1. Introduction
The Neoproterozoic–Early Palaeozoic palaeoposition of South China is uncertain and controversial (Li, Zhang & Powell, Reference Li, Zhang and Powell1995; Li et al. Reference Li, Li, Zhou and Kinny2002, Reference Li, Bogdanova, Collins, Davidson, De Waele, Ernst, Fitzsimons, Fuck, Gladkochub, Jacobs, Karlstrom, Lu, Natapov, Pease, Pisarevski, Trane and Vernikovski2008; Evans et al. Reference Evans, Li, Kirchvink and Wingate2000; Yang et al. Reference Yang, Sun, Yang and Pei2004; Yu et al. Reference Yu, O'Reilly, Wang, Griffin, Zhang, Wang, Jiang and Shu2008). This is partially due to the sparsity of high quality palaeomagnetic data; only three high-quality palaeomagnetic poles for the Neoproterozoic (748 Ma), Middle Cambrian and Middle Silurian have been obtained from South China owing to late widespread remagnetization (Evans et al. Reference Evans, Li, Kirchvink and Wingate2000; Yang et al. Reference Yang, Sun, Yang and Pei2004). Using detrital zircon from late Neoproterozoic paragneisses and migmatites, Yu et al. (Reference Yu, O'Reilly, Wang, Griffin, Zhang, Wang, Jiang and Shu2008) proposed a northern Gondwana palaeoposition for South China in the Neoproterozoic, which conflicts with the ‘missing link’ model in which South China was between Laurentia and Australia based on geochronological, stratigraphic and tectonothermal correlations (Li, Zhang & Powell, Reference Li, Zhang and Powell1995; Li et al. Reference Li, Li, Zhou and Kinny2002, Reference Li, Bogdanova, Collins, Davidson, De Waele, Ernst, Fitzsimons, Fuck, Gladkochub, Jacobs, Karlstrom, Lu, Natapov, Pease, Pisarevski, Trane and Vernikovski2008). In most early Palaeozoic palaeobiogeographic models, South China is an isolated continental block that had varied faunal links but seems close to peri-Gondwana (Fortey & Cocks, Reference Fortey and Cocks2003; Rong et al. Reference Rong, Chen, Su, Ni, Zhan, Chen, Fu, Li and Fan2003). However, Arenigian brachiopods and acritarchs of South China show a high degree of endemism and strong faunal differences compared with those of the peri-Gondwanan province (Xu & Liu, Reference Xu and Liu1984; Playford, Ribecai & Tongiorgi, Reference Playford, Ribecai and Tongiorgi1995).
A provenance study of the voluminous late Neoproterozoic (Sinian)–Ordovician deepwater terrigeneous siliciclastic rocks in the Wuyishan Fold Belt is crucial to understanding the sedimentary and tectonic history of South China. In this paper, we report the detrital zircon U–Pb ages of six samples from five sedimentary units in this succession obtained by LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry). We compare our results with published data from time-equivalent strata of the Gondwanan and eastern Laurentian margins to provide a new perspective on the continental affinity of South China.
2. Geological setting
South China is divided into two parts by the Jiangshao Fault Zone: the Yangtze craton to the northwest and the Wuyishan Fold Belt to the southeast (Fig. 1a). In the Yangtze craton, the Precambrian basement rocks record extensive rift-related magmatism (c. 850–720 Ma) in the development of the Neoproterozoic continental rifts of South China, which has been interpreted as episodic events during the break-up of the Rodinia supercontinent (e.g. Li et al. Reference Li, Bogdanova, Collins, Davidson, De Waele, Ernst, Fitzsimons, Fuck, Gladkochub, Jacobs, Karlstrom, Lu, Natapov, Pease, Pisarevski, Trane and Vernikovski2008). These basement rocks are largely overlain by late Neoproterozoic–Ordovician shallow-marine platform strata with abundant evaporites (mainly carbonates) in the Yangtze craton as well as some siliciclastic input on the southeastern margin (Fig. 2a; e.g. Liu & Xu, Reference Liu and Xu1994). The facies assemblages of the cover strata developed in the Yangtze depositional domain represent a typical passive margin setting in the late Neoproterozoic–Ordovician (Fig. 2a; BGMRJX, 1984; Liu & Xu, Reference Liu and Xu1994).
