Petrogenesis of Himalayan Leucogranites: Perspective From a Combined Elemental and Fe‐Sr‐Nd Isotope Study

Abstract

The petrogenesis of Himalayan leucogranites remains crucial for understanding the thermal and tectonic evolution of the Himalayan orogen. To understand whether they are largely pristine melts of crustal anatexis or have experienced a high degree of fractional crystallization (FC), we present Fe isotopic data of 30 representative Himalayan leucogranites and 9 local metasedimentary rocks. Excepting three garnet leucogranites with low δ 56 Fe (−0.04‰-0.06‰) that are likely affected by garnet accumulation, tourmaline, and two-mica leucogranites have largely homogeneous δ 56 Fe from 0.13‰ to 0.24‰ irrespective of their highly variable SiO 2 , MgO, and FeOt contents. Combined with observed mineral assemblages and available fractionation factors, this does not support a high degree of FC (with or without assimilation) in their petrogenesis. The elevated δ 56 Fe relative to the supposed source rocks, represented by metasedimentary rocks and/or metabasite with a δ 56 Fe value of 0.10‰, by ∼0.07‰, may reflect Fe isotope fractionation during crustal anatexis. This study indicates most leucogranites can provide robust constraints on the conditions of crustal anatexis and thus the thermal and tectonic evolution of the Himalayan orogen.Plain Language Summary The Himalayas have been formed from the collision between two tectonic plates. During their formation, the rocks of the continental crust have melted to form leucogranites which potentially provide important information on how the collision process evolves. Several recent studies of rare-element mineralization associated with these granites have argued that the magmas result from extensive removal of early formed minerals during the cooling of the magma (fractional crystallization [FC]) which, if true, would undermine their usefulness as monitors of the collisional process. In this study, we address this issue through a geochemical approach that combines isotopic data from iron, strontium, and neodymium. Whereas Sr and Nd give information on the source of the magmas, the isotopes of Fe will remain largely unfractionated if the granites result simply from melting the crust but fractionate significantly during FC. Our results reveal very limited fractionation of Fe isotopic compositions for two types of leucogranites, which is inconsistent with the model requiring a high degree of FC but supports the interpretation that they represent largely unfractionated crustal melts. Our study therefore confirms that Himalayan leucogranites can provide reliable probes for the thermal and tectonic evolution of the Himalayan crust. SHI ET AL.

In-situ boron isotope and chemical composition of tourmaline in the Gyirong pegmatite, southern Tibet: Implications for petrogenesis and magma source

“…The Gyirong leucogranite shows high ( 87 Sr/ 86 Sr) i ratios (0.7548-7586), high Rb contents, and low Sr and Ba contents (Gao et al, 2017;Wang et al, 2017), which can be ascribed to fluid-absent melting of muscovite in metasedimentary sources. This interpretation is also supported by recent Mg-Fe-Sr-Nd isotope studies (e.g., Tian et al, 2020;Shi et al, 2021). Note that some authors found that the Himalayan leucogranites do not record any mantle material contribution to the sources (Inger and Harris, 1993;opkinson et al, 2017).…”

Magnesium Isotopes Archive the Initial Carbonate Abundances of Metasedimentary Rocks Prior to Thermal Decarbonation

“…Here we report elemental and Mg isotopic data of 9 Himalayan carbonate‐free metasedimentary rocks and 23 Himalayan carbonate‐bearing metasedimentary rocks which were collected from Bhutan and Nepal Himalaya (Groppo et al., 2021; Rolfo et al., 2017; Shi et al., 2021; Table S1 in Supporting Information ), to establish the correlation between bulk Mg isotopic compositions and ICAs. In order to reveal the elemental and isotopic characteristics of the present carbonates and silicate components in the carbonate‐bearing rocks, five samples were selected to conduct leaching experiments.…”

Boron and Molybdenum Isotope Evidence for Source‐Controlled Compositional Diversity of Cenozoic Granites in the Eastern Tethyan Himalaya

“…Similarly, the geochemical compositions of anatectic melts cannot unequivocally discriminate dehydration versus water‐fluxed melting and low‐degree partial melting versus fractional crystallization (Bartoli, 2021; Guo & Wilson, 2012). Recent studies have suggested that a number of stable isotope systems are useful in tracing the sources and petrogenetic processes of anatectic granites, which complement traditional geochemical approaches (Fan et al., 2021; Shi et al., 2021; Wang, Zhang, et al., 2020; Wang, Hao, et al., 2020). The stable B and Mo isotope systems have been used to study mantle heterogeneity and fluid or melt recycling during subduction‐related processes (Chen et al., 2022; Freymuth et al., 2015; König et al., 2016; Li et al., 2021).…”