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Two Types of mafic rocks in southern Tibet: A mark of tectonic setting change from Neo-Tethyan oceanic crust subduction to Indian continental crust subduction

https://doi.org/10.1016/j.jseaes.2019.103883Get rights and content

Highlights

  • The mantle source for Eocene mafic rocks was the asthenospheric mantle with the recycled oceanic crust.

  • The mantle source for Miocene mafic rocks was the (asthenospheric or lithospheric) mantle with continental crust.

  • Evolution from oceanic to continental crust subduction in southern Tibet was recorded in the Cenozoic mafic rocks.

Abstract

We collated existing data for the Eocene Langshan mafic rocks (Eocene mafic rocks) and the Miocene potassic–ultrapotassic mafic rocks (Miocene mafic rocks) in southern Tibet to investigate the tectonic transition from Neo-Tethyan oceanic crust subduction to Indian continental crust subduction. The Eocene mafic rocks have high Na2O contents (K2O/Na2O = 0.03–0.2) and show OIB-like trace element patterns (e.g., positive Nb and Ta anomalies) and depleted radiogenic Sr–Nd isotope compositions (87Sr/86Sr of apatite = 0.7031, εNd(t) = +5.1 to +6.1). In contrast, the Miocene mafic rocks have high K2O contents (K2O/Na2O = 1.9–8.5) and exhibit arc-like trace element patterns (enrichment in LILEs and depletion in HFSEs) and enriched radiogenic Sr–Nd isotope compositions (87Sr/86Sr = 0.7115–0.7362, εNd(t) = −16 to −12.4). The mantle source for the Eocene mafic rocks was generated by reactions between asthenospheric mantle wedge and felsic melts from subducted Neo-Tethyan oceanic crust (outside the field of rutile stability). In contrast, the mantle source of the Miocene mafic rocks was generated by reactions between asthenospheric (or lithospheric) mantle wedge and felsic melts from subducted Indian continental crust. Taking into account the regional tectonic evolution, we propose that break-off of the Neo-Tethyan oceanic slab and roll-back and/or break-off of the Indian continental slab were the most likely geodynamic mechanisms that led to the production of the Eocene and Miocene mafic rocks, respectively. Therefore, the transition from the Eocene to Miocene mafic rocks in southern Tibet provides an opportunity to understand the tectonic transition from Neo-Tethyan oceanic to Indian continental crust subduction.

Introduction

The subduction of continental crust in continental collisional orogens follows the subduction of oceanic crust to mantle depths, closure of the oceanic basin, and eventual continent–continent collision (e.g., Ernst, 2005, Castro et al., 2013, Gerya, 2014, Dash et al., 2015). These sequential processes are significant not only for high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism of subducted crustal rocks (e.g., Chopin, 2003, Ernst and Liou, 2008, Zheng et al., 2012) but also for the recycling of crustal materials into the deep mantle (e.g., Zindler and Hart, 1986, Willbold and Stracke, 2010, Zheng, 2012, Zhao et al., 2013). In an orogen formed by continent–continent collision, the subduction of oceanic crust would have predated the subduction of continental crust, and because oceanic crust subjected to eclogite-facies metamorphism has a high density, it can pull the continental crust into the mantle where it experiences UHP metamorphism (Forsyth and Uyeda, 1975). Thus, in this scenario, both oceanic and continental crustal materials would be recycled into the mantle during continent–continent collision. It is assumed that oceanic crust undergoes metamorphic dehydration during subduction and that aqueous fluids derived from the subducted oceanic crust alter the peridotite of the overlying mantle wedge, thus generating the mantle source for oceanic arc basalts (e.g., Kelemen et al., 2003, Schmidt and Poli, 2003, Spandler and Pirard, 2013). In this process, the residual oceanic crust is subducted farther into the deep mantle to provide the mantle source for intraplate oceanic island basalts (OIBs), which are enriched in high-field-strength elements (e.g., Nb and Ta) and have depleted Sr–Nd isotopic values (Hofmann and White, 1982, Hofmann, 1997, Stracke et al., 2003, Chauvel et al., 2008). The subducted continental crust may become dehydrated and partially melted at depths of 80–130 km in a continental subduction zone (e.g., Zhang et al., 2008, Chen et al., 2013a, Chen et al., 2013b, Hermann et al., 2013). Thus, the fluids or melts derived from the subducted continental crust would also metasomatize the peridotite of the mantle wedge that overlies the continental slab (Zheng et al., 2012). An enriched mantle (ultramafic metasomatite), generated by reactions between mantle peridotite and the hydrous felsic melts derived from the partial melting of the deeply subducted continental crust, provides a likely source of the post-collisional potassic–ultrapotassic mafic rocks that are found in continent–continent collisional orogens, and these mafic rocks exhibit arc-like trace element patterns and enriched Sr–Nd isotope compositions (Prelević et al., 2008, Prelević et al., 2013, Guo et al., 2013a, Guo et al., 2013b, Guo et al., 2015, Zhao et al., 2013, Zhao et al., 2015). The existence of two types of crust–mantle interaction (i.e., oceanic crust–mantle and continental crust–mantle) during continent–continent collision means that the geochemical transition from OIB-like basalts to post-collisional potassic–ultrapotassic mafic rocks could provide a window into the processes involved during these two types of slab–mantle interaction.

