i.e. Early Paleozoic ‘Caledonian’ and Permian-Triassic ‘Indosinian’) is debated (i.e. Zhang et al., 1984; Ren, 1991).

The aim of this paper is to propose some answers to the questions of timing, tectonic style and geodynamic interpretation of the geologic development of south China since the Proterozoic. It will be focused mainly on the Proterozoic events recorded in the middle Jiangnan belt, for which a tentative geodynamic model will be suggested.

General geological setting of the Jiangnan belt

The middle part of the Jiangnan belt can presently be divided into several blocks, bounded by major faults (Fig. 1). It covers about 20,000 km2 of northern Jiangxi, western Zhejiang-southern Anhui, and a part of eastern Hunan provinces. We focus on the northern Jiangxi area

223

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Collision of Yangzi and Cathaysia blocks 225

(Fig. 2). In this area, the pre-Sinian basement, composed of metamorphic rocks of greenschist facies and strongly deformed melanges, is cropping out, together with Sinian and post-Sinian strata, weakly deformed and weakly or non-metamorphosed.

Two main opposite tectonic interpretations have been advocated for this region. Hsii et al. (1988, 1990) reinterpreted the geological map at 1:500,000 and suggested that the Proterozoic Banxi Group, renamed the Banxi melange, was tectonically overlying, as a large rootless nappe, the Paleozoic to Triassic strata, which would crop out systematically within tectonic windows. Furthermore, the Precambrian granites and gneiss should in turn tectonically overlie the Banxi Group, forming nappes and klippes at the top of the structural pile. In this view, the area is a late Mesozoic collisional suture zone, and the largely unfossiliferous Banxi ophiolitic melange might be as young as Mesozoic, The collision should have occurred during the Late Cretaceous (Hsii et al., 1988) or the Jurassic (Hsii et al., 1990) with a northwestern vergence of thrusting. This interpretation has been already criticized, based on field data (e.g. Rowley et al., 1989; Charvet and Faure, 1989; Rodgers, 1989; Gupta, 1989; Shu et al., 1991).

On the contrary, according to the classical view, linked with the concept of south China paraplatform or platform (e.g. Wang, 1986; Hamilton, 1979), the Precambrian granites, and the Proterozoic metamorphic rocks and sedimentary strata constitute the basement, overlain unconformably by the Sinian and/or Paleozoic strata. Then, the suture, if any, is of Late Proterozoic or Caledonian age.

Our field data allow us to rule out the former interpretation (Shu et al., 1991). Indeed, strata up to Upper Jurassic are strongly folded and involved in thrusting, also documented by drilling, and give evidence for an important Indosinian and Yenshanian tectonism (e.g. Zhu, 1983; Wang, 1986; Shu, 1991). Sometimes, the Middle-Late Proterozoic greenschists are overthrust along low-angle faults onto the Late Paleozoic to Early Mesozoic rocks. But, these individual thrusts, generally south-verging, are rooted, and only brittle deformation occurs along the thrust planes, which cut at high angle the schistosity of Proterozoic rocks (Shu et al., 1991; Figs 3 and 4). The hypothesis combining two large rootless nappes of Banxi melange and Precambrian crystalline basement on one hand, and tectonic windows of Paleozoic and Mesozoic rocks on the other, is not valid. However, locally, the younger thrusting does put Late Paleozoic strata above Jurassic ones, as stated by Hsii et al. (1988) near Leping. Although these younger thrusts are typical of thin-skinned tectonics, they argue against the concept of a paraplatform which would have been stable since the Early Paleozoic.

As shown by the unconformities (of Sinian and Late Paleozoic strata, respectively, Figs 3 and 4), and cross-cut relationships of granitic bodies (Figs 3 and 5; Shu, 1991; Chen et al., 1991), the deep-seated deformation and the schistosity are restricted to Proterozoic and Early Paleozoic rocks. Most of the radiometric ages of HP/LT metamorphic rocks and ophiolitic remnants cluster within the Late Proterozoic; some metamorphic ages of greenschists reach the Early Paleozoic (Shu and Zhou 1988; Shu et al., 1991; Chen et al., 1991; Xu et al., 1992). So, if the Jiangnan belt represents a suture zone, this suture was

likely due to a Late Proterozoic or Caledonian tectonic event.

