Elsevier

Lithos

Volumes 314–315, August 2018, Pages 88-99
Lithos

Diverse magma sources for the Himalayan leucogranites: Evidence from B-Sr-Nd isotopes

https://doi.org/10.1016/j.lithos.2018.05.022Get rights and content

Highlights

  • Tourmaline B isotopes can be used to trace the magma sources of leucogranites.

  • A Sr-Nd-B isotopic system indicates an event anataxis of Paleoproterozoic rock units in central Higher Himalayan.

  • The anataxis might be related to the depression of the Xainza-Dinggye rift.

Abstract

The Himalayan orogen is featured by widespread S-type leucogranites (ca. 45 to 9 Ma) formed primarily from the partial melting of metapelites (800–480 Ma), and minor from the partial melting of amphibolite with subordinate metapelites. The Ama Drime gneiss and Mabja leucogranite pluton are both located at the footwall of the NS-trending Xainza–Dinggye normal fault in the central part of the Himalayan orogen, with published εNd(t) values of −21.0 to −19.6 and − 19.3 to −18.3 and 87Sr/86Sr(t) values of 0.90954–0.92574 and 0.84853–0.85474, respectively. In this study, we presented new SHRIMP zircon U-Pb ages and laser ablation-multi-collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) tourmaline boron isotope data on the metamorphic rocks and leucogranites of the Himalayan orogen. The weighted mean 207Pb/206Pb age of the zircon cores of the Ama Drime gneiss (T0446-2-3) is 1854 ± 4 Ma. The tourmaline δ11B values of the Ama Drime gneiss (T0446-1-6) are −17.6 to −14.3‰, similar to those of the Mabja leucogranite (T0436-4, −18.9 to −17.4‰). In contrast, the tourmaline δ11B values of the Quedang metapelite (T0389-18) and its partial melting product – Malashan leucogranite (T0659-12A-3) are substantially higher, i.e., −15.3 to −12.5‰ and − 16.2 to −8.0‰, respectively. The tourmaline δ11B values of the Yadoi leucogranite (T0321-4), derived from partial melting of amphibolite, range from −8.4 to −5.4‰. Therefore, the tourmaline B-isotopes and Sr-Nd-isotopes results are consistent, and may have reflected an E-W extension along the southern Tibetan rift system, indicating a late Miocene anatexis in the North Himalaya region. The melts derived from the partial melting of metapelites and mature crustal materials (e.g., Paleoproterozoic Ama Drime gneiss) ascended along the N-S trending Xainza-Dinggye normal fault, and were subsequently emplaced in the Tethyan Himalaya sequence.

Introduction

The Himalayan Orogen is one of the world's most famous collisional belts, and was formed by the India-Asia continent-continent collision at or before ~50 Ma (e.g., Ding et al., 2016; Rowley, 1996; Yin and Harrison, 2000). Since the India–Asia collision, the Himalayan Orogen has experienced episodic melting of crustal rocks, forming various types of granites and migmatites (Gao and Zeng, 2014; Harris and Massey, 1994; Le Fort et al., 1987; Zeng et al., 2011, Zeng et al., 2015). Previous studies of the Himalayan leucogranites demonstrated that these granites were largely younger than 27 Ma, and primarily formed via muscovite dehydration melting (Harris and Massey, 1994; King et al., 2011; Knesel and Davidson, 2002; Patino Douce and Harris, 1998; Yang and Jin, 2001; Zhang et al., 2004), or fluid-present melting of the HCS (High Himalayan Crystalline Sequence) metapelites during rapid exhumation (Gao et al., 2017; Prince et al., 2001). In addition, granitoids older than 35 Ma from the Yadoi gneiss dome and Ramba gneiss dome are characterized by peraluminous granites with relatively high Na/K and Sr/Y ratios (Zeng et al., 2009). These granites were likely derived from the partial melting of a source consisting mainly of amphibolite with subordinate pelitic gneiss under thickened crustal conditions (Aikman et al., 2008; Liu et al., 2014; Zeng et al., 2009, Zeng et al., 2011, Zeng et al., 2015). In the central Himalayan Orogen, leucogranites from the Mabja gneiss dome are characterized by extremely radiogenic Sr (87Sr/86Sr(t) =0.84853–0.85474) and non-radiogenic Nd (εNd(t) = −19.3 to −18.3) isotopes (Hu et al., 2016; Zhang et al., 2004), which are substantially different from other Himalayan leucogranites (Sr(87Sr/86Sr(t) = 0.72401–0.79703) and non-radiogenic Nd (εNd(t) = −16.8 to −12.1)) that derived from partial melting of metapelites (Guo and Wilson, 2012). This suggests that these leucogranites may have formed from the anatexis of mature and ancient crustal materials, such as the Lesser Himalayan Paleoproterozoic rock units (Murphy, 2007; Guo and Wilson, 2012).