(a) Main tectonic elements of South China. JFZ – Jiangshao Fault Zone, WFB – Wuyishan Fold Belt. (b) Geological map of study area in the Wuyishan Fold Belt, showing sampling localities. (c) Stratigraphic column of the sampling area with marked sample positions.
(a) Schematic lithofacies palaeogeographic map of South China during the early Palaeozoic, showing two principal stratigraphic regions, the sampling area, and a proposed source terrane with a Grenville-age province in a highly uncertain location. (b) Interpretation of cross-section A–A′. Location of section is indicated in (a).
Southeast of the Jiangshao Fault Zone, the Wuyishan Fold Belt (also known as the South China Caledonian-period orogenic belt) has been identified as an early Palaeozoic (c. 460–400 Ma) orogenic belt with a regional unconformity between the Early Palaeozoic sequences and the Middle–Upper Devonian coarse clastic sediments (Fig. 1b, c; Huang, Reference Huang1945; BGMRJX, 1984; BGMRGD, 1988; Guo et al. Reference Guo, Shi, Lu, Ma, Dong and Yang1989; Shu, Reference Shu2006; Li et al. Reference Li, Li, Wartho, Clark, Li, Zhang and Bao2010). In contrast to the late Neoproterozoic–Ordovician strata on the Yangtze craton, coeval sedimentary rocks in the Wuyishan Fold Belt were intensely deformed during the early Palaeozoic orogenic event, with widespread tight to isoclinal folding, greenschist- to amphibolite-facies metamorphism, intrusions of granites and the formation of migmatites at c. 460–400 Ma (e.g. Guo et al. Reference Guo, Shi, Lu, Ma, Dong and Yang1989; Shu, Reference Shu2006; Li et al. Reference Li, Li, Wartho, Clark, Li, Zhang and Bao2010). This early Palaeozoic orogenic belt, which covers the southeastern half of South China (Fig. 1a), overlaps with the regional extent of the late Neoproterozoic–early Palaeozoic basin, which started as a failed continental rift and was subject to thick sedimentation (Li et al. Reference Li, Li, Wartho, Clark, Li, Zhang and Bao2010), here termed the Southeast Basin (Fig. 2a, b). The late Neoproterozoic–Ordovician strata deposited in the Southeast Basin are mainly composed of extensive ~10 km thick deepwater terrigenous clastic turbidites; the clastic rocks of this succession coarsen southeastward/coastward and reveal a general palaeocurrent direction towards the northwest (Fig. 2a, b; BGMRGD, 1988; Liu & Xu, Reference Liu and Xu1994).
3. Methods and results
Six fine- to medium-grained meta-sandstone samples were collected from the Upper Neoproterozoic (J198–1), Lower Cambrian (J200), Middle Cambrian (J186, J188), Upper Cambrian (J192) and Middle Ordovician (J191) strata in the Chongyi district of the southern Jiangxi Province within the Wuyishan Fold Belt (Fig. 1b). The upper Neoproterozoic–Ordovician strata are successive and have undergone greenschist-facies metamorphism (BGMRJX, 1984). These samples were processed by standard methods to separate zircons, and backscattered electron (BSE) imaging was conducted to spot target detrital zircons for U–Pb geochronology analysis. The BSE imaging and LA-ICP-MS U–Pb geochronology were carried out in the State Key Laboratory for Mineral Deposits Research, Nanjing University. Isotopic ratios were calculated using GLITTER (ver. 4.4) All zircon ages described here exclude analyses with >10% discordance or >10% reverse discordance. Details of the sample descriptions and preparation, analysis procedure, full tables of results, and exact sampling localities are shown in Appendix 1, available online at http://journals.cambridge.org/geo.
