Is Himalayan leucogranite a product by in situ partial melting of the Greater Himalayan Crystalline? A comparative study of leucosome and leucogranite from Nyalam, southern Tibet

Abstract

Widespread leucogranites in the Himalayan orogenic belt are thought to have originated by in situ partial melting of the Greater Himalayan Crystalline (GHC) when it underwent high-grade metamorphism during Cenozoic orogenesis. Therefore, the leucogranites and associated migmatites can be used to constrain the exhumation history of the GHC. However, the petrogenetic relationship between the GHC, leucogranites, and migmatites is not well-constrained. As such, we carried out a detailed petrographic, mineralogical, geochronological, and geochemical study of leucosomes and leucogranites from the Nyalam region in southern Tibet. Monazite U–(Th)–Pb dating indicates that anatexis of the GHC occurred during the late Eocene and Miocene (40–14 Ma), whereas leucogranite emplacement occurred from 27 to 14 Ma. There are marked differences between the leucosomes in migmatites and leucogranites in terms of field geology, mineralogy, and geochemistry, suggesting different origins. The leucosomes occur mainly as pockets or are interlayered with melanosomes in stromatic metatexites. The leucosomes contain oligoclase and Fe-rich biotite, and have whole-rock compositions with high K2O contents (4.8–7.4 wt%) and K2O/Na2O ratios (1.35–2.97), positive Eu anomalies (Eu/Eu* = 0.93–2.61), and low rare-metal (Li, Be, Cs, Sn, and Ta) contents. These features are consistent with an origin by muscovite dehydration melting of the GHC. However, the leucogranites intrude the GHC and occur as small plutons along the South Tibetan Detachment System. In contrast to the leucosomes, the plagioclase and biotite in the leucogranites are albite and siderophyllite, respectively. The leucogranites have relatively low K2O (4.3–4.7 wt%) contents and K2O/Na2O ratios (1.04–1.24), high rare-metal contents, and marked negative Eu anomalies (Eu/Eu* = 0.47–0.70), indicating an origin by extensive fractional crystallization. We propose that the leucogranites were magmas produced in the deeper GHC during peak metamorphism, and subsequent extensive fractional crystallization occurred during long-distance, upward migration of magma along the South Tibetan Detachment System (STDS) during exhumation of the GHC. During these processes, the high-grade metamorphic rocks of the GHC were partially melted, resulting in the formation of leucosomes within migmatites. Therefore, the leucosomes and leucogranites have different origins and should be considered separately in studies of Himalayan orogenesis.