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