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Gondwana Research

Volume 43, March 2017, Pages 149-163
Gondwana Research

Ages and Hf isotopes of detrital zircons from Paleozoic strata in the Chagan Obo Temple area, Inner Mongolia: Implications for the evolution of the Central Asian Orogenic Belt

https://doi.org/10.1016/j.gr.2016.08.004Get rights and content

Highlights

  • The Ordovician-Permian strata mainly sourced from the Mongolian arcs and Northern Orogen

  • Provenance variations and εHf values indicate the opening of a back-arc basin (Hegenshan)

  • Source rocks of the Carboniferous-Permian sediments were mainly juvenile

  • The juvenile crust was related to the subduction of the Paleo-Asian Ocean and Hegenshan Ocean

Abstract

The Central Asian Orogenic Belt (CAOB), as one of the largest accretionary orogens in the world, was built up through protracted accretion and collision of a variety of terranes due to the subduction and closure of the Paleo-Asian Ocean in the Neoproterozoic to Early Mesozoic. Located in the Uliastai continental margin of the southeastern CAOB, the Chagan Obo Temple area is essential for understanding the tectonic evolution of the southeastern part of the CAOB and its relation with the “Hegenshan Ocean”. In this study, detrital zircon U-Pb geochronology coupled with Hf isotopic analysis was performed on Paleozoic sedimentary strata in this area. Most detrital zircons from the studied samples possess oscillatory zoning and have Th/U ratios of 0.4-1.73, indicative of an igneous origin. Detrital zircons from the Ordovician to Devonian sedimentary strata yield a predominant age group at 511-490 Ma and subordinate age groups at 982-891 Ma, 834-790 Ma and ~ 574 Ma, and have a large spread of εHf(t) values (-20.77 to + 16.94). Carboniferous and Early Permian samples yield zircon U-Pb ages peaking at ~ 410 Ma and ~ 336 Ma, and have dominantly positive εHf(t) values (+ 1.30 to + 14.86). Such age populations and Hf isotopic signatures match those of magmatic rocks in the Northern Accretionary Orogen and the Mongolian arcs. A marked shift of provenance terranes from multiple sources to a single source and Hf isotope compositions from mixed to positive values occurred at some time in the Carboniferous. Such a shift implies that the Northern Accretionary Orogen was no longer a contributor of detritus in the Carboniferous to Early Permian, due to the opening of the “Hegenshan Ocean” possibly induced by the slab rollback of the subducting Paleo-Asian Ocean.

Introduction

The Central Asian Orogenic Belt (CAOB), also termed the Altaids, is one of the largest accretionary orogenic collages in the world. It is bounded to the north by the Siberian craton, to the west by the East European Block and to the south by the Tarim and North China cratons (Fig. 1). (Zonenshain et al., 1990, Mossakovsky et al., 1993, Sengör et al., 1993, Jahn et al., 2000, Badarch et al., 2002, Xiao et al., 2003, Xiao et al., 2004, Jahn, 2004a, Jahn et al., 2004b, Briggs et al., 2007, Windley et al., 2007, Jian et al., 2008, Xiao et al., 2009a, Xiao et al., 2009b, Xiao et al., 2010, Xiao et al., 2013, Eizenhöfer et al., 2014, Xiao and Santosh, 2014, Xiao et al., 2015, Eizenhöfer et al., 2015a, Eizenhöfer et al., 2015b). This vast orogen was built up through interplay of a series of tectonically related island arcs, fore-arc or back-arc basins, ophiolitic belts, blueschist belts, and remnant microcontinents, resulting from Neoproterozoic to Mesozoic oceanic subduction, closure, accretion and collision of allochthonous microcontinents. (Wang and Liu, 1986, Shao, 1989, Tang, 1990, Sengör and Natal’in, 1996a, Sengör and Natal’in, 1996b, Xu and Chen, 1997, Xiao et al., 2003, Xiao et al., 2004, Li, 2006, Xiao et al., 2009a, Xiao et al., 2009b, Xiao et al., 2010, Kröner et al., 2011, Xu et al., 2013, Zhao et al., 2013, Xiao and Santosh, 2014, Han et al., 2015, Xiao et al., 2015, Han et al., 2016a, Han et al., 2016b). The accretionary and collisional processes, starting at ~ 750 Ma coevally with the breakup of Rodinia and possibly lasting till early Triassic (Eizenhöfer et al., 2014), resulted in (1) the closure of the Paleo-Asian Ocean (PAO), (2) the assembly of Eurasia, including the amalgamation of the Tarim and North China blocks in the south, the East Europen and Siberian blocks in the north, and also microcontinents, terranes and arcs in or between blocks, and (3) considerable Phanerozoic juvenile crustal growth (Sengör et al., 1993, Heubeck, 2001, Xiao et al., 2003, Torsvik and Cocks, 2004, Xiao et al., 2004, Xiao et al., 2009a, Xiao et al., 2009b, Xiao et al., 2010, Han et al., 2011, Xiao et al., 2013, Eizenhöfer et al., 2014, Xiao and Santosh, 2014, Han et al., 2015, Xiao et al., 2015, Zhu et al., 2015, Zhang et al., 2015a, Han et al., 2016a, Han et al., 2016b).

