Magmatic Processes and Conditions of Ashikule Volcanic Cluster, NW Tibetan Plateau

Abstract

Although magmatism at mid-ocean ridges and subduction zones are reasonably well understood, our understanding of magmatic processes beneath thick continental plateaus is lacking. On the northwestern Tibetan Plateau, the Ashishan and Wulukeshan volcanoes of the Ashikule Volcanic Cluster represent the youngest post-collisional volcanism. Despite being the most recently active volcanism in China, minimal research has been conducted on their magmatic processes because of remoteness and harsh field conditions. In this work, new whole-rock analyses (major, trace, and isotopic), mineral compositions (major, trace, and isotopic), and crystal size distributions are measured to reveal magmatic processes beneath northwest Tibetan Plateau. Magmatic equilibrium temperature, pressure, oxygen fugacity, and crustal thickness are estimated from mineral chemistry. Single-grain plagioclase Sr isotopes are powerful and sensitive for identifying open-system magmatic processes, as initially shown in Speetjens, 2018. Here I present more single-grain high-precision 87Sr/86Sr analysis of additional plagioclase grains, but also pyroxene grains of Ashikule volcanic rocks, to evaluate open-system magmatic processes at a mineral grain scale. This data shows that 36% of plagioclase crystals were derived from country rocks, as these xenocrystic plagioclase grains have significantly different 87Sr/86Sr ratios from the host rocks. In contrast, all pyroxene grains have Sr isotopic compositions similar to the host rocks, suggesting that these pyroxenes are phenocrysts crystallized from the host magma. Here I report new zircon 238U-206Pb ages and trace element compositions from Ashishan. U-Pb ages from Ashishan indicate multiple age populations with young phenocrystic (0.23±0.33 Ma and 0.45±0.46 Ma) and older (4.09±0.55 to 1,731.9±23.7 Ma) xenocrystic zircons. The older zircons likely were entrained in the magma during ascension through assimilation of crustal material. In addition to xenocrystic plagioclase and zircon detected by Sr and U-Pb radiogenic isotopes, xenocrystic garnet and sillimanite are also observed under microscope. Furthermore, curved crystal size distributions also support magma assimilation processes. Based on trace element chemistry of phenocrystic zircons, the Ashishan lavas were sourced from highly reducing magmas with oxygen fugacity relative to the fayalite-magnetite-quartz buffer (FMQ) ranging from delta FMQ of -1.3 to -2.1, which is more reducing than subduction zone magmas. Mineral crystallization temperatures for Ashishan lavas are 1071-1110 C for pyroxene, and 828-840 C for phenocrystic zircons, and 768-774 C for xenocrystic zircons.