Petrogenesis And A Vertical Compositional Section Of The Early Mesozoic Gangdese Arc Crust In Southern Tibet

Vertical composition and differentiation mechanism of magmatic arcs are critical to understanding the formation and evolution of continental crust.As a prominent part of the Transhimalayan batholith,the Gangdese Batholith(?1500 km in length)in southern Tibet consists of subduction-and collision-related magmatites,and thus is an ideal area to study the composition and differentiation mechanism of magmatic arcs.To reveal the petrogenesis of the Early Mesozoic Gangdese Arc and to construct its vertical compositional section,we report,for the first time,a systematic dataset of petrology,mineralogy,geochronology,and geochemistry for the Cuijiu Igneous Complex in the eastern Gangdese Arc of southern Tibet The Cuijiu Igneous Complex intrudes the Late Devonian-Early Carboniferous gneissic granites and includes a wide variety of rocks from ultramafic-mafic and felsic cumulates to mafic to felsic non-cumulate plutonic rocks.In addition,the diorite contains abundant mafic microgranular enclaves(MMEs).Cumulate and non-cumulate rocks can be distinguished by combining petrography and bulk-rock composition.Titanite and zircon U-Pb dating of twenty samples show that the varying lithologies within this complex crystallized contemporaneously at ca.200 Ma.The non-cumulate plutonic rocks that contain adakitic rocks were formed through hornblende-dominated fractional crystallization from co-genetic primitive basaltic magmas,which were derived from partial melting of a mantle wedge metasomatized by subduction-related fluids.The ultramafic-mafic cumulates,felsic cumulates and MMEs share similar isotopic compositions and crystallization order of minerals to the non-cumulate plutonic rocks,representing complementary compositions of the fractional crystallization processes.Field observations,preservation of abundant zircon xenocrysts captured from the Late Devonian-Early Carboniferous gneissic granites by the monzogranitic dikes and cumulate tonalites,and whole-rock Sr-Nd-Pb-Hf isotopic and zircon Hf isotopic data,all indicate minor wall-rock assimilation was also involved in the generation of the complex.Petrological and geochemical signatures call upon a polybaric AFC as the dominant process to generate the Early Mesozoic Gangdese Arc.Abundant whole-rock geochemical data show the Early Mesozoic Gangdese Batholith is less evolved than its volcanic counterpart,indicating the batholith might be slightly cumulative on average.The assemblage and crystallization order of minerals as well as Al-in-Hb geobarometry reveal that the ca.200 Ma Cuijiu Igneous Complex crystallized at a range of pressures between 10 and 2.5 kbar.This result allows us to propose that the Early Mesozoic Gangdese crust with normal thickness(?3 5 km),in which the middle to lower part(20-35 km)was dominated by hornblende-rich rocks with minor diorites and tonalites intruding the metamorphosed Late Paleozoic igneous rocks,followed upward by a thick granitoid batholith(4-20 km)and by volcano-sedimentary rocks(0-4 km).