Despite extensive investigations, many unresolved issues still remain, especially regarding when and how the Paleo-Asian Ocean subducted and closed to form the CAOB. Available models emphasizing different accretionary processes include: (1) progressive subduction and accretion associated with the closure of a single major ocean (e.g., Sengör et al., 1993, Sengör and Natal’in, 1996a, Sengör and Natal’in, 1996b, Yakubchuk, 2004), (2) accretion of accretionary complexes onto microcontinents or within oceanic domains (e.g., Mossakovsky et al., 1993, Fedorovskii et al., 1995), and (3) punctuated subduction and collision of several microcontinents and arcs with bidirectional orogenic polarity (e.g., Coleman, 1989, Mossakovsky et al., 1993, Filippova et al., 2001, Xiao et al., 2003, Xiao et al., 2004, Briggs et al., 2007, Windley et al., 2007, Kelty et al., 2008, Xiao et al., 2009a, Xiao et al., 2009b, Xiao et al., 2010). In addition, controversy has also surrounded the timing of final closure of the Paleo-Asian Ocean, with some researchers invoking that the collision between North China and Siberia occurred during the late Devonian to early Carboniferous (e.g., Tang, 1990, Shao, 1991, Xu and Chen, 1997), whereas other workers, based on the recognition of Permian calc-alkaline magmatism, paleontology and ophiolites, argue that the intervening ocean between the North China craton (NCC) and the South Mongolian microcontinent was not closed until the late Permian or early Triassic (e.g., Wang and Liu, 1986, Hsü et al., 1991, Sengör and Natal’in, 1996a, Sengör and Natal’in, 1996b, Xiao et al., 2003, Xiao et al., 2004, Miao et al., 2008, Xiao et al., 2009a, Xiao et al., 2009b, Xiao et al., 2010, Xiao et al., 2013, Eizenhöfer et al., 2014, Xiao et al., 2015, Eizenhöfer et al., 2015a, Eizenhöfer et al., 2015b).

Compared with the western segment of the CAOB, several fundamental questions remain ambiguous in the eastern part due to lack of insufficient studies, especially on the Inner Mongolia terrane in China. The Inner Mongolia terrane occupies an intermediate position between Siberia and North China (Fig. 1). It records wealthy information regarding how the two blocks amalgamated, and is crucial not only to differentiating between the above models, but also to understanding the architecture of the CAOB, such as the tectonic nature and emplacement of the Hegenshan ophiolite and its interaction with the surrounding tectonic entities.

