北拉萨地块东段早白垩世麦巴岩体成因及构造背景OA

Early Cretaceous magmatic rocks are widely exposed in the northern Lhasa block,but the deep geodynamic setting in which they formed is still debated.In this paper,we study the granodiorite and tonalite from the Maiba pluton in the Zuogong area,located in the eastern section of the northern Lhasa block.We focus on the petrography,geochronology,geochemical characteristics,and zircon Hf isotopes in the rocks.LA-ICP-MS dating of the granodiorite and tonalite yields crystallization ages of 130.2～127.0 Ma and 121.8 Ma,respectively,suggesting that the Maiba pluton formed during the early Cretaceous.The granodiorite has an SiO2 content of 63.09%～69.28%,an all-alkali content(Na2O+K2O)of 5.13%～7.79%,an Al2O3 content of 13.27%～15.96%,and an aluminum saturation index(A/CNK)of 0.84～1.04;the corresponding values for tonalite are 60.92%～62.79%,4.50%～5.77%,15.45%～17.32%,and 0.81～1.00.These geochemical characteristics indicate that granodiorite and tonalite are metaluminous to weakly peraluminous high-K calc-alkaline I-type granite.Both granodiorite and tonalite are relatively enriched in light rare earth elements and large ion lithophile elements(Rb,Th,U,and K),but depleted in heavy rare earth elements and high field strength elements(Ta,Nb,and Ti).They exhibit high Mg#values(42～65)and show significant variation in zircon Hf isotopic compositions[εHf(t)=-11.65 to-5.74],indicating that their parental magmas originated from a mixed source of ancient lower crust and mantle.Combined with previous research findings,it is suggested that the Maiba pluton formed at the end of the southward subduction of the Bangong-Nujiang Ocean.The rollback of the subducting slab caused asthenospheric upwelling,which triggered partial melting of the ancient lower crust,and its subsequent mixing with mantle-derived melts formed the parental magma of the Maiba pluton.