Five age histograms of the six samples are presented in Figure 3a. Of the detrital zircon ages, 687 range from 3447±36 to 506±8 Ma. All samples yield primary age peaks at c. 1200–950 Ma, and present a series of detrital age peaks in the Palaeo- to Mesoproterozoic (2050–1550 Ma). Peaks at c. 1400 Ma are noticeable in J198–1, J186, J188, J192 and J191. Five Phanerozoic samples show a cluster ranging from 2700–2400 Ma, mainly early Palaeoproterozoic (c. 2470 Ma). Minor grains from 900–500 Ma are also observed. Rare analysed grains (ten) yield concordant ages older than 3000 Ma. Detrital zircons of all samples exhibit large compositional variations of Th and U, and most zircons (~84%) have Th/U >0.3; only 30 grains (~4%) are below 0.1 (online Appendix 1). The Th/U ratio analysis and BSE images suggest that most detrital zircons of these samples are magmatic (with high Th and U contents and Th/U ratio), but they crystallized from complex compositions under different conditions. A remarkably uniform age signature of the detritus is revealed for different stratigraphic levels of the thick succession, indicating a common sediment source throughout the duration of deposition (c. 150 Ma, from late Neoproterozoic to Ordovician).
Histograms displaying U–Pb detrital zircon age distributions of Neoproterozoic to early Palaeozoic sedimentary samples. (a) Six samples from the Wuyishan Fold Belt in the present study. (b) Samples from the Gondwanan margin. Source of data: G1 (Copalayo Formation) – DeCelles, Carrapa & Gehrels (Reference Decelles, Carrapa and Gehrels2007); G2 (Kahar, Bayandor, Barut, Lalun, Mila formations) – Horton et al. (Reference Horton, Hassanzadeh, Stockli, Axen, Gillis, Guest, Amini, Fakhari, Zamanzadeh and Grove2008); G3 (Tethyan and Lesser Himalayan sample) – Myrow et al. (Reference Myrow, Hughes, Paulsen, Williams, Parcha, Thompson, Bowring, Peng and Ahluwalia2003); G4 (Tumblagooda Sandstone) – Cawood & Nemchin (Reference Cawood and Nemchin2000); G5 (Argentine Metamorphics, Wynyard Metamorphics) – Fergusson et al. (Reference Fergusson, Carr, Fanning and Green2001, Reference Fergusson, Henderson, Fanning and Withnall2007); G6 (Douglas Conglomerate, Starshot, Douglas, Dick formations) – Goodge, Williams & Myrow (Reference Goodge, Williams and Myrow2004). (c) Samples from the eastern Laurentian margin. Source of data: L1 (central Blue Ridge, Dahlonega gold belt, eastern Blue Ridge and Sauratown Mountains window) – Bream et al. (Reference Bream, Hatcher, Miller, Fullagar, Tollo, Corriveau, McLelland and Bartholomew2004); L2 (Unicoi, Erwin, Hardystron formations) – Eriksson et al. (Reference Eriksson, Campbell, Palin, Allen and Bock2004); L3 (South Brook, Summerside, Blow-Me-Down, Bradore, Hawke Bay formations) – Cawood & Nemchin (Reference Cawood and Nemchin2001); L4 (Argyll Group, Southern Highland Group) – Cawood et al. (Reference Cawood, Nemchin, Smith and Loewy2003). s – number of samples; n – number of analyses.
4. Discussion
4.a. An outboard source terrane with a Grenville-age province
All samples share a common dominant (~37%) age population ranging from 1200 to 950 Ma (Fig. 3a). These dominant concordant ages are consistent with the Grenville-age provinces formed during the period 1300–900 Ma, owing to the convergence of the Rodinia supercontinent (Hoffman, Reference Hoffman1991). Based on Sm–Nd dating using whole-rock samples, feldspar and pyroxene from a gabbro unit in an ophiolite suite in NE Jiangxi Province (with a internal isochron age of 1.034±0.024 Ga; Chen et al. Reference Chen, Foland, Xing, Xu and Zhou1991), a Grenville-age orogen has been suggested along the southeastern edge of the Yangtze craton as a result of the continental collision during amalgamation of South China (Li, Zhang & Powell, Reference Li, Zhang and Powell1995; Li et al. Reference Li, Li, Zhou and Kinny2002). However, Yu et al. (Reference Yu, O'Reilly, Wang, Griffin, Zhang, Wang, Jiang and Shu2008) argued that granitoids or high-grade metamorphic rocks with Grenville ages that could denote a continental-collision setting were absent in that region. Therefore, using the fact that the rocks with precise Grenvillian U–Pb ages were not reported within the southeastern half of South China, Yu et al. (Reference Yu, O'Reilly, Wang, Griffin, Zhang, Wang, Jiang and Shu2008) suggested a southern derivation of the numerous euhedral Grenville-age detrital zircons obtained from the late Neoproterozoic paragneisses and migmatites in the Wuyishan Fold Belt. In this regard, our study reveals a similar scenario to Yu et al. (Reference Yu, O'Reilly, Wang, Griffin, Zhang, Wang, Jiang and Shu2008). Moreover, in the present study, the late Neoproterozoic–Ordovician lithofacies palaeogeography of South China provides palaeogeographic constraints on the source direction from which these thick siliciclastic sediments in the Southeast basin were mainly derived: the southeast (Fig. 2a; Liu & Xu, Reference Liu and Xu1994). This northwestward sedimentary transport is also supported by several palaeocurrent indicators preserved in the strata (BGMRJX, 1984; BGMRGD, 1988; Liu & Xu, Reference Liu and Xu1994). Thus, our results indicate the existence of a terrane with a Grenville-age province to the southeast of South China, which acted as a sustained (c. 150 Ma) source terrane for the voluminous siliciclastic succession of Upper Neoproterozoic to Ordovician (Fig. 2a, b), although the exact distance and extension direction of the Grenville-age province have not been identified.