In a previous paper (Shu et al., 1991), we interpreted the Nanchang-Wanzai sinistral ductile shear zone, in the middle segment of the Jiangnan belt, as the result of a Caledonian transpressive deformation, post-dating a Late Proterozoic orogeny. In the next section, we will focus attention on both sides of the Dongxiang-Shexian fault zone, or Northeastern Jiangxi Fault (NEJF), where relics of a convincing Late Proterozoic melange-bearing suture zone are preserved (Shu 199 1; Shi et al., 1994; Xu, 1988; Xu et al., 1992).

Evidence of a Late Proterozoic collisional event in northeastern Jiangxi

Figure 5 presents a summary of the stratigraphy of northeastern Jiangxi-southern Anhui area. The NEJF, which is trending northeast-southwest, separates two different blocks or ‘terranes’. To the northwest lies the ‘Jiuling-Zhanggong terrane’ (Shu, 1991; Shi et al., 1994) or ‘Northeastern Jiangxi terrane’ (Xu, 1988; Xu et al., 1992). Both Jiuling and Huaiyu sequences are capped with an unconformity by the Sinian strata (Fig. 5).

The Jiuling block is made mainly of continent-derived, slightly metamorphosed middle to upper Proterozoic turbidites and littoral sedimentary rocks. In its southern part, the flysch formations contain mafic-ultramafic blocks dated at 1515 + 241 Ma (916 Geological Brigade in Shu, 1991). The age of the Jiuling basement is given by fossils: algae of 1700 Ma (Shu et al., 1994), and by radiometric datings: from 1930 f 162 Ma (Sm-Nd on a sandy phyllite) to about 700-800 Ma (Shu et al. 1989; Xu et al., 1992).

To the southeast of the fault, the ‘Huaiyu terrane’ (Shu, 1991; Shi et al., 1994) or ‘Jiangxi-Anhui terrane’ (Xu, 1988) is made of Middle-Late Proterozoic metavolcanic rocks, including an ophiolitic melange dated at about 930-1000 Ma (Shu, 1991; Chen et al., 1991, Xing et al., 1992), and of turbiditic flysch. The volcanic rocks include mainly basalt, diabase in the Middle Proterozoic, suggesting an affinity of oceanic or back-arc basin crust, and basalt, andesite and rhyolite in the Late Proterozoic. These rocks are generally metamorphosed to greenschist facies, but a strip of blueschist facies ophiolitic melange occurs along the fault (Shu and Zhou, 1988; Xu et al., 1992). The age of the Huaiyu sequence ranges from 1401 Ma (Rb-Sr on phyllite, Jiangxi Bureau of Geology and Mineral Resources, 1984) to 700-800 Ma. A dacite located at the top of the Huaiyu sequence gives an age of 8 17 + 83 Ma (Rb-Sr isochron, Shu and Zhou, 1988).

At outcrop scale as in thin sections, there is evidence of polycyclic deformation and polymetamorphic evol- ution. In the Dongxiang-Shexian ductile shear zone for instance (Shi et al., 1994) the regional schistosity trending N45” to 60”E, is cross-cutting the east-west trending foliation and early folds Fl of blueschist. Also, the changing composition of amphiboles from core to rim, the different 40Ar/3gAr ages argue for a polymeta- morphic event (Xu et al., 1992).

The best evidence for a Late Proterozoic collisional suture is provided by the occurrence and age of ophiolitic remnants and high pressure schists.

226 J. Charvet et al.

Collision of Yangzi and Cathaysia blocks 227

Ophiolitic relics Additional evidence for the suturing age may be taken from the high pressure schists.

