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

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.