Triassic turbidites in the West Qinling Mountains, NW China: Part of the collisional Songpan-Ganzi Basin or an active forearc basin?

https://doi.org/10.1016/j.jseaes.2020.104366Get rights and content

Highlights

  • Triassic sandstones in the West Qinling Mts. mainly comprise feldspathic litharenite and lithic arkose.

  • Sediments are mainly derived from ca. 273 Ma, 435 Ma and 1869 Ma sources.

  • Deposition occurred in a forearc basin in response to northward subduction.

Abstract

Lower to Middle Triassic clastic rocks in the West Qinling Mountains along NE margin of the Qinghai-Tibet Plateau are generally regarded as part of the Songpan-Ganzi flysch Basin. However relatively little attention has been paid to the age and provenance of these units. New petrological and geochemical results demonstrate that these sediments accumulated along an active continental margin and are dominated by feldspathic litharenite and lithic arkose with low mineral and compositional maturity. They were derived primarily from a continental arc source dominated by intermediate to felsic igneous rocks, with a minor contribution from older metamorphosed and sedimentary sources. Turbidite samples yielded two primary detrital zircon U-Pb age populations of ca. 273 Ma and ca. 435 Ma, which is different from the Early Triassic (ca. 252 Ma) and Middle Silurian (ca. 427 Ma) age populations that dominate the Songpan-Ganzi Basin. These data together with paleocurrent results indicate that the South Qilian Belt was the primary source origin because this belt contains both early Paleozoic Andean-type igneous and magmatic rocks and has a basement of Precambrian metamorphosed rocks. Regionally, an Andean-type arc traverses the South Qilian belts and extends into the West Qinling Mountains and Kunlun Orogen, which formed by north-directed subduction of the Paleo-Tethyan Ocean during the Early to Middle Triassic. Voluminous detritus that originated from the Qilian and Kunlun orogens as they were uplifted and eroded was transported to the south and deposited in the forearc area in front of the A’nimaqen-Mianlue suture.

Introduction

Clastic sediments are composed of different minerals and detritus derived from pre-existing source rocks. Their compositions are closely related to provenance and the tectonic settings of sedimentary basins. Studies of the provenance of siliciclastic rocks can provide robust information for understanding basin development by linking sediment supply to exhumation episodes and tectonic evolution (e.g., Japsen et al., 2007; Weislogel et al., 2006; Higgs and King, 2018, Sun et al., 2016, Weislogel et al., 2010) and allows the reconstruction of sediment transport routes (e.g., Dickinson and Gehrels, 2009; Olivarius et al., 2014, Yan et al., 2014, Pe-Piper et al., 2016). Diverse techniques based on petrographical and geochemical criteria have been successfully used in ancient rocks for this purpose (e.g., Dickinson and Gehrels, 2008, Ryan and Williams, 2007, She et al., 2006, Sun et al., 2016, Yan et al., 2008, Yan et al., 2012, Yan et al., 2014). In addition, chemically immobile elements in siliciclastic rocks such as rare earth elements (REE) and some trace elements of Th, Zr, Hf, and Sc also provide information on the degrees and types of weathering, which is useful in developing an understanding of the regional setting of basin development.

Triassic flysch in the West Qinling Mountains is traditionally regarded as a component of the collisional Songpan-Ganzi Basin in the NE Tibetan Plateau (Brugier et al., 1997, Enkelmann et al., 2007, Nie et al., 1994, Weislogel et al., 2010, Zhou and Graham, 1996). These turbidite deposits have commonly been interpreted as the erosional products associated with uplift during closure of the Paleo-Tethyan Ocean basin between the North China-Tarim plate and Qaidam block to the north, the South China plate to the east, and the Qiangtang terrane and Yidun arc terrane to the south (Sengör et al., 1988, Stampfli and Borel, 2002, Weislogel, 2008, Li et al., 2017, Li et al., 2018; Fig. 1a). Although these sediments have been studied before, most work focuses on interpretation of detrital zircons in sandstones from the southern Songpan-Ganzi Basin and less attention has been paid in the West Qinling Mountains. There is still no agreement on the provenance and depositional setting of the Songpan-Ganzi Basin. In contrast, a distinct synthesis based on petrology and geochemistry of ophiolite and the associated rocks in the West Qinling Mountains and Kunlun Orogen (Xu et al., 1996, Xiao et al., 2002, Bian et al., 2004, Guo et al., 2012, Li et al., 2013, Li et al., 2014) infers that a Triassic Andean-type continental margin (Yan et al., 2012, Yan et al., 2014) developed in response to the northward subduction of the Paleo-Tethyan Ocean. However, Li et al., 2017, Li et al., 2018 suggested that all the continental blocks in China were assembled to form the Supercontinent Pangea around ~250 Ma, indicating that the Paleo-Tethyan Ocean closed prior to 250 Ma and the Lower to Middle Triassic sediments in the Songpan-Ganzi Basin were not related to subduction of the Paleo-Tethyan Ocean.