More than 100 ophiolitic mafic-ultramafic pods are distributed along the NEJF (Fig. 6). The isotopic datings give the following ages:

Age of blueschist facies rocks

929 + 34 Ma, near Dexing (Xu, 1988) and 1034 + 16 Ma near Yiyang (Sm-Nd on gabbro, unpublished data in Shu, 1991);

935 + 10 Ma for the Fuchuan ophiolite gabbro, southern Anhui (Sm-Nd internal isochron, Chen et al., 1991; Xing et al., 1992);

1034 * 24 Ma for the Zhangsudun ophiolite gabbro (Sm-Nd internal isochron, Chen et al., 1991; Xing et al., 1992);

1024 + 30 Ma for the Fuchuan ophiolite (Sm-Nd, Zhou et al., 1989); and

Shu and Zhou (1988) discovered glaucophane-bearing rocks for the first time, in the Huaiyushan area, within the ophiolitic melange containing blocks dated at 929 + 34 Ma (Xu, 1988). Glaucophane schist and blueschist facies rocks are found along the NEJF zone, especially in Xiwan, Raoer and Zhangsudun. The paragenesis includes: jadeite- glaucophane-aragonite- albiteepidote (Zhou et al., 1989). The initial N70”E trending schistosity is often cut by the secondary schistosity of greenschists. The glaucophane is some- times retromorphosed to albite-actinolite.

1154 + 43 Ma for the Zhangsudun ophiolite (Sm-Nd, Zhou et al., 1991).

So, on average, the ophiolitic relics, especially the two main ones of Fuchuan and Zhangsudun may be considered as having the same Sm-Nd age of around 1 Ga, within realistic uncertainties (Chen et al., 1991). Of course, this age represents the time of the oceanic lithosphere formation, not the time of emplacement and suturing. However, in the Proterozoic as during the Phanerozoic, the abduction was not so much younger than the lithosphere formation, even in the Early Proterozoic (Picard et al., 1990). An upper limit is given by the Sinian unconformity (around 800 Ma) and the intrusions of Precambrian granites cross-cutting the ophiolitic melange, like the Shexian granite, dated at 930 Ma (U/Pb on zircons, Xing et al., 1989).

Two glaucophane-rich samples were collected from Xiwan and their K and Ar isotopic composition measured by J. L. Zimmermann (Centre de Recherches Petrographiques et Geochimiques, CNRS, Nancy, France) in April 1990 (Shu, 1991; Shu et al., 1993; Table 1). The average value is an age of 866 f 14 Ma. That was the first dating of a Late Proterozoic HP metamorphic event in the Jiangnan region. However, as polymetamorphism is obvious, this apparent age might be the result of later disturbances, and slightly rejuvenated. Xu et al. (1992) provide 40Ar/3gAr datings on amphibole from the blueschists cropping out in the same area: 901 + 19 Ma, believed to be the formation age of the amphibole, 857 + 18 Ma, 792 + 24 Ma and 791 + 93 Ma, interpreted as the ages of successive metamorphic events or disturbance. Zhou et al. (1989) got a 799 Ma age on glaucophane.

On the geochemical point of view, these ophiolites Thus, an age of about 870 to 900 Ma is now the oldest show some similarity to island-arc volcanics (Xu and available. This is not compatible with a 900-950 Ma age Qiao, 1989; Chen et al., 1991) and/or small oceanic basin of the peraluminous granitoids, retained to be reliable by (Shu, 1991; Shu et al., 1995). Chen et al. (1991). Because the blueschist foliation is cut

Nanyue Xiushui Shihuajian Yifeng

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Shu, 1991). (1) Fig. 3. Schematic cross-section of Jiuling segment of the Jiangnan belt (modified after Conglomerate; (2) limestone; (3) metasandstone; (4) phyllite; (5) greenschist; (6) meta-volcanoclastic rock; (7) Late Proterozoic granite; (8) Upper Paleozoic; (9) Lower Paleozoic; (10) Sinian System; (11) Xiushui Group of Middle Proterozoic; (12) upper assemblage of Middle Proterozoic Jiuling Group; (13) lower assemblage

of Middle Proterozoic Jiuling Group; (14) thrust fault.

J. Charvet et al.

Jiyhipai Fengkuang Wanpchun

SE NW

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0 C

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serbitechbrite phyllite

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m Lower assemblage of Jiuling Group

m Upper assemblage of Jiuling Group

m Oiiong Group

m slate or phyllite m Shanshu Group

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Fig. 4. Schematic cross-sections showing some sequences of Jiuling and Huaiyu domains in northeast Jiangxi (modified after Shu, 1991).

locally by the regional schistosity, which is in turn for solving it. However, if we assume as reliable the cross-cut by the granites, the age of blueschist 930 Ma age of the Shexian granite (U/Pb on zircons), metamorphism should be older than the age of granitic similar to the 937 k 6 Ma age of the Jiuling granite intrusions. This discrepancy is maybe due to later (40Ar/39Ar on biotite), the HP metamorphism of disturbance of K-Ar system. More datings are necessary blueschist and the subsequent emplacement of the

