Elsevier

Lithos

Volumes 290–291, October 2017, Pages 1-17
Lithos

Origin of the mafic microgranular enclaves (MMEs) and their host granitoids from the Tagong pluton in Songpan–Ganze terrane: An igneous response to the closure of the Paleo-Tethys ocean

https://doi.org/10.1016/j.lithos.2017.07.019Get rights and content

Highlights

  • The MMEs were mainly formed by melting of metasomatized lithospheric mantle wedge.

  • The host granitoids were derived from partial melting of lower crust in arc setting.

  • Our results indicate that magma mixing was linked to the break-off of Paleo-Tethys slab.

Abstract

The Songpan–Ganze terrane is mainly composed of a Triassic sedimentary sequence and late Triassic–Jurassic igneous rocks. A large number of plutons were emplaced as a result of tectono-magmatic activity related to the late stages of Paleo-Tethys ocean closure and ensuing collision. Granitoids and their hosted mafic enclaves can provide important constraints on the crust–mantle interaction and continental crustal growth. Mesozoic magmatism of Songpan–Ganze remains enigmatic with regard to their magma generation and geodynamic evolution. The Tagong pluton (209 Ma), in the eastern part of the Songpan–Ganze terrane, consists mainly of monzogranite and granodiorite with abundant coeval mafic microgranular enclaves (MMEs) (ca. 208–209 Ma). The pluton comprises I-type granitoid that possesses intermediate to acidic compositions (SiO2 = 61.6–65.8 wt.%), high potassium (K2O = 3.2–4.1 wt.%), and high Mg# (51–54). They are also characterized by arc-type enrichment of LREEs and LILEs, depletion of HFSEs (e.g. Nb, Ta, Ti) and moderate Eu depletions (Eu/Eu* = 0.46–0.63). Their evolved zircon Hf and whole-rock Nd isotopic compositions indicate that their precursor magmas were likely generated by melting of old lower continental crust. Comparatively, the MMEs have lower SiO2 (53.4–58.2 wt.%), higher Mg# (54–67) and show covariation of major and trace elements, coupled with field and petrographic observations, such as the disequilibrium textures of plagioclase and amphibole, indicating that the MMEs and host granitoids were originated from different magma sources but underwent mafic–felsic magma mixing process. Geochemical and isotopic data further suggest that the precursor magma of the MMEs was formed in the continental arc setting, mainly derived from an ancient metasomatized lithospheric mantle wedge.

The Triassic granitoids from the Songpan–Ganze terrane show remarkable temporal–spatial-petrogenetic affinities to the counterparts of subduction zones in the Yidun and Kunlun arc terranes, plausibly support a double-sided subduction of the Paleo-Tethys ocean. The mixing mechanism for the formation of the Tagong pluton was likely associated with the break-off of a subducted slab of the Paleo-Tethys ocean, which triggered subsequent upwelling of hot asthenosphere beneath accreted arc fragments and induced lithospheric mantle-derived magmas suffice to underplate and mix with the lower crust-derived felsic magma. Collectively, the late Triassic igneous rocks record significant crustal growth and continental development as response to the final demise of the Paleo-Tethys ocean (ca. 210 Ma), and marks the last episode of orogenic magmatism in the Songpan–Ganze terrane after which the region entered into post-orogenic phase of evolution.

Introduction

The Songpan–Ganze terrane (SGT), was formed principally during the closure of the Paleo-Tethys ocean and subsequent amalgamation among the North China Block (NCB) and Kunlun arc terrane in the north, the South China Block (SCB) in the east, and the Qiangtang Block and Yidun arc terrane in the south (Fig. 1a; Roger et al., 2010). The characteristic Triassic flysch sediments in the Songpan–Ganze with a vast exposure of > 220,000 km2 and thickness ranging from 5 to 15 km, were mainly derived from the elevated orogeny and adjacent continents (Huang and Chen, 1987, Nie et al., 1994, Weislogel et al., 2010). Different tectonic settings have been proposed for their deposition, including: (1) a rift basin (Chang, 2000, Chung and Jahn, 1995, Song et al., 2004); (2) a back-arc or fore-arc basin (Ding et al., 2013, Pullen et al., 2008, Yuan et al., 2010); and (3) a remnant ocean basin (Zhou and Graham, 1996).

