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 epsilon(Nd)(t) values of -21.0 to -19.6 and -19.3 to -18.3 and Sr-87/Sr-86(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 (207)pb/(206)pb age of the zircon cores of the Ama Drime gneiss (T0446-2-3) is 1854 +/- 4 Ma. The tourmaline delta B-11 values of the Ama Drime gneiss (T0446-1-6) are -17.6 to -14.3 parts per thousand, similar to those of the Mabja leucogranite (T0436-4, -18.9 to -17.4 parts per thousand). In contrast, the tourmaline delta B-11 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 delta B-11 values of the Yadoi leucogranite (T0321-4), derived from partial melting of amphibolite, range from -8.4 to -5.4 parts per thousand. 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. (C) 2018 Elsevier B.V. All rights reserved.