Collision of Yangzi

ophiolitic melange should have occurred before 93O- 940 Ma. That is, a suturing episode took place not so late after the oceanic lithosphere production (around 1 Ga).

and Cathaysia blocks 229

Kinematic constraints for the Late Proterozoic collisional event

The sense of displacement, the vergence of nappes, can be deduced from the kinematic analysis, as seen in the

In general, pre-Sinian rocks display a regional

next section. greenschist metamorphism, except the aforementioned blueschist blocks in the ophiolitic melange preserved

I sandy sl&?& phyllites and tuffaceous phyllites.

pedite slates, sandy and n6ceous slates.

partly, silty phylhtes an carbrwceolls slates. producing algae.

IKS, meta-t&fires and meta-basalt thin layers.

Stratigraphic columnar section in the middle part of the Jiangnan Composite Terrane

0 6OOm

A’-“- \ I- 0.

B

I! 3

Fig. 5. Synthetic stratigraphic columnar sections of Jiuling and Huaiyu sequences in the middle part of Jiangnan belt (modified after Shu, 1991).

230 J. Charvet et al.

Ii

7 Foliation attitude

- Lineation with its plunge.

3* Sense of shear 303

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Fig. 6. Structural sketch of the NEJF zone area (modified after Shu, 1991 and Xu et al., 1992). (1) Mesozoic strata; (2) Paleozoic sedimentary rocks; (3) Early-Late Proterozoic basalt-andesite-tufT series and latest Proterozoic sedimentary rocks (Sinian); (4) Middle to Early-Late Proterozoic spilite, basalt, keratophyre and ophiolitic melange; (5) Eary-Middle Proterozoic phyllite; (6) ophiolitic pods; (7) schist and phyllite; (8) spilite and keratophyre; (9) basalt and andesite; (10) conglomerate; (11) granite; (12) Mesozoic granite; (13) mylonite

zone. Sense of shear: a, late stage; b, early stage.

along the NEJF zone (Shu, 1991; Shu et al., 1995; rather scattered due to later folding (from north-south Xu et al., 1992). An early flat-lying foliation is cut by a to nearly east-west), marked at various scal_es: steep younger one, linked to a strike-slip shear zone. elongated and stretched minerals, recrystallization tails, The average attitude of the folded early foliation is boudins, sheath-like folds etc. Kinematic indicators a northeast trend with a northwest dip of about show a non-coaxial deformation with a sense of shear 30” to 40”. This foliation bears a stretching lineation, from northwest to southeast, top to the southwest (Shu,

Collision of Yangzi and Cathaysia blocks

1991; Xu et al., 1992; personal unpublished data). The initial thrust motion indicates a northwestward under thrusting of the Huaiyu domain beneath the Jiuling domain.

Thus, a change of setting took place around 1500 Ma ago, likely due to a first suturing. No clear structures can be assigned to this event, so far.

In summary, a polystage Late Proterozoic collisional event is recorded in the middle Jiangnan belt. It comprises a southeastward abduction and nappe

Post-dating the nappe emplacement, a younger deformation occurred along the NEJF shear zone,

emplacement corresponding to the suturing episode,

leading to a steep northeast trending and southeast dipping foliation, which bears a subhorizontal lineation

around 950 + 40 Ma, followed by intrusions of S-type

(Fig. 6). In the southwest segment of the shear zone, the kinematic study shows a sinistral ductile strike-slip

grantoids at 930 f. 20 Ma, and an intracontinental

shearing, based on various kinematic indicators (Shu, 1991; Shi et al., 1994). In the northeastern segment of

shortening along some ductile shear zones like the NEJF

this zone, in the Shexian area, the system grades into a

sinistral transpressive shear zone, which was active

series of several low-angle ductile faults, dipping southeast, including a granitic mylonite belt and several mylonite belts made of metavolcanic rocks. The

about 770-800 Ma ago.

kinematic analysis shows a northwest-directed sense of shear (Xu et al., 1992). The white mica, developed as the typical metamorphic mineral in the granite mylonite, provided an 40Ar/39Ar age of 769 Ma, similar to a K/Ar age of 768.9 Ma and a whole-rock Rb/Sr age of 768.5 Ma (Xu et al., 1992). This age fits with the Rb/Sr age of 766 Ma yielded by the Shiersan mylonitic granite (Shi et al., 1994). As the Sinian and Early Paleozoic strata are not affected by mylonitization, this sinistral transpressive deformation likely took place around 770 Ma, at a late stage of the collisional event.

