Origin Of The Late Early Cretaceous Granodiorite And Associated Dioritic Dikes In The Hongqilafu Pluton,Northwestern Tibetan Plateau: A Case For Crust-mantle Interaction

We present a detailed study of geochronology,mineral chemistries,bulk-rock major and trace element abundances,and Sr-Nd-Hf isotope compositions of the granodiorite and associated dioritic dikes in the Hongqilafu pluton at the northwestern margin of the Tibetan Plateau.The granodiorite and dioritic dikes yielded zircon U-Pb ages of ~104 Ma and ~100 Ma,respectively.The dioritic dikes comprise varying lithologies of gabbroic diorite,diorite porphyry and granodiorite porphyry,exhibiting a compositional spectrum from intermediate to felsic rocks.Their mineral compositions display disequilibrium features such as large major element compositional variations of plagioclase,clinopyroxene and amphibole crystals.These dioritic dikes are enriched in incompatible elements(Ba,Rb,Th,U,K)and Sr-Nd-Hf isotopes(87Sr/86Sri: 0.7066 to 0.7071,?Nd(t):-5.3 to-7.4,?Hf(t):-3.6 to-6.2).We suggest that the dioritic dikes were most likely derived from partial melting of mantle wedge metasomatized by the subducted/subducting seafloor with a sediment component,followed by AFC processes with fractional crystallization of clinopyroxene,amphibole and plagioclase and assimilation of lower continental crust.The mantle-wedge derived magma parental to the dioritic dikes underplated and induced the lower continental crust to melt,forming the felsic crustal magma parental to the granodiorite with mantle melt signatures and having more enriched isotope compositions(87Sr/86Sri: 0.7087 to 0.7125,?Nd(t):-9.5 to-11.6,?Hf(t):-10.3 to-14.1)than those of the dioritic dikes.The Hongqilafu pluton is thus the product of mantle-crust interaction at an active continental margin subduction setting over the period of several million years.This understanding further indicates that the closure timing of the Shyok back-arc basin and the collision between the Kohistan-Ladakh Arc and the Karakoram Terrane may have taken place later than ~100 Ma.