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

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.