An Early Paleozoic remobilization

The southern border of the Jiangnan belt shows the overprint of an Early Paleozoic compression.

In Sanxianling area, to the southwest of Dexing, the Wannian ductile shear zone, trending east-west, is made of Middle Proterozoic Jiuling Group turbidites. It shows a stretching lineation trending roughly north-south. The

To the south of the Jiuling Mountain, runs the east-northeast-west-southwest trending

vergence is still debated: southward (Xu et al., 1992) or

Nanchang- Wanzai ductile shear zone, about 300 km long and

northward (Shu, 1991). Anyway, two 40Ar/39Ar plateau

2-5 km wide. The kinematic study shows that it worked as a sinistral transpressive shear zone, during the Early Paleozoic (Shu er al., 199 1). The age is constrained by

ages were obtained on 2 M-phengite: 428.9 f 0.8 Ma

the following data: mylonitization affects Late Protero-

and 428.1 f 0.5 Ma, which argue for a Paleozoic

zoic Jiuling granite (935-937 f 6 Ma, ““Ar/39Ar on biotite, Jiangxi Bureau of Geology and Mineral

‘Caledonian’ overprint (Xu ef al., 1992).

Resources, 1984), the Early Paleozoic rocks, and cross-cuts the regional greenschists; it does not affect the Late Paleozoic strata and the mylonites are, intruded by the 390-Ma-old Devonian adamellite (K/Ar on biotite). In addition, a 2 M-phengite from the granitic mylonite yielded an 40Ar/39Ar plateau age of 510 f 4 Ma (Xu et al., 1994); a K/Ar 502 Ma age was obtained on biotite (Shu et al., 1995). These mylonites cross-cut the greenschists which show an earlier north to south thrusting (Shu, 1991).

Remnants of a Middle Proterozoic collisional event

The existence of an older suture, lately reworked during the Late Proterozoic collision, is shown by the relics of a dismembered ophiolitic melange in the Jiuling terrane (Fig. 5). This melange, present in the southern part of Jiulingshan, is poorly dated at 15 15 + 241 Ma (Rb/Sr on spilite), and shows island-arc affinities (Zhang et al., 1984; Guo et al., 1985; Shu, 1991). It is overlain by the Late-Middle Proterozoic foreland-basin turbid- ites and continent-derived elastic rocks of the Xiushui Group (Wang, 1986).

Thus, an Early Paleozoic compressive event, leading to ductile deformation, overprinting the late Proterozoic structures, is well documented on the southern border of the Jiangnan belt. But no trace of Caledonian suture occurs in this belt, where the Early Paleozoic compression induced intracontinental shortening and shearing. If this event was linked to a collision, which is likely, the suture has to be found further south and east, between the Jiangnan and Wugong-Wuyi blocks. Our preliminary results (Charvet, Faure, Lu, Shu, 1993, unpublished data) indicate that the northwestern part of Wuyishan is a good candidate, in agreement with some previous works (e.g. Zhang et al., 1984). In the Jiangnan area, the Early Paleozoic tectonism induced the last ductile thrusting. Younger ones, involving the Late

231

Table 1. Results of K/Ar radiometric dating of two glaucophanes from Xiwan near Dexing

Radiogenic MAr*

content Grain (x 1o-6 Atmospheric Age

Sample size K (%) mole/g-‘) “Ar (%) 40Ar/“‘K (Ma) Jx-12 very fine 0.22 9.416 17 0.0645 Jx-13 0.18 mm 0.22 9.518 14.8 0.0647

Radioactive constants: @ = 4.962 x 1O-‘o an-‘, ly = 0.581 x 1O-1o an-‘, “K = 0.01167 K%.

864.5 &- 19 867.5 f 13

232 J. Charvet et al.

Paleozoic strata, were marked by brittle structures, especially during the Yenshanian cycle.

