In situ boron isotopic analyses of tourmalines from Neogene magmatic rocks in the northern and southern margins of Tibet: Evidence for melting of continental crust and sediment recycling

Abstract

Sediment recycling has important impacts on the differentiation of the continental crust, and is considered to mainly occur in arc settings. In collisional orogenic belts, however, sediment recycling processes are comparatively less well understood. In general, sedimentary rocks or tourmaline with continental affinities have boron (B) isotope compositions that differ from marine sedimentary rocks, magmatic rocks undergoing seawater alteration or tourmaline originating from these rocks or fluids. This study uses laser-ablation multi-collector, inductively coupled plasma-mass spectrometry (LA-MC-ICPMS) for in-situ B isotope analyses of tourmaline from the northern and southern margins of Tibet. Tourmalines from the Hudongliang Pliocene two mica rhyolites in the Songpan-Gangzi block and the Cuonadong Miocene two mica granites in the Himalayan Block have δ11B values of −10.47 ± 0.54‰ and −12.48 ± 1.04‰, respectively. Combined with petrological and Nd–Sr isotope data, we suggest that these strongly peraluminous magmatic rocks from the northern and southern margins of Tibet were mainly generated by partial melting of subducted or overridden metasedimentary rocks with continental affinities due to the contraction caused by the collision between the Indian and Asian plates. Given the widespread occurrences of strongly peraluminous Neogene magmatic rocks in the southern and northern margin of Tibet, we suggest that the recycling of sediments with continental affinity played an important role in the evolution of continental crust during the collisional orogenic process.

Keywords

• Tibet
• Peraluminous magmatic rocks
• Sediment recycling
• In-situ B isotope
• Tourmalines