In order to address this controversy, we assess the provenance of Triassic flysch in the West Qinling Mountains focusing on the geochemistry of fine-grained siliciclastic rocks and two detrital zircon U-Pb dating samples. Using these new data in combination with regional geology and previous published data, we discuss the tectonic setting in which deposition occurred and the associated evolution of the Paleo-Tethyan Ocean in the West Qinling Mountains between the North and South China plates.

Section snippets

Geological setting

The West Qinling Mountains is a geologically significant portion of the Qinling Orogenic Belt. This belt extends for more than 1000 km across eastern Asia and separates the North China Plate to the north from the South China Plate to the south. To the north and west, the West Qinling Mountains connect with the Qilian and Kunlun orogenic belts (Fig. 1b), respectively. To the south, they are separated from the Songpan-Ganzi Basin by a latest middle-Triassic ophiolitic mélange along the

Sampling and analytical methods

In order to further define the composition of Lower- and Middle-Triassic sediments and their potential source rocks, thin-sections of fine- and coarse-grained sandstone and mudstone samples were observed under a polarizing microscope. A total of 32 representative samples including 18 mudstones, 11 siltstones, and three lithic arkoses from around Tongren area (Fig. 1c) were selected for geochemical analysis, and two medium-grained arkose samples (LW1 and TR7) were collected for detrital zircon

Sandstone petrology

Sandstones in the Longwuhe Group are dominated by lithic arkose and litharenite containing abundant volcanic and metamorphic fragments. Those from the Gulangdi Formation consist mainly of feldspathic litharenite and lithic arkose, characterized by abundant feldspar, volcanic, and granitic fragments with minor carbonate and metamorphic fragments (Fig. 3). Siltstone contains abundant fine-grained feldspar and minor quartz and volcanic fragments, and mudstone is dominated by detrital sericite with

Geochemistry

The analyzed samples have a similar average major element geochemistry, indicating a similar mineralogy. Four siltstone samples from the Longwuhe Group exhibit lower SiO2 (43.34–49.59%) and Al2O3 (12.86–14.39%) abundance and higher abundance of CaO (7.09–17.72%) and loss on ignition (LOI; 7.21–14.59%), indicating the presence of detrital grains with carbonate and/or calcareous cement. Other samples have a variable and higher range of SiO2 content (51.93–61.16%) and exhibit variable negative

Paleoweathering conditions

Provenance and the intensity of weathering in the source region are the main factors that control the chemical and mineralogical composition of siliciclastic rocks. Generally, large ionic radius cations such as Cs, Rb and Ba are relative immobile during chemical weathering processes and preferentially fixed in weathering profiles by adsorption clays, while smaller cations like Na, Ca and Sr are selectively leached from them (McLennan et al., 1993). Therefore, the effects of alteration have to

Provenance

Al2O3/TiO2 ratios of Lower to Middle Triassic sediments (18–25) in this study are similar to intermediate-felsic igneous rocks (19–28; Girty et al., 1996), and their REE patterns are also comparable to felsic igneous rocks showing high LaN/YbN ratios and negative Eu anomalies (Rollinson, 1993). On the discrimination plot of Floyd et al. (1989) (Fig. 9a), almost all samples plot in the magmatogenic greywacke field associated with felsic to intermediate igneous sources.

Generally, basic volcanic

Conclusions

  • (1)

    Lower to Middle Triassic turbidites in the West Qinling Mountains mainly consist of feldspathic litharenite and lithic arkose with low mineral and compositional maturity.

  • (2)

    Detrital sediments originated from a primary continental arc source dominated by intermediate to felsic igneous rocks, with a minor contribution from older metamorphosed and sedimentary sources.

  • (3)

    Detrital zircon dating indicates that Early- and Middle-Triassic turbidites of the West Qinling Mountains are dominated by detritus

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.

Acknowledgments

Professor Shu Sun is a prestigious sedimentologist, who made important contributions to Geoscience. We would like to commemorate Mr. Sun Shu with this article. Support for this work was provided by the National Natural Science Foundation of China (Grants 41672221, 41872241, and 41702239) and China Geological Survey (Grants DD20190006, DD20160201-04). We greatly appreciate constructive and helpful reviews by Professor Sanzhong Li, an anonymous reviewer and comments by Editor Professor Mei-Fu

References (83)

  • S.Z. Li et al.

    Triassic southeastward subduction of North China Block to South China Block: insights from new geological, geophysical and geochemical data

    Earth Sci. Rev.

    (2017)
  • S.Z. Li et al.

    Closure of the Proto-Tethys Ocean and Early Paleozoic amalgamation of microcontinental blocks in East Asia

    Earth Sci. Rev.

    (2018)
  • X.W. Li et al.

    Petrology and geochemistry of the early Mesozoic pyroxene andesites in the Maixiu Area, West Qinling, China: products of subduction or syn-collision?

    Lithos

    (2013)
  • L. Li et al.

    Late Permian-early Middle Triassic back-arc basin development in West Qinling, China

    J. Asian Earth Sci.

    (2014)
  • X.W. Li et al.

