Diverse magma sources for the Himalayan leucogranites: Evidence from B-Sr-Nd isotopes
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).
References (77)
- et al.
Evidence for Early (>44 Ma) Himalayan crustal thickening, Tethyan Himalaya, southeastern Tibet
Earth Planet. Sci. Lett.
(2008) - et al.
TEMORA 1: a new zircon standard for Phanerozoic U-Pb geochronology
Chem. Geol.
(2003) - et al.
Secular boron isotope variations in the continental crust: an ion microprobe study [J]
Earth Planet. Sci. Lett.
(1992) - et al.
Early Eocene (c. 50 ma) collision of the India and Asian continents: constrains from the north Himalayan metamorphic rocks, southeastern Tibet
Earth Planet. Sci. Lett.
(2016) - et al.
Beryllium and boron in subduction zone minerals: an ion microprobe study
Geochim. Cosmochim. Acta
(1993) - et al.
Fluxed melting of metapelite and the formation of Miocene high-CaO two-mica granites in the Malashan gneiss dome, southern Tibet
Geochim. Cosmochim. Acta
(2014) - et al.
Oligocene crustal anatexis in the Tethyan Himalaya, southern Tibet
Lithos
(2016) - et al.
Crustal architecture of the Himalayan metamorphic front in eastern Nepal
Gondwana Res.
(2006) - et al.
Geochemical constraints on the bimodal origin of High Himalayan leucogranites
Lithos
(1995) - et al.
The Himalaya leucogranites: constrains on the nature of their crustal source region and geodynamic setting
Gondwana Res.
(2012)
In situ LA-MC-ICP-MS boron isotope and zircon U-Pb age determinations of Paleoproterozoic borate deposits in Liaoning Province, northeastern China
Ore Geol. Rev.
Nd isotopic data reveal the material and tectonic nature of the main central thrust zone in Nepal Himalaya
Tectonophysics
Ab initio prediction of equilibrium boron isotope fractionation between minerals and aqueous fluids at high P and T
Geochim. Cosmochim. Acta
Kinematic evolution of the Ama Drime detachment: insights into orogen-parallel extension and exhumation of the Ama Drime Massif, Tibet-Nepal
J. Struct. Geol.
Crustal generation of the Himalayan leucogranites
Tectonophysics
Evolution of North Himalayan gneiss domes: structural and metamorphic studies in Mabja Dome, southern Tibet
J. Struct. Geol.
The South Tibet detachment shear zone in the Dinggye area: time constraints on extrusion models of the Himalayas
Earth Planet. Sci. Lett.
Petrogenesis of the Ramba leucogranite in the Tethyan Himalaya and constraints on the channel flow model
Lithos
Two contrasting eclogite types in the Himalayas: implications for the Himalayan orogeny
J. Geodyn.
Boron isotope systematics of hydrothermal fluids and tourmalines: a synthesis
Chem. Geol.
The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes
Earth Planet. Sci. Lett.
Age of initiation of collision between India and Asia: a review of stratigraphic data
Earth Planet. Sci. Lett.
The overview of the stratigraphy and tectonics of the Nepal Himalaya
J. Asian Earth Sci.
Eocene north-south trending dikes in Central Tibet: new constrains on the timing of east-west extension with implications for early uplift?
Earth Planet. Sci. Lett.
The geochemical cycle of boron: constraints from boron isotope partitioning experiments between mica and fluid
Lithos
Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation
Earth Sci. Rev.
Mid-Eocene high Sr/Y granites in the Northern Himalayan Gneiss Domes: melting thickened lower continental crust
Earth Planet. Sci. Lett.
High-pressure melting of metapelite and the formation of Ca-rich granitic melts in the Namche Barwa Massif, Southern Tibet
Gondwana Res.
Structure and geochronology of the southern Xainza–Dinggye rift and its relationship to the south Tibetan detachment system
J. Asian Earth Sci.
Causes and consequences of protracted melting of the mid-crust exposed in the North Himalayan antiform
Earth Planet. Sci. Lett.
Tectonics of the northern Himalaya since the India-Asia collision
Gondwana Res.
Structure and evolution of the Himalaya–Tibet orogenic belt
Nature
Late Cenozoic right-lateral strike-slip faulting across southern Tibet
J. Geophys. Res.
Normal faulting in central Tibet since at least 13.5 Myr ago
Nature
The south Tibetan detachment system, Himalayan orogen: extension contemporaneous with and parallel to shortening in a collisional mountain belt
Spl. Paper Geol. Soc. Am.
Evidence for Tibetan plateau uplift before 14 Myr ago from a new minimum age for east-west extension
Nature
Tectonic implications of U-Pb zircon ages of the Himalayan Orogenic belt in Nepal
Science
Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences of the age and source of Himalayan leucogranites
Contrib. Mineral. Petrol.
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2023, GeochemistryCitation Excerpt :These leucogranites have significant variations of mineral assemblages and geochemical compositions (Guo and Wilson, 2012; Visona and Lombardo, 2002; Wang et al., 2017; Wu et al., 2020; Wu et al., 2015). More and more studies attributed this variation to magmatic differentiation (Chakraborty and Upadhyay, 2020; Gao et al., 2021; Guo and Wilson, 2012; Hu et al., 2018; Patiño Douce and Harris, 1998; Scaillet et al., 1995; Thomas et al., 2005; Veksler, 2004; Wang et al., 2019; Webster, 2004; Wu et al., 2020; Wu et al., 2015; Xie et al., 2020). However, the melt evolution and element migration processes in Himalayan leucogranite belt during magmatic differentiation are still ambiguous (Wang et al., 2017; Wu et al., 2020; Wu et al., 2015).
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2022, Journal of Asian Earth SciencesCitation Excerpt :In general, inherited zircon is a key indicator for the identification of source rocks (Gao et al., 2021c). All inherited zircons from the monzonites are characterized by subhedral to euhedral prismatic form and clear oscillatory zoning and have crystallization ages of ∼ 1800 Ma, which is consistent with the morphological features and ages of the zircons from the ADM orthogneisses (Fig. 12b–12c; Cottle et al., 2009; Hu et al., 2018). Considering that the monzonites are directly emplaced in the orthogneisses, their inherited zircons and monazites might result from the contamination of country rocks.