Mesozoic folding and thrusting and the problem of Mesozoic collision

As stated in the geological setting section, folding and thrusting involving Mesozoic strata are noteworthy in south China, in particular within and to the south of the Jiangnan belt (Figs 3 and 6). Several sheets, comprising Proterozoic greenschists, are thrust along low-angle faults over the Late Paleozoic and Early Mesozoic strata. Particularly, the Upper Triassic strata: coal-bear- ing conglomerates, sandstones and shales of the Anyuan Formation, are involved. This is documented by mapping and also by drilling (Shu, 1991; Shu et al., 1991). The Jurassic conglomerates are also tightly folded and uprighted, and affected locally by reverse faults. On the contrary, the Upper Cretaceous red beds are overlying different units with a conspicuous angular unconformity, and are generally only gently tilted. Thus, the main shortening occurred prior to Late Cretaceous and cannot correspond to a Late Cretaceous collision (Hsii et al., 1988). It should not be overlooked that the fault offsets range from a few hundred metres to several kilometres and, indeed, younger rocks appear sometimes in tectonic windows, beneath the Proterozoic green- schists. It is the case for the Carboniferous limestones in Shixan (Hsti et al., 1988, 1990; and personal data). Also small Mesozoic granitic intrusions can be cut and overthrust onto Mesozoic strata, like in the Zhimusan copper mine, near Pingxiang (Shu, 1991). But, these thrust contacts are in a general sense of brittle style, linked to fracture and cataclasis. There is no large klippes of mylonitic Precambrian granites as suggested by Hsii et al. (1988). The field data show that Sinian strata unconformably overlie those granites (Lu, 1964; Shu, 1991; Chen et al., 1991). The Proterozoic schistosity is cut at high angle by fault planes (Shu et al., 1991). These thrusts display the characteristics of rather superficial thin-skinned tectonics, with ramps, maybe linked to a basal decollement of the cover, and corresponding to an intracontinental shortening. A Late Mesozoic suture zone can hardly be found in south China, from Jiangnan belt to the coast area.

But, as exposed elsewhere (Charvet et al., 1994), a collision occurred, after a westward subduction episode, between the China-Indochina block and the West Philippines Block ?, around the Early-Late Cretaceous boundary. The suture zone is now in the Philippines archipelago, due to Tertiary drifting subsequent to the South China sea opening. This collisional event is a good candidate for the responsibility of the Late Mesozoic deformation of south China.

A tentative model of the Proterozoic evolution of the Jiangnan belt: initial welding of Yangzi plate

and Cathaysia

The plate tectonic settings of the Jiuling and Huaiyu blocks are determined after petrotectonic assemblages, sedimentologic analysis of strata, and geochemistry of

magmatic rocks (Shu, 1991). The timing and kinematic constraints were exposed in earlier sections. Using all these data, may we propose a tentative geodynamic model.

The main basis for the reconstruction can be summarized as follows, regarding the successive plate tectonic settings:

A Jiuling passive margin at more than 1500 Ma, with mature flyschs deposit, with detrital supply coming from the Yangzi old land;

A Jiuling arc-trench system around 1500 Ma, docu- mented by volcanic rock affinities, and volcanic arc-derived elastic rocks;

A Jiuling back-arc basin about 1500 Ma, filled with arc-derived detritus supplied from the south;

A small Huaiyu oceanic basin around 1000 Ma, documented by the tholeiitic ophiolitic remnants dated at 1150-930 Ma, siliceous rocks, deep-water turbidites; and

A Huaiyu volcanic arc of about 900?-818 Ma, based on oceanic crust, and contaminated by continental crust in the last stage.