Typical Himalayan leucogranites comprise mainly quartz, plagioclase, K-feldspar and muscovite, and there are also some plutons of two-mica, tourmaline-bearing, and/or garnet-bearing leucogranites (Gao et al., 2016, Gao et al., 2017; Gao and Zeng, 2014; Guo and Wilson, 2012; Zhang et al., 2004). Tourmaline and muscovite have high boron contents (Domanik et al., 1993), thus a number of Himalayan leucogranites are B-rich and offer an excellent opportunity to investigate their B-isotope compositions. Tourmaline is a common borosilicate mineral in many granitoids, especially the highly fractionated ones. Tourmaline is derived primarily from the partial melting of B-rich clay minerals and crystallized during the late granitic fractionation stages (e.g., Jiang et al., 2003; Yang et al., 2015), or it may be captured from the wall rocks (e.g., Hu et al., 2015; Marschall et al., 2006). Tourmaline is a unique recorder of its geological evolution, and displays remarkable stability irrespective of large variations in host-rock compositions, pressure, temperature and fluid compositions during metamorphism (e.g., Marshall and Jiang, 2011; Van Hinsberg et al., 2011). Boron isotope ratios in tourmaline have been identified as sensitive indicators of subduction-slab components in arc-related and granitic magmas (Jone et al., 2014; Palmer, 1991). The B-isotope compositions of an S-type granitic pluton were determined by its source regions (Jiang and Palmer, 1998; Yang et al., 2015). Therefore, variations in tourmaline B-isotope compositions in leucosomes and leucogranite formed in similar P-T conditions may indicate diverse melting sources for the magma.

In this study, we conducted LA–MC–ICP–MS (laser ablation-multi-collector- inductively coupled plasma-mass spectrometry) analysis to determine the B-isotope compositions of tourmalines from the leucogranites and metamorphic rocks (e.g., Mabja leucogranite) of the Himalayan Orogen. In addition, we integrated SHRIMP zircon U-Pb and tourmaline EPMA (electron probe microanalysis) data to evaluate the possibility of partial melting of the Paleoproterozoic orthogneiss to form the late Miocene leucogranites.

Section snippets

Structural geology of the Himalayan orogen

The 2500-km-long Himalaya Orogen is bounded by the Main Front Thrust to the south and the Yalung-Tsangpo Suture to the north. Geographically, this orogen can be subdivided from north to south into four tectonic units: the Tethyan Himalayan Sequence (THS), the High Himalayan Crystalline Sequence (HCS), the Lesser Himalayan Sequence (LHS) and the sub-Himalaya (SH) (Fig. 1). These units are separated from north to south by the South Tibetan Detachment System (STDS), Main Central Thrust (MCT) and

SHRIMP zircon U-Pb dating

Zircons from the Ama Drime gneiss (T0446-2-3) were separated using standard heavy-liquid and magnetic techniques, and later handpicked under a binocular microscope. Selected zircon grains together with the zircon standard 91,500 were embedded in 25-mm epoxy discs and ground to approximately half of their thickness. The internal zoning of the zircons was revealed by CL (cathodoluminescence) imaging, using an FEI PHILIPS XL30 SFEG SEM (scanning electron microscope) set at 15 kV and 120 μA, with a

Age of the Ama Drime gneiss

Zircon grains from the Ama Drime gneiss (T0446-2-3) are euhedral to subhedral, long or stubby prismatic, and 200–250 μm long and 80–100 μm wide. A large number of zircon grains contain inherited core and oscillatory zoning (Fig. 6a). Sixteen analyses were performed on the rim and three on core. The discordant values of inherited core (T0446-2-3-3.1, T0446-2-3-4.1 and T0446-2-3-5.1) range from 87–96%, and thus do not yielded a meaningful result. 13 analyses on the zircon rims yielded U and Th

Tectonic slice of Paleoproterozoic strata and its anatexis

The magmatic oscillatory zircon rims of the Ama Drime gneiss (T0446-2-3) show a weighted mean 207Pb/206Pb age of 1854 ± 4 Ma (Fig. 6), implying that these orthogneisses have the geochemical nature of LHS (2.6–0.8 Ga) (Goscombe et al., 2006; Hu et al., 2016; Liao et al., 2008) and the tectonic nature of HCS (amphibolite-facies metamorphism) (Searle et al., 2008). The Ulleri Formation is a semi-continuous unit that occurs throughout Nepal, and is interpreted to have formed at ca. 1850 Ma (

Conclusions

The late Miocene Mabja leucogranitic pluton exhibits similar B-Sr-Nd isotope characteristics to those of the Lesser Himalayan Sequence, which indicates that the former was derived from a source dominated by Paleoproterozoic orthogneiss with minor meta-pelite. The melt may have ascended along a NS-trending normal fault and emplaced in the Tethyan Himalaya sequence. Tourmaline B-isotope characteristics of the Himalayan orogen are similar to the whole-rock Sr-Nd isotopic system. The Himalayan

Acknowledgements

This work was supported by the National Natural Science Foundation of China (41425010 and 41503006).

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