4.b. Gondwanan affinity of South China
The Grenville-age provinces were mainly seated in Gondwana and eastern Laurentia in the early Palaeozoic when the two continents were largely isolated (e.g. Hoffman, Reference Hoffman1991; Li et al. Reference Li, Bogdanova, Collins, Davidson, De Waele, Ernst, Fitzsimons, Fuck, Gladkochub, Jacobs, Karlstrom, Lu, Natapov, Pease, Pisarevski, Trane and Vernikovski2008), so we compare our results with published data from coeval strata of the Gondwanan and eastern Laurentian margins to evaluate the continental affinity of South China (Fig. 3). Our comparison reveals that the distinctive 650–500 Ma age component that is well represented in the Gondwanan margin samples (DeCelles, Carrapa & Gehrels, Reference Decelles, Carrapa and Gehrels2007; Horton et al. Reference Horton, Hassanzadeh, Stockli, Axen, Gillis, Guest, Amini, Fakhari, Zamanzadeh and Grove2008; Myrow et al. Reference Myrow, Hughes, Paulsen, Williams, Parcha, Thompson, Bowring, Peng and Ahluwalia2003; Cawood & Nemchin, Reference Cawood and Nemchin2000; Fergusson et al. Reference Fergusson, Carr, Fanning and Green2001, Reference Fergusson, Henderson, Fanning and Withnall2007; Goodge, Williams & Myrow, Reference Goodge, Williams and Myrow2004) is poorly represented in our samples as well as eastern Laurentian margin samples (Bream et al. Reference Bream, Hatcher, Miller, Fullagar, Tollo, Corriveau, McLelland and Bartholomew2004; Eriksson et al. Reference Eriksson, Campbell, Palin, Allen and Bock2004; Cawood & Nemchin, Reference Cawood and Nemchin2001; Cawood et al. Reference Cawood, Nemchin, Smith and Loewy2003, Reference Cawood, Nemchin, Strachan, Prave and Krabbendam2007) (Fig. 3). This remarkable difference can be attributed to the Pan-Gondwanaland event representing the assembly of Gondwana and the subsequent orogenic belts along the Gondwanan margin (e.g. Pampean, Ross-Delamerian; Goodge, Williams & Myrow, Reference Goodge, Williams and Myrow2004; Horton et al. Reference Horton, Hassanzadeh, Stockli, Axen, Gillis, Guest, Amini, Fakhari, Zamanzadeh and Grove2008). Moreover, Gondwanaland is also characterized by widespread intrusions of 650–500 Ma alkaline rocks and carbonatites (Veevers, Reference Veevers2007). However, none of the structural deformations or igneous activities of these 650–500 Ma thermo-tectonic events, which are extensive in much of Gondwana, have been identified in South China, which experienced continuous sedimentation in the late Neoproterozoic–Ordovician and witnessed the subsequent formation of the Wuyishan Fold Belt at c. 460–400 Ma (BGMRJX, 1984; BGMRGD, 1988; Guo et al. Reference Guo, Shi, Lu, Ma, Dong and Yang1989; Shu, Reference Shu2006; Li et al. Reference Li, Li, Wartho, Clark, Li, Zhang and Bao2010). Thus, the geological and geochronological evidence presented in our study is difficult to reconcile with those Gondwanan affinity models for South China. For instance, the late Neoproterozoic northern Gondwanan palaeoposition model (Yu et al. Reference Yu, O'Reilly, Wang, Griffin, Zhang, Wang, Jiang and Shu2008), which is based on isotopic analysis and comparisons between detrital zircon ages of late Neoproterozoic metamorphic rocks in the Wuyishan Fold Belt and the pre-Ordovician passive margin strata in northern Gondwana (e.g. G3 in Fig. 3b; Myrow et al. Reference Myrow, Hughes, Paulsen, Williams, Parcha, Thompson, Bowring, Peng and Ahluwalia2003), failed due to the recognition of the early Palaeozoic North India Orogen (c. 530–470 Ma, see Cawood, Johnson & Nemchin, Reference Cawood, Nemchin, Strachan, Prave and Krabbendam2007) along northern Gondwana.
4.c. Laurentian affinity of South China
A noticeable peak at c. 1400 Ma in our five samples coincides with the age of the trans-Laurentian granite–rhyolite province as well as the granodiorites (1436±7 Ma) exposed in Hainan Island of South China, which have been proposed as important pieces of evidence for connecting South China and Laurentia (Li et al. Reference Li, Li, Zhou and Kinny2002, Reference Li, Bogdanova, Collins, Davidson, De Waele, Ernst, Fitzsimons, Fuck, Gladkochub, Jacobs, Karlstrom, Lu, Natapov, Pease, Pisarevski, Trane and Vernikovski2008). In addition, the detrital zircon signatures of the analysed samples, particularly the presence of a series of detrital age peaks in the Palaeo- to Mesoproterozoic (2050–1550 Ma), the paucity of mid-Mesoproterozoic detritus (1400–1250 Ma) and the evidence for a Grenvillian (c. 1200–950 Ma) tectonothermal event, show several similarities to coeval strata preserved in the eastern Laurentian margin (Fig. 3c; Bream et al. Reference Bream, Hatcher, Miller, Fullagar, Tollo, Corriveau, McLelland and Bartholomew2004; Eriksson et al. Reference Eriksson, Campbell, Palin, Allen and Bock2004; Cawood & Nemchin, Reference Cawood and Nemchin2001; Cawood et al. Reference Cawood, Nemchin, Smith and Loewy2003, Reference Cawood, Nemchin, Strachan, Prave and Krabbendam2007). These correlation and age signatures suggest that Laurentia could be regarded as the potential source terrane.
The Neoarchaean–Early Palaeoproterozoic (2700–2400 Ma) age cluster in the Phanerozoic samples has a peak at around 2470 Ma. These old grains record the c. 2.5 Ga global tectonics environmental change and submarine–subaerial magmatism (e.g. Bleeker, Reference Bleeker2003; Barley, Bekker & Krapez, Reference Barley, Bekker and Krapez2005). The same detrital age group revealed in late Neoproterozoic metamorphic rocks from the Wuyishan Fold Belt had been regarded as another correlation with India and East Antarctica (Yu et al. Reference Yu, O'Reilly, Wang, Griffin, Zhang, Wang, Jiang and Shu2008). However, the magmatism at c. 2.5 Ga was suggested to be global in nature (Heaman, Reference Heaman1997). Large igneous provinces at 2500–2450 Ma also occur in the Superior and Wyoming cratons (Barley, Bekker & Krapez, Reference Barley, Bekker and Krapez2005). Therefore, the presence of several Neoarchaean (c. 2.5 Ga) rocks documented in Laurentia (e.g. Friend & Kinny, Reference Friend and Kinny1995; Heaman, Reference Heaman1997; Schultz et al. Reference Schultz, Chacko, Heaman, Sandeman, Simonetti and Creaser2007) increases the possibility that the 2700–2400 Ma detritus originated from Laurentia.