Large volume of late Triassic–early Jurassic (~ 230–190 Ma) granitoids intruded the early–middle Triassic strata in the Songpan–Ganze terrane. Because the origin of these granitoids was closely associated with the final closure of Paleo-Tethys ocean and the formation of the related Triassic orogen, these rocks have been used to constrain the tectonic setting of the region (de Sigoyer et al., 2014, Wang et al., 2011, Xiao et al., 2007, Yuan et al., 2010, Zhang et al., 2006, Zhang et al., 2007, Zhang et al., 2014). However, due to the paucity of coeval mafic magmas, controversy exists on the petrogenesis and geodynamic setting of these Triassic magmatic activities. Abundant mafic microgranular enclaves (MMEs) occur in these granitic intrusions, but little attention was paid previously.

In order to better constrain the geochemical processes during the Paleo-Tethys ocean closure, we conducted a combined study of petrology, geochemistry, zircon U–Pb–Hf and whole-rock Sr–Nd isotopic analysis on the MMEs and their host granitoids of the Tagong pluton in the east Songpan–Ganze terrane. Our results shed light on the complicated petrogenesis of these rocks and related tectonic evolution.

Section snippets

Geological background

The Songpan–Ganze terrane (SGT) is a triangular tectonic junction, bounded by the Anyimaqen–Kunlun–Muztagh suture (AKMs) in the north, the Jinsha–Ganze–Litang suture (JSs-GLs) in the southwest, and the NE-trending Longmenshan Thrust Belt in the east (Fig. 1a; Bruguier et al., 1997, Xu et al., 1992, Yin and Harrison, 2000). The Anyimaqen–Kunlun–Muztagh suture separates the SGT from the Kunlun terrane to the north, the Jinsha–Ganze–Litang suture (JSs-GLs) separates the SGT from the

Zircon U–Pb and trace element analysis

Zircon crystals were separated using conventional density and magnetic techniques and hand-picked under a binocular microscope. They were mounted in epoxy resin, polished and vacuum-coated with a 50 nm layer of carbon, and then examined in reflected and transmitted lights, and cathodoluminescence (CL) images at the University of Hong Kong in order to select coherent domains within grains for analysis. Both in-situ U–Pb dating and trace element analysis of zircon were performed using an Agilent

Monzogranite XITG-01

Zircons from monzogranite sample XITG-01 are commonly transparent, euhedral, ranging in size from 90 to 180 μm. A few subhedral–subrounded grains occur, possibly with inherited or xenocrystic origins. Most zircon grains exhibit strong oscillatory zoning with variable Th (35–261 ppm) and U (99–1214 ppm) contents and Th/U ratios (0.22–0.64), indicating an igneous origin. Twenty analytical spots give concordant 206Pb/238U ages between 204.4 ± 2.85 Ma and 213.3 ± 2.63 Ma and yield a weighted mean of 208.9 ± 

The age of the Tagong pluton

Euhedral zircons from samples collected from different parts of the Tagong pluton, including the host rock and MMEs, all give concordant ages ranging from 203 to 233 Ma. This indicates that the host granites and the MMEs are coeval, and a weighted mean age of 209 ± 1 Ma (N = 77, MSWD = 0.43) records the emplacement age of the Tagong pluton. The inherited or xenocrystic zircon cores yield an older weighted mean age of 226 ± 2 Ma (N = 16, MSWD = 0.7), which is consistent with that obtained by Xiao et al. (2007)

Conclusions

  • (1)

    The Tagong I-type potassium-rich granitoids and MMEs give zircon 206Pb/238U ages of 208.4 ± 3 Ma and 208.9 ± 3 Ma, respectively. They represent coeval late Triassic mafic-felsic magmatic activities and consequent magma mixing in the eastern Songpan–Ganze terrane.

  • (2)

    The precursor magma of the MMEs was mainly originated from a lithospheric mantle wedge modified by subduction process. The host granitoids were derived from partial melting of amphibolitic lower crust.

  • (3)

    Our data suggest that the generation of

Acknowledgements

This work was supported by the “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences (XDB03010600) and the National Basic Research Program of China (973 Program, No. 2014CB440801). The work is a contribution to IGCP-592 by the Joint Laboratory of Chemical Geodynamics between HKU and CAS (Guangzhou Institute of Geochemistry) and National Key Technology R&D Program of China (No. 2016YFC0601005).

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