Genesis and multi-stage evolution of crustal magma: A case study of the Late Jurassic Xurucuo plutonic complex in the South Lhasa terrane, Tibet

Abstract

Genesis and evolution mechanisms of granites provide clues regarding the development of architecture and composition of continental crust. Lithologically zoned granitoid complexes and mafic microgranular enclaves (MMEs) are common and enlightening in understanding of genesis and, particularly the dynamic evolution of granitoid magmas, as well as mantle–crust interaction. The Late Jurassic Xurucuo plutonic complex (XPC) in the South Lhasa is diverse in lithology, being composed of zoned monzogranite, granodiorite, quartz diorite, diorite, and MME. Here, zircon UPb ages, oxygen and hafnium isotopes, and whole-rock and mineral geochemistries of the XPC were determined in order to reveal the genesis of the zoned pluton, and further, the dynamic processes of the crustal magmas. The XPC granitoids are high-K calc-alkaline and metaluminous to moderately peraluminous, I-type and arc-related magmatic rocks. These rocks were dated to be 159–154 Ma, with zircon δ18O values of 6.87–8.83‰ and negative εHf(t) values of −18.4 to −9.7. The concomitance of different lithologies and consistent O − Hf isotopic compositions suggest a prolonged cognate evolution of the magmas. The lithological and geochemical features of these granitoids indicate that they were generated from the reworking of crystal magma mush at shallow crustal levels aided by magma replenish, mixing and subsequent fractional crystallization. Combining our results with those of previous studies for the Late Jurassic magmatism in the Lhasa terrane, we propose an integrated model wherein mantle-derived magma intruded into crustal mush during the northward subduction of the Yarlung-Zangbo Neo-Tethyan oceanic lithosphere beneath the Lhasa terrane.