Detrital zircons preserved in sedimentary sequences carry important information on the assembly/dispersion history of geological entities (e.g., Cawood et al., 2012, Han et al., 2015, Zhang et al., 2015b, Zhang et al., 2015c, Zhang et al., 2016). Detrital U-Pb zircon geochronology provides insights into geological correlations between discrete terranes, and thus has been extensively employed as a reliable approach in tectonic reconstructions (e.g., Haughton et al., 1991, Eizenhöfer et al., 2014). Specifically, geological correlations between discrete terranes can be established by comparing age distributions between depositional areas and possible provenance terrane candidates.

In this study, detrital U-Pb zircon geochronology coupled with Hf isotopic analysis was performed on the Ordovician and Early Permian strata from the Chagan Obo Temple area in central Inner Mongolia of southeastern CAOB (Fig. 1). Due to poor exposure conditions, few detailed geochronological and geochemical data have been reported for this area. Integrating new data, we attempt to decipher the sedimentary record and major sedimentary provenance terranes, and discuss the Paleozoic evolution of the Uliastai continental margin arc in the southeastern CAOB. Our results support the punctuated collision and accretionary model and allow an interpretation of back-arc extension origin for Carboniferous-Permian opening of the “Hegenshan Ocean”, and imply that the final closure of the PAO postdates the Early Permian.

Section snippets

Regional geology

The CAOB in Chinese Inner Mongolia, also referred to “Manchurides” (Sengör and Natal’in, 1996a, Sengör and Natal’in, 1996b) or “Great Hinganling-Inner Mongolian orogenic belt” (Yin and Nie, 1996), is characterized by a ENE-trending tectonic collage composed of remnants of ophiolites, arcs, accretionary wedges and associated volcano-sedimentary rocks.

In the Inner Mongolia, the CAOB can be subdivided into several tectonic zones. They are, from south to north, the Southern Accretionary Orogen, the

Sample preparation

Zircons were separated using conventional heavy liquid and magnetic techniques before handpicking under a binocular microscope at the Langfang Regional Geological Survey, Hebei Province, China. Randomly selected zircon grains were mounted in epoxy resin and polished to expose their interior. Handpicked zircons were photographed under transmitted and reflected light under optical microscope before Cathodoluminescence (CL) imaging using a FEI Quanta 400 FEG environmental scanning electron

Results

The zircon U-Pb ages and in situ Hf isotopes are illustrated in Fig. 7, Fig. 8, Fig. 9, and presented in Supplementary Table 1, Supplementary Table 2. To ensure the efficient quality, U-Pb analyses showing large variations of signals or large age discordance (not between 90% and 120%) were discarded.

Constraints on the depositional ages

The depositional age of the Wubinaobao Formation has been previously considered to be Early-Middle Ordovician based on association of fossils and regional lithological correlations (IMBGMR (Inner Mongolian Bureau of Geology and Mineral Resou, Zhao et al., 2014). Our new dating results show the youngest age components of 443 ± 6 Ma for sample 13EH10A and 475 ± 10 Ma for sample 13EH12B, respectively. These age determinations define the maximum depositional age, indicating that the Wubinaobao

Conclusions

Based on systematic U-Pb and Hf isotopic study of detrital zircons from Late Ordovician-Early Permian sedimentary rocks of the Chagan Obo Temple area, and on comprehensive comparisons with previous published data in adjacent regions, we can draw the following main conclusions:

  • (1)

    Detrital zircons from four Ordovician-Devonian samples yield U-Pb ages clustering around ca. 510-490 Ma, 830-790 Ma and 980-890 Ma with relatively less Meso-Proterozoic to Archean ages, and have a large spread of εHf(t)

Acknowledgements

We would like to thank journal reviewers Shuanhong Zhang and an anonymous reviewer for their constructive and thoughtful comments and suggestions. This work was funded by a NSFC Project (41190075) entitled “Final Closure of the Paleo-Asian Ocean and Reconstruction of East Asian Blocks in Pangea”, which is the fifth project of NSFC Major Program (41190070) “Reconstruction of East Asian Blocks in Pangea”, Hong Kong RGC GRF (HKU7063/13P and 17301915), HKU Seed Funding (201311159126) and Small

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