The Tibetan Plateau is an ideal location to investigate processes associated with continent–continent collision (Allégre et al., 1984, Molnar et al., 1993). For example, southern Tibet records the subduction of Neo-Tethyan oceanic lithosphere, the subsequent collision of the Indian and Asian continents, and subduction of the India continental lithosphere (e.g., Chung et al., 2005). However, it is commonly difficult to identify the processes associated with the transition from the subduction of the Neo-Tethyan oceanic crust to the subduction of the Indian continent during the evolution of this orogen (Chung et al., 2005 and references therein). Recently, the Eocene (∼45 Ma) Langshan mafic rocks (gabbros) have been discovered in the Gyangze region of southern Tibet and have been shown to have clear OIB-like characteristics (Ji et al., 2016). Unlike these OIB-like magmatic rocks, which are of limited extent in southern Tibet, the Miocene (23–8 Ma) post-collisional potassic–ultrapotassic mafic rocks are widely distributed within the Lhasa Block of southern Tibet (Turner et al., 1996, Miller et al., 1999, Ding et al., 2003, Williams et al., 2004, Zhao et al., 2009).

In this study, we collated existing data on the Langshan mafic rocks (gabbros) (herein, Eocene mafic rocks) and the Miocene (23–8 Ma) Xuruco Lake–Dangre Yongcuo Lake (XDY) potassic–ultrapotassic mafic rocks (herein, Miocene mafic rocks) in southern Tibet to investigate (1) the mantle source of both group of rocks and (2) whether the transition from Eocene to Miocene mafic rocks records the tectonic evolution from subduction of Neo-Tethyan oceanic crust to subduction of Indian continental crust.

Section snippets

Geology of the study area

The Tibetan Plateau comprises the Songpan–Gangzi, Qiangtang, Lhasa, and Himalaya blocks from north to south. The Lhasa block is bounded by the Bangong–Nujiang suture zone (BNSZ) to the north and the Indus–Yarlung Zangbo suture zone (IYZSZ) to the south (Fig. 1a). The BNSZ formed from the Middle Jurassic to the Early Cretaceous (Yin and Harrison, 2000, Kapp et al., 2007, Zhu et al., 2011, Zhu et al., 2013, Zhu et al., 2016, Pan et al., 2012 Zhang et al., 2012, Fan et al., 2014) and the IYZSZ

Magma sources

The data used in this study for the Eocene mafic rocks were obtained from Ji et al. (2016), and the data for the Miocene mafic rocks were obtained from Liao et al., 2002, Ding et al., 2003, Ding et al., 2006, Gao et al. (2007), Zhao et al. (2009), Guo et al., 2013a, Guo et al., 2013b. To exclude crustal contamination and obvious fractional crystallization, we used only those samples with MgO > 6.5 wt%. Details regarding data selection are given in Supplementary text 1. The ages and geochemistry

Transition from sodic OIB-like magmas to potassic–ultrapotassic mafic rocks

Recent studies have ascribed the change from sodic OIB-like basalts (mafic dikes) to potassic mafic intrusive rocks in the Hong’an–Dabie orogens of east-central China to mantle sources with different types of recycled crustal material (oceanic crust versus continental crust) (Dai et al., 2012, Dai et al., 2015, Dai et al., 2017, Zhao et al., 2013, Zhao et al., 2015, Zheng et al., 2015, Zheng and Chen, 2016). These rocks are related to the closure of the Paleo-Tethys, which involved first the

Implications for the tectonic evolution from oceanic crust to continental crust subduction

It has previously been thought that the Late Triassic–Early Jurassic (210–174 Ma) magmatic belt in southern Tibet recorded the onset of the subduction of Neo-Tethyan oceanic crust (Zhang et al., 2007, Yang et al., 2008, Zhu et al., 2008, Zhu et al., 2011, Ji et al., 2009, Guo et al., 2013a, Kang et al., 2014 Song et al., 2014, Meng et al., 2015). However, the recent discovery of Middle–Late Triassic (237–212 Ma) volcanic rocks in the southern Lhasa subterrane has revealed that the northwards

Conclusions

The Eocene mafic rocks (gabbros) and Miocene Xuruco Lake–Dangre Yongcuo Lake potassic–ultrapotassic mafic rocks in southern Tibet exhibit significantly different geochemical features, which indicates that they originated from two types of mantle source. The Eocene mafic rocks exhibit OIB-like trace element patterns and depleted Sr–Nd radiogenic isotope compositions, consistent with a mantle source that was generated by the reaction of felsic melts derived from subducted Neo-Tethyan oceanic

Declaration of Competing Interest

The authors declared that they have no conflicts of interest to this work.

Acknowledgments

This research was supported by the National Key Research and Development Project of China (project 2016YFC0600305) and the Natural Science Foundation of China (41573024, 41602341, and 41873037). We thank the journal editor and two anonymous reviewers for their constructive and thoughtful comments.

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