The successive stages of geodynamic evolution are summarized in Fig. 7. The orientation is supposed to be the same as today, although this was unlikely to be the case according to some paleomagnetic data (Shu et al., 1995). That means that no rotation of blocks was taken into account. Before 1500 Ma (stage l), the Jiuling continental passive margin (‘southern’ edge of the Yangzi block) is bounded by Huanan ‘old’ ocean (proto-Huanan). At around 1500 Ma (stage 2) the Jiuling arc, continent-based, is working, due to a ‘northward’ subduction of the proto-Huanan ocean. A continental fragment, detached from Gondwana, enter- ing the subduction zone, will lead to a first collision, and ophiolitic melange emplacement. The new continent splits and, around 1100-1000 Ma (stage 3), the Huaiyu oceanic basin opens. At around 1000-950 Ma (stage 4), the convergence phase starts and the Huaiyu arc develops, first on an oceanic lithosphere, progressively changed to intermediate. At around 950 f 40 Ma (stage 5), the collision begins, due to the underthrusting of Cathaysia beneath the Huaiyu arc and oceanic nappes, and the Jiuling continental domain. The oceanic remnants previously brought to depth give the blueschist melange nappes, on which the Huaiyu arc volcanics are overthrust. The late stage of volcanism shows contami- nation by continental material, maybe due to the beginning of Cathaysia underthrusting. Soon after nappes emplacement, generally of greenschist facies, S-type peraluminous granites are intruded into the structural pile, due to crust anatexis. The intra- continental shortening will continue, with the onset of strike-slip shearing and steep schistosity, until about 770-800 Ma.

Basically, this model is similar to the one proposed by Shu et al. (1995) and Xing et al. (1992). The timing is slightly changed, regarding the beginning of collision, based on the age relationship between nappe emplacement and granitic intrusions, assuming a 950-900 Ma age for the latter. Also, the hypothesis of an intra-oceanic subduction at the onset of the Huaiyu arc seems a more elegant solution, explaining better the geologic features of the Huaiyu block and the collision.

Collision of Yangzi

Conclusions

and Cathaysia blocks 233

beneath the Yangzi plate, and the welding left an oceanic suture in the Jiangnan belt.

The tectonic development of south China was polycyclic. The two main divisions proposed by Huang (1945) are still partly valid. They correspond to the Yangzi and Cathaysia blocks. The amalgamation of- these two blocks definitely took place during the Late Proterozoic, due to the succession of subduction and collision marked by:

This main event was probably preceded by a similar collision and suturing during the Middle Proterozoic, remnants of which are now reworked in the southern

HP/LT metamorphism; abduction of ophiolite and ophiolitic melange; thrusting of greenschist nappes; emplacement of collisional S-type granites.

The vergence was southeast-directed (with respect to the present position of units), the Jiuling domain (southern margin of the Yangzi plate) overthrusting the oceanic Huaiyu domain (part of Huanan ocean). In other words, the Cathaysia plate has been underthrust

Jiuling sequence. Then, a strong remobilization occurred during the

Early Paleozoic, leading to transpressive ductile defor- mation, when a new suturing took place to the southeast of the Jiangnan belt.

After this ‘Caledonian’ orogeny, south China was not a stable platform. In particular, strong thin-skinned folding and thrusting occurred during the Mesozoic, prior to the unconformity of Late Cretaceous red beds. We suggest that this intracontinental shortening was the response to the collision of China-Indochina upper plate with the west Philippines Block (lower plate).

The effects of an Indosinian event, which are obvious (unconformity of Upper Triassic), were not considered

1 -Passive continental margin stage (>7500 Ma )

N Yangzi block Jiuling passive margin

S Huanan old ocean

2- Active continental margin (ca - 1500 Ma) I

back-arc basin t S

Jiulin ‘8 Huanan old ocean .__.._.. _ _ ____ i 0

P

3-Small ocean basin stage (ca- 1 100- 1000 Ma )

NW Jiuling continental Huaiyu ocean basin t+ SE

Fig. I (caption overleaf)

234 J. Charvet et al.

4- Island-arc stage (ca- 1000-950 Ma )

Huaiyu Cathaysia

5- Jiangnan collision stage (ca-950-900 Ma)

NW

Fig. 7. Tentative geodynamic model of the geological evolution of Jiangnan area from Middle to Late Proterozoic.

in this paper, because the location of the suture is still very unclear for us, although it is likely to be in the coastal or sea area (e.g. Zhang et al., 1984), nor the effects of Himalayan collision, which seem to be limited in the studied region.

South China is actually a composite block, comprising the relics of Middle Proterozoic, Late Proterozoic, Early Paleozoic and maybe Late Paleozoic sutures.

Acknowledgements-We thank sincerely Professor Ren Jishun for his kind invitation to present a paper in this volume in honour of Professor Huang Jiqing.

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