Petrogenesis And Tectono-magmatic Evolution Of The Early Jurassic-eocene Granites From The Jiacha Area,eastern Gangdese Belt

The Gangdese magmatic belt,located along the southern margin of the Lhasa terrane in the southern Tibet,records the subduction process of the Tethys Ocean,Indo–Asian collision and the crustal growth and reworking of the Lhasa terrane.Granites,closely coexisting with gabbros,are the dominant rock types among the Gangdese igneous assemblage which provide an ideal subject for the study of the evolution of the tectono-magmatism of the Gangdese belt.In this paper,systematical studies including petrography,lithogeochemical(major and trace elements),as well as LA-ICP-MS zircon U–Pb ages and Hf isotopic compositions were carried on the Early Jurassic-Eocene granites and related gabbros from the Jiacha area in the eastern part of the Gangdese belt,in order to constrain the characteristics of their temporal and spatial distributions,rock associations,magma sources and tectonic settings,and to discuss the Early Jurassic-Eocene tectono-magmatism evolution of the Gangdese belt and the history of the crustal growth and reworking of the Gangdese belt.An association of Early Jurassic high-Mg diorites,calc-alkaline quartz diorites and granodiorites,adakitic quartz diorites and tonalities are occurred in the Jiacha area.The mafic enclaves are randomly distributed in the adakitic quartz diorite.LA-ICP-MS zircon U–Pb dating results show that these intrusions formed in the Early Jurassic(202–179Ma).The high-Mg diorites are characterized by similar features of the sanukite which were probably produced by partial melting of mantle peridotites metasomatized by subducted slab-derived aqueous fluids and melts.The calc-alkaline quartz diorites and granodiorites are medium-to high-K calc-alkaline I-type granitoids,which were mostly formed by partial melting of the lower crust with various contribution of mantle derived magmas.The adakitic quartz diorites are characterized by high Mg O(2.63 to 3.46 wt.%)and Mg#(48–54)values and high ?Hf(t)values(6.6 to 13.4),which were probably produced by partial melting of subducted oceanic slab with a mantle contribution.The adakitic tonalites have very low abundances of compatible elements and relatively low ?Hf(t)values(3.5 to 10.3),and are interpreted to have formed by partial melting of Neoproterozoic mafic lower crust.Upwelling asthenosphere,triggered by rollback of the subducting Bangong–Nujiang(Meso-Tethys)oceanic plate,provided the necessary heat for metasomatized mantle wedge,slab and lower crust melting,resulting in the geochemical diversity of the coexisting intrusive rocks.LA-ICP-MS zircon U-Pb dating yielded the crystallization age of 87–75Ma of the Late Cretacous monzogranites,quartz diorites and gabbros in the Jiacha area.The monzogranites exhibits similar geochemical characteristics of adakitic rocks,with very low abundance of compatible elements(e.g.,Cr = 0.30–12.4ppm,Ni = 0.52–9.87ppm)and varying low ?Hf(t)values from 4.3 to 9.2.The adakitic monzogranites were probably derived from partial melting of a thickened lower crust.The quartz diorites and gabbros are enriched in large ion lithophile elements and depleted in high field strength elements,with depleted Hf isotopic compositions(?Hf(t)= 8.1 to 10.1).The parental magmas of quartz diorites and gabbros were probably derived from the subducted slab-derived fluids metasomatized mantle wedge,and experienced subsequently fractional crystallization.The Late Cretacous intrusive rocks were suggested to be formed in the tectonic setting of northward subduction of the Neo-Tethyan oceanic lithosphere.The Paleocene-Eocene(66–41Ma)is the most intense stage among the magmatic activity process of the Gangdese belt and the igneous rocks are characterized by the widespread granites and coexisting basic intrusive rocks as well as the large distributed Linzizong volcanic succession.Zircon LA-ICP-MS dating results indicate that granodiorites,monzogranites including enclave and gabbros association in the Jiacha area were synchronously emplaced at 50–47Ma.The parent magma of the gabbros was probably generated by partial melting of the depleted mantle.The granodiorites and monzogranites were probably formed by fractional crystallization of metasomatized mantle wedge derived magmas with crustal assimilations in deep crust.A synthese of the Paleocene-Eocene(66–41Ma)intrusive rocks and contemporaneous Linzizong volcanic rocks shows that both of them are characterized by variation range of petrochemistry composition from basic to acidic rocks.Their ?Hf(t)or ?Nd(t)values become more wide with the increase of Si O2 content.They further show the geochemical characteristics and magma evolutionary trend similar to the continental magmatic arcs.The volcanic rocks have higher fractional degree relative to the intrusive rocks.We suggest that Gangdese belt may be still the active continental margin during northward subduction of the Neo-Tethyan Oceanic plate.All the Early Jurassