Early history of the eastern Sibao Orogen (South China) during the assembly of Rodinia: New mica 40Ar/39Ar dating and SHRIMP U–Pb detrital zircon provenance constraints

doi:10.1016/j.precamres.2007.05.003

Precambrian Research

Volume 159, Issues 1–2, 25 October 2007, Pages 79-94

https://doi.org/10.1016/j.precamres.2007.05.003 Get rights and content

Abstract

The Sibao Orogen in South China is one of the poorest known Grenville-aged orogenic belts through which the Neoproterozoic supercontinent Rodinia assembled. We report here the first UV laser spot ⁴⁰Ar/³⁹Ar mica and SHRIMP U–Pb zircon ages from a rare Grenville-aged metamorphic complex, the Tianli Schists, in the eastern Sibao Orogen. Our U–Pb zircon provenance ages indicate that the protolith of the Tianli Schists was a clastic sedimentary succession most likely derived from the Yangtze Block. The depositional age of the protolith is younger than 1530 Ma, as constrained by the youngest detrital zircon grains, but is older than 1040 Ma as constrained by the oldest ⁴⁰Ar/³⁹Ar muscovite ages. The Yangtze Block provenance for the Tianli Schists suggests that the Sibaoan ophiolitic complexes in northeastern Jiangxi, the ca. 970 Ma Xiwan adakitic granite intrusions, and the ca. 900 Ma(?) Xiwan blueschists, all to the northwest of the study region, were likely formed during the closure of a back-arc basin along the margin of the Yangtze Block. Our in situ UV laser ⁴⁰Ar/³⁹Ar results from S₁ and S₂ muscovites suggest that the Tianli Schists underwent metamorphism and deformation at 1042 ± 7 Ma to 1015 ± 4 Ma, the oldest known metamorphic event in the eastern Sibao Orogen. Muscovite/biotite cooling ages of ca. 968 ± 4 and 942 ± 8 Ma are recorded by deformed and recrystallised muscovite and biotite, respectively, indicating tectonic reactivation before 900 Ma, during the later stages of the Sibao Orogeny. Together with previous results from the western Sibao Orogen, our work suggests that the closure of the ocean between the Yangtze and Cathaysia Blocks during the assembly of Rodinia was diachronous: ≥1000 Ma at the western Sibao Orogen and ca. 900 Ma at the eastern Sibao Orogen.

Introduction

Worldwide occurrences of Grenville orogenic events at ca. 1300–900 Ma have commonly been associated with continental collisions during the assembly of the Neoproterozoic supercontinent Rodinia, and the orogenic belts have been used as one of the key features in piecing together Rodinia (e.g., Moores, 1991, Dalziel, 1991, Hoffman, 1991, Clarke et al., 1995, Karlstrom et al., 1999, Burrett and Berry, 2000, Fitzsimons, 2000, Boger et al., 2001, Jacobs et al., 2003). As an alternative to the conventional SWEAT model of western Rodinia (e.g., Moores, 1991), Li et al. (1995), and Z.X. Li et al., 2002, Z.X. Li et al., 2003b suggested that South China could have been situated between western Laurentia and Australia–East Antarctica in Rodinia (Fig. 1a). If such a model is correct, the Grenville-aged Sibao Orogen that runs across central South China (Fig. 1b) would have recorded the assembly process of Rodinia close to the present west margin of Laurentia. Based on different understandings of the Sibao orogenesis and Neoproterozoic magmatic events in South China, others believe that the South China Block was not a part of Rodinia at all (e.g., Zhou et al., 2002), or was on the periphery of Rodinia (e.g., Zhao and Cawood, 1999).

Despite its potential significance in late-Precambrian global tectonics, the Sibao Orogen is perhaps one of the poorest known Grenville-aged orogenic belts in the world. This was partly due to the sporadic exposure of the orogen (Fig. 1b), and the limited number of high-precision geochronological studies on the orogen. Questions that are yet to be systematically addressed include (1) how many major tectonic units (i.e., arc terranes, micro-continental terranes) are there in the orogen, (2) what are the ages and crustal affinities of these units, and (3) what are the metamorphic and structural characteristics of these units and when and how were they joined together.

We report in this paper the first high-resolution ultra-violet (UV) laser spot ⁴⁰Ar/³⁹Ar mica dating and Sensitive High-Resolution Ion Micro-Probe (SHRIMP) U–Pb zircon provenance ages on the largest Sibaoan meta-sedimentary complex in eastern Sibao Orogen—the Tianli Schists (Fig. 1, Fig. 2). The provenance data enable us to establish the Yangtze Block affinity of the complex, which is consistent with the back-arc nature of the Sibaoan northeastern Jiangxi ophiolites (e.g., Zhou, 1989) and the Xiwan blueschists (Zhou, 1997) (Fig. 1b). The UV laser ⁴⁰Ar/³⁹Ar analyses provide us the first high-quality ca. 1040 Ma metamorphic age for the eastern Sibao Orogen, and evidence for ca. 1015 Ma, ca. 970 Ma and ca. 940 Ma reactivations during the late Sibao Orogeny.

Section snippets

Geological background and sampling

The South China Block consists of two major Precambrian blocks: the Yangtze Block to the northwest and the Cathaysia Block to the southeast (present coordinates), with the late-Mesoproterozoic to earliest Neoproterozoic Sibao Orogen situated between them (Fig. 1b). Precambrian basement outcrops in South China are few; Fig. 1b shows the 11 best known pre-850 Ma outcrops and the known ages (with metamorphic ages shown in italics).

The Tianli Schists outcrop in the eastern segment of the Sibao

Experimental techniques and results of in situ UV laser ⁴⁰Ar/³⁹Ar dating and electron microprobe analyses

Polished 200–300 μm thick sections of samples 04SC66, 04SC67 and 04SC68, mounted on glass slides using Canada Balsam resin were prepared and the rock sections were then removed from the glass slides by soaking in acetone. The samples underwent ultrasonic treatment in methanol and subsequently deionised water. Approximately 10 mm × 10 mm regions containing areas of interest were broken off from the polished thick sections. Samples were individually wrapped in aluminium foil, and all the samples were

Experimental techniques and results of SHRIMP U–Pb dating

Zircon concentrates were extracted from samples 04SC66 (28°28′13.1″N, 118°08′23.9″E) and 04SC67 (28°28′15.2″N, 118°08′32.6″E) at the MinSep Laboratories at Denmark, Western Australia, using standard density and magnetic separation techniques. Zircon grains, together with zircon standards TEMORA (for calibrating U–Th–Pb ratios; ²⁰⁶Pb/²³⁸U = 0.0668, corresponding to 417 Ma; Black et al., 2003) and CZ3 (for calibrating absolute U abundances; U = 550 ppm), were mounted in epoxy mounts and all zircons

Summary of tectonothermal events in the eastern Sibao Orogen

The debate regarding the timing of amalgamation between the Yangtze and Cathaysia Blocks to form the coherent South China Block, and its role in the assembly of Rodinia, has largely stemmed from (1) the lack of information regarding the Sibaoan metamorphism, and (2) variable interpretations of Neoproterozoic magmatism. The Tianli region provides not only rare Sibaoan metamorphic rocks that have enabled us to obtain high-precision geochronological data regarding its crustal affinity and

Conclusions

We report here the first in situ UV laser ⁴⁰Ar/³⁹Ar and SHRIMP U–Pb zircon provenance analyses of the oldest Grenville-aged metamorphic rocks, the Tianli Schists, in the eastern Sibao Orogen, South China. Our sedimentary provenance data indicate that the protolith of the Tianli Schists was a clastic sedimentary succession, probably formed at the southern continental shelf of the Yangtze Block between ca. 1530 Ma and 1042 Ma. It was metamorphosed at 1042–1015 Ma, and suffered reactivations before

Acknowledgements

We thank Wuxian Li, Shihong Zhang and Yuzhang Zhou for assistance during the fieldwork, Mathew Greentree and Bert De Waele for assisting the SHRIMP analyses, A. Frew with SEM imaging, and Guochun Zhao and an anomalous reviewer for constructive reviews. The ⁴⁰Ar/³⁹Ar data presented in this study were undertaken at the Western Australian Argon Isotope Facility, operated by a consortium consisting of Curtin University and the University of Western Australia. The electron microprobe analyses were

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¹: Previously at: Tectonics Special Research Centre, The University of Western Australia, Crawley, WA 6009, Australia.

²: Present address: School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ 85287-1404, USA.

³: Present address: Fugro Airborne Surveys, Locked Bag 6, Wembley, WA 6014, Australia.

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Early history of the eastern Sibao Orogen (South China) during the assembly of Rodinia: New mica 40Ar/39Ar dating and SHRIMP U–Pb detrital zircon provenance constraints

Abstract

Introduction

Section snippets

Geological background and sampling

Experimental techniques and results of in situ UV laser 40Ar/39Ar dating and electron microprobe analyses

Experimental techniques and results of SHRIMP U–Pb dating

Summary of tectonothermal events in the eastern Sibao Orogen

Conclusions

Acknowledgements

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Earth Planet. Sci. Lett.

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Earth Planet. Sci. Lett.

Early Paleozoic tectonism within the East Antarctic Craton: the final suture between East and West Gondwana?

Geology

Proterozoic Australia–Western United States (AUSWUS) fit between Laurentia and Australia

Geology

Magmatism along the southeast margin of the Yangtze block: Precambrian collision of the Yangtze and Cathysia block of China

Geology

Early history of the eastern Sibao Orogen (South China) during the assembly of Rodinia: New mica ⁴⁰Ar/³⁹Ar dating and SHRIMP U–Pb detrital zircon provenance constraints

Experimental techniques and results of in situ UV laser ⁴⁰Ar/³⁹Ar dating and electron microprobe analyses