The mid-Neoproterozoic (c. 850–720 Ma) detritus is presented in this study with the exact positions of the individual peaks varying between samples (Fig. 3a). Detritus of this age is contemporaneous with extensive Neoproterozoic rift-related magmatism documented in the Yangtze craton (e.g. Li et al. Reference Li, Bogdanova, Collins, Davidson, De Waele, Ernst, Fitzsimons, Fuck, Gladkochub, Jacobs, Karlstrom, Lu, Natapov, Pease, Pisarevski, Trane and Vernikovski2008), suggesting that the Neoproterozoic rift-related magmatism may have also occurred on the proposed source terrane (Fig. 2b). However, this result provides only weak constraints on the continental affinity of South China owing to the widespread Neoproterozoic continental rifting records across the globe during the break-up of Rodinia (Li et al. Reference Li, Bogdanova, Collins, Davidson, De Waele, Ernst, Fitzsimons, Fuck, Gladkochub, Jacobs, Karlstrom, Lu, Natapov, Pease, Pisarevski, Trane and Vernikovski2008). Nevertheless, these continental rifting intrusions in the Laurentian margin (e.g. Dalziel & Soper, Reference Dalziel and Soper2001; Heaman, LeCheminant & Rainbird, Reference Heaman, LeCheminant and Rainbird1992), Neoproterozoic grains in sedimentary rocks (e.g. Bream et al. Reference Bream, Hatcher, Miller, Fullagar, Tollo, Corriveau, McLelland and Bartholomew2004; Cawood et al. Reference Cawood, Johnson and Nemchin2007) and the comparable Neoproterozoic rifting history make the Laurentian affinity of South China possible.
In addition, the remarkably uniform age signature of the detritus within these strata (Fig. 3a), which have depositional ages spanning up to 150 Ma, suggests extensive reworking and homogenization of detritus before deposition and/or a uniform age character of rock units within the source terrane so that changes in specific source areas with time had little effect on the age distribution. We note that the geology of Laurentia is consistent with the latter point, and the Laurentian affinity of South China meets several requirements of significant tectonic events and provenance characteristics as follows: (1) the distribution of Grenville-age province at c. 1.2–1.0 Ga, the transcontinental magmatic belt at c. 1.4 Ga, and Archaean rocks at c. 2.5–2.7 Ga in Laurentia; (2) the Neoproterozoic rift events in both South China and Laurentia; (3) the relative quiescence with continuous sedimentation during the late Neoproterozoic–Ordovician; and (4) the subsequent orogenic event at c. 460–400 Ma (e.g. Wuyishan Fold Belt, Caledonides). If this correlation is correct, then the implication is that South China was a continental block that did not drift away from Laurentia during the formation of the Southeast Basin during the ongoing fragmentation of Rodinia, received thick sediments in the late Neoproterozoic–Ordovician (Fig. 2a, b), and collided with Laurentia to form the Wuyishan Fold Belt in late Ordovician–Silurian time. These hypotheses can be tested by further systematic structural, metamorphic, stratigraphic, geochronological and geodynamic investigations focused on the orogenic mechanism of Wuyishan Fold Belt, and more evidence is needed to establish the exact palaeogeographic reconstruction of these two continents during this time.
5. Conclusions
South China experienced continuous sedimentation during the late Neoproterozoic–Ordovician prior to the development of the Wuyishan Fold Belt (c. 460–400 Ma). In this paper, U–Pb detrital zircon ages were obtained from the voluminous Upper Neoproterozoic–Ordovician siliciclastic strata within the Wuyishan Fold Belt, and we draw the following conclusions:
(1) Coupled with lithofacies palaeogeography and palaeocurrent evidence for South China, the predominant c. 1200–950 Ma ages in our samples indicate the existence of an outboard provenance terrane with a Grenville-age province to the southeast.
(2) South China lacks both geochronological and field evidence for a Pan-Gondwanaland (c. 650–500 Ma) event in a comparison between our samples and coeval Gondwanan margin samples, arguing strongly against the previously suggested peri-Gondwanan setting of South China.
(3) Similarities between our new geochronological results and existing data from Laurentian margin samples, together with the integrated provenance and tectonic analyses of the two continents, suggest that Laurentia could be regarded as the potential source terrane for the Southeast Basin of South China during late Neoproterozoic–Early Palaeozoic times.
Acknowledgements
We sincerely thank Liangshu Shu, Jinhai Yu and Zhenyu Yang for their helpful suggestions; we are also grateful to Judith Hubbard and Qin Wang for their assistance in improving the manuscript. This research was supported by grants from the NSF of China (Grant No. 40634022 and 40672132) and the National S&T Major Project of China (2008ZX05003-001 and 2008ZX05009-001).
