Origin of K-feldspar megacrysts of Quxu batholith in Gangdese belt, South Tibet: Implication for magma rejuvenation in a crystal mush reservoir

The origin of the K-feldspar megacrysts is still controversial and how magmas move in crystal-rich reservoir and the mechanism of the reactivation of crystal mushes after rheological locking are still elusive. K-feldspar megacryst in granitoids is a good candidate to constrain these issues. We carried out an integrated study of field geology, crystal size distribution (CSD), LA-ICP-MS zircon UPb geochronology, whole-rock geochemistry, and in-situ major and trace elements and Pb isotopic compositions of K-feldspar megacrysts from the monzogranites and mafic microgranular enclaves (MMEs) of the Quxu batholith in the Gangdese belt, southern Tibet. The Quxu monzogranites have a magma crystallization age of ∼50 Ma. Some elements (e.g., Al, Sr, Cr and Ni) of the Quxu monzogranite and MMEs show scatter and no linear variations with increasing SiO2, indicating that simple magma mixing between mafic and felsic endmembers is unconspicuous for the formation of the Quxu monzogranites. The K-feldspar megacrysts from the Quxu monzogranites and MMEs can be divided into three types: normal zoning, reverse zoning and oscillatory zoning or no zoning. In-situ major and trace element of the K-feldspar megacrysts, combined with their variation in-situ Pb isotopic composition (206Pb/204Pb = 18.41–18.80, 207Pb/204Pb = 15.42–15.75 and 208Pb/204Pb = 38.29–39.06), indicate that the Quxu monzogranites were formed by the assembly of multiple heterogeneous magma pulses. The CSD results suggest that K-feldspar megacrysts have a longer residence time of 0.14–0.33 k.yr to 4.1–52 k.yr. We suggest that K-feldspar megacrysts could grow prolongedly via crystals transfer or magma injection of different batches in melt-present environments, and magma mush in cold-stored could be rejuvenated by the repeatedly recharging of hot magma pulses.