    Petrology, geochemistry and geochronology of the magmatic suite from the Jianzha Complex, central China: petrogenesis and geodynamic implications

    J. Asian Earth Sci.

    (2014)
  • S.M. McLennan et al.

    Geochemical and Nd-Sr isotopic composition of deep-sea turbidites: crustal evolution and plate tectonic associations

    Geochim. Cosmochim. Acta

    (1990)
  • M. Olivarius et al.

    Provenance signal variations caused by facies and tectonics: Zircon age and heavy mineral evidence from Miocene sand in the north-eastern North Sea Basin

    Mar. Pet. Geol.

    (2014)
  • G. Pe-Piper et al.

    Quaternary evolution of the rivers of northeast Hainan Island, China: tracking the history of avulsion from mineralogy and geochemistry of river and delta sands

    Sed. Geol.

    (2016)
  • B.P. Roser et al.

    Provenance signatures of sandstone–mudstone suites determined using discriminant function analysis of major-element data

    Chem. Geol.

    (1988)
  • R.L. Rudnick et al.

    The composition of the continental crust

  • Z. She et al.

    Provenance of the Triassic Songpan-Ganzi flysch, west China

    Chem. Geol.

    (2006)
  • G.M. Stampfli et al.

    A plate tectonic model for the Paleozoic and Mesozoic

    Earth Planet. Sci. Lett.

    (2002)
  • Y. Tang et al.

    Mesozoic-Cenozoic evolution of the Zoige depression in the Songpan-Ganzi flysch basin, eastern Tibetan Plateau: constraints from detrital zircon U-Pb ages and fission-track ages of the Triassic sedimentary sequence

    J. Asian Earth Sci.

    (2018)
  • A.L. Weislogel

    Tectonostratigraphic and geochronologic constraints on evolution of the northeast Paleotethys from the Songpan-Ganzi complex, central China

    Tectonophysics

    (2008)
  • A. Basu et al.

    Reevaluation of the use of undulatory extinction and polycrystallinity in detrital quartz for provenance interpretation

    J. Sediment. Petrol.

    (1975)
  • BGMRGP (Bureau of Geology and Mineral Resources of Gansu Province), 1989. Regional Geology of Gansu Province....
  • BGMRQP (Bureau of Geology and Mineral Resources of Qinghai Province), 1991. Regional Geology of Qinghai Province....
  • M.R. Bhatia

    Plate tectonics and geochemical composition of sandstones

    J. Geol.

    (1983)
  • M.R. Bhatia et al.

    Trace element characteristics of graywackes and tectonic setting of sedimentary basins

    Contrib. Miner. Petrol.

    (1986)
  • O. Brugier et al.

    U-Pb dating on single zircon grains from the Triassic Songpan-Garze flysch (Central China): provenance and tectonic correlations

    Earth Planet. Sci. Lett.

    (1997)
  • E.Z. Chang

    Geology and tectonics of the Songpan-Ganzi fold belt, southwestern China

    Int. Geol. Rev.

    (2000)
  • B. Chen et al.

    Geotectonics of the Nujiang-Lancangjiang-Jinshajiang Region

    (1987)
  • W.R. Dickinson et al.

    Sedimentary delivery to the Cordilleran foreland basin: insights from U-Pb ages of detrital zircons in Upper Jurassic and Cretaceous strata of the Colorado Plateau

    Am. J. Sci.

    (2008)
  • W.R. Dickinson et al.

    U-Pb ages of detrital zircons in Jurassic eolian and associated sandstones of the Colorado Plateau: evidence for transcontinental dispersal and intraregional recycling of sediment

    Geol. Soc. Am. Bull.

    (2009)
  • L. Ding et al.

    Provenance analysis of the Mesozoic Hoh-Xil-Songpan-Ganzi turbidites in northern Tibet: implications for the tectonic evolution of the eastern Paleo-Tethys Ocean

    Tectonics

    (2013)
  • Y. Dong et al.

    Subduction and accretionary tectonics of the East Kunlun Orogen, western segment of the Central China Orogenic System

    Earth Sci. Rev.

    (2017)
  • E. Enkelmann et al.

    How was the Triassic Songpan-Ganzi basin filled? A provenance study

    Tectonics

    (2007)
  • C.M. Fedo et al.

    Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance

    Geology

    (1995)
  • Feng, Y., Cao, X., Zhang, E., 2002. Orogenic Belt of the Western Qinling. Xi’an Cartography Publishing House, Xi’an,...
  • P.A. Floyd et al.

    Tectonic environments of Devonian Gramscatho basin, south Cornwall: framework mode and geochemical evidence from turbiditic sandstones

    J. Geol. Soc.

    (1987)
  • P.A. Floyd et al.

    Geochemistry and provenance of Rhenohercynian synorogenic sandstones: implications for the tectonic environment discrimination

    Geol. Soc. Spec. Pub.

    (1991)
  • Cited by (0)

    View full text