Elsevier

Precambrian Research

Volume 351, December 2020, 105947
Precambrian Research

Geochemical and zircon U-Pb-Hf isotopic study of metasedimentary rocks from the Huangyuan Group of the Central Qilian block (NW China): Implications for paleogeographic reconstruction of Rodinia

https://doi.org/10.1016/j.precamres.2020.105947Get rights and content

Highlights

  • Metasedimentary rocks in Huangyuan Group represent a sedimentary series with a long history of deposition.

  • The deposition started at ca. 1317 Ma in an oceanic island arc-related basin that developed through a transitional continental arc-related basin into an active continental marginal basin at ca. 927 Ma.

  • The 1795–1321 Ma detritus was sourced from juvenile arc crust at the margin of the Indian or the Western Australian craton.

  • The source rocks for 1317–913 Ma detritus were arc magmatic rocks formed by oceanic lithosphere subduction at the margin of Rodinia.

Abstract

We present a systematic study of micaschists and felsic gneisses from the Huangyuan Group of the Central Qilian block in NW China, with aims to unravel the connection with the Rodinia supercontinent. The micaschists have detrital zircon ages of 2895–928 Ma that peaking at 1.80–1.40 Ga. They show strongly increasing zircon εHf(t) values of −8.1 to +12.1 from 1.6 Ga to 1.4 Ga. Detrital zircon ages from the felsic gneisses are dominantly 960–913 Ma with εHf(t) values of −0.1 to −10.7. The micaschists have a wide range of whole-rock major element compositions, and the felsic gneisses have higher SiO2 contents, combined with lower other major element contents than those of the micaschists. All samples have trace element compositions consistent with upper continental crustal origin. The protoliths of the micaschists are dominantly shales and minor wackes with maximum depositional ages from ca. 1317 to 928 Ma. The protoliths of the felsic gneisses are mostly wackes with a maximum depositional age of ca. 927 Ma. The source materials for these metasedimentary rocks originated from intermediate to felsic igneous rocks. The variable maximum depositional ages of the metasedimentary rocks in the Huangyuan Group indicate that their protoliths constituted a sedimentary series with a long history of deposition starting at ca. 1317 Ma in an oceanic island arc-related basin that developed through a transitional continental arc-related basin into an active continental marginal basin at ca. 927 Ma. It is inferred that the 1795–1321 Ma detritus was sourced from juvenile arc crust at the margin of the Indian or the Western Australian craton. The source rocks for 1317–913 Ma detritus were arc magmatic rocks formed during assembly of Rodinia. A sequence of initial intra-oceanic subduction (ca. 1317–967 Ma) and continuous oceanic crust-continent subduction with formation of a mature continental arc (ca. 967–896 Ma) at the margin of Rodinia during the formation of the Central Qilian block is suggested.

Introduction

The tectonic framework of China consists of three major Precambrian blocks (the North China, South China, and Tarim cratons) amalgamated with numerous smaller Precambrian blocks during Phanerozoic orogenies (Zhao and Cawood, 2012, Zheng et al., 2013). As one of these small Precambrian blocks, the Central Qilian block is located in the NW-SE striking Qilian orogen, which is a part of the Central China orogenic system (Li et al., 1978) and located at the junction of the North China craton in the northeast, the South China craton in the southeast, and the Tarim craton in the northwest (Song et al., 2013; Fig. 1a). The tectonic affinity of the Central Qilian block has long been controversial. Some researchers proposed that the Central Qilian block was rifted off the North China craton and later collided with it again (Zuo and Liu, 1987, Xia et al., 1991, Feng and He, 1996). More studies suggest it is related to the South China craton, instead (Wan et al., 2000, Wan et al., 2003, Wan et al., 2006, Xu et al., 2007, Tung et al., 2007, Yan et al., 2015, Xia et al., 2016). Recently, Wu et al. (2017) proposed that the Central Qilian block was originated from the Tarim craton.

Abundant records of magmatic and metamorphic ages of 1000–900 Ma in the Central Qilian block reflect an early Neoproterozoic basement (Guo et al., 1999, Guo et al., 2000, Tung et al., 2012, Tung et al., 2013, Song et al., 2012, Huang et al., 2015, Yan et al., 2015, Li et al., 2018a, Li et al., 2020). However, many Paleo- to Meso-proterozoic detrital zircon U-Pb ages were also obtained from metasedimentary basement rocks in the Central Qilian block, e.g., 1.8–1.1 Ga in the Tuolai Group and 1.8–1.6 Ga in the Huangyuan Group (Tung et al., 2007). It is widely accepted that the 1.8–1.3 Ga and the 1.0–0.9 Ga are two significant periods in global tectonic events, related to the breakup of the Columbia supercontinent (Zhao et al., 2004, Zhao et al., 2011 and references therein) and assembly of the Rodinia supercontinent (Zhao et al., 2018, Li et al., 2008), respectively. The North China craton and the South China craton are considered to have been part of both Columbia and Rodinia (e.g., Li et al., 1999, Li et al., 2002, Rogers and Santosh, 2002, Zhai et al., 2003, Zhao et al., 2003, Zhao et al., 2004). However, whether the adjacent Central Qilian block was also associated with Columbia or Rodinia, or both, is poorly constrained.

The Huangyuan Group is located in the central-eastern part of the Central Qilian block and was identified as the lower part of the Precambrian basement of the block (BGMR-GP, 1989, BGMR-QP, 1991). It is composed of widespread garnet-bearing quartzofeldspathic gneisses, garnet-bearing schists, leptynite and intercalated lenses or layers of marble, quartzite, amphibolite and magmatic veins (BGMR-QP, 1991). The whole-rock geochemistry of metasedimentary and metavolcanic rocks indicates that the Huangyuan Group formed in a continental rift setting (Bai et al., 1998) or an arc-related basin (Guo et al., 1999). A large span of Paleo- to Neo-proterozoic detrital zircon U-Pb ages arose from the group (Lu et al., 2009, Tung et al., 2007). Guo et al. (2000) interpreted the Huangyuan Group as Neoproterozoic basement based on LA-ICPMS zircon U-Pb dating on a metatuff (910 ± 7 Ma) and a granitic gneiss (917 ± 12 Ma). However, other Neoproterozoic (884 Ma; Tung et al., 2007) and even Mesoproterozoic (1246 Ma; Lu et al., 2009) ages have also been interpreted to represent deposition of the protoliths of the metasedimentary rocks in the group.

Based on detailed fieldwork and systematic sample collection, we present zircon U-Pb-Hf isotopic compositions and whole-rock geochemical date of metasedimentary rocks from the Huangyuan Group in the Central Qilian block. Our data provide new constraints on their depositional age, provenance and tectonic setting, and shed new light on reconstruction models of the Rodinia supercontinent and connection with the Columbia supercontinent.

Section snippets

Geological setting

The Central Qilian block is one of a series of continental fragments at the junction between the South China, North China and Tarim cratons. From northeast to southwest, the sequence is made up of the Alxa block, the North Qilian accretionary belt, the Central Qilian block, the South Qilian accretionary belt, the Quanji block, the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt and the Qaidam block (Fig. 1b; Song et al., 2013, Song et al., 2017, Yan et al., 2015, Xia et al., 2016, Xu et

Zircon cathode luminescence (CL) imaging

Sample processing for zircon separation involved crushing, initial heavy liquid and subsequent magnetic separation. Representative zircon grains were hand-picked and mounted on adhesive tape, embedded in epoxy resin, polished down to about half their size and photographed in reflected and transmitted light. Zircon structures were studied by CL imaging of zircon grains using a Gatan MonoCL4 instrument mounted on an FEI Nova NanoSEM 450 scanning electron microscope at the Key Laboratory of

Petrology

Rocks of the Huangyuan Group are well exposed in the northwestern part of Xining City in Qinghai Province. It comprises mainly deformed micaschists and felsic gneisses, with intercalated lenticular or quasi-lamellar amphibolites, layered quartzites and banded marbles (Li et al., 2019). Garnet-bearing schist is interbedded with felsic leptynite (Fig. 2a), suggesting a sedimentary protolith. Most schists and gneisses contain aluminium rich minerals, such as garnet, staurolite and sillimanite (

Depositional ages of the protoliths

The youngest age peak of detrital zircon grains in a sample suite can be used to constrain the maximum depositional age of sedimentary rocks (Dickinson and Gehrels, 2009). Detrital zircon ages extracted from micaschists in the Huangyuan Group range from 2895 to 928 Ma, and the youngest zircon ages in each sample are variable from 1317 to 928 Ma (Fig. 4a-i). The felsic gneiss sample from the Huangyuan Group in this study has detrital zircon ages predominantly of 960–913 Ma that peaking at

Conclusion

  • (1)

    The micaschists in the Huangyuan Group have detrital zircon ages of 2895–928 Ma that peaking at ca. 1.80–1.40 Ga with two stage Hf model ages of 3423–1407 Ma. They show strongly increasing zircon εHf(t) values of −8.1 to + 12.1 from ca.1.6 Ga to 1.4 Ga, indicating voluminous juvenile materials were added to the crust during this period. Detrital zircon ages from the felsic gneisses are dominantly 960–913 Ma and peak at 927 Ma with εHf(t) values of −0.1 to −10.7 and two-stage model ages of

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are indebted to graduate students Xin Tong and Limin Zhao of the School of Earth Sciences, China University of Geosciences (Wuhan) for help with field work and sample preparation. We are grateful to Editor Guochun Zhao for his supportive editorial handling and two anonymous reviewers for their careful reviews, which greatly helped us to improve the paper. This study was funded by grants from the National Natural Science Foundation of China (41872063, 41930215, 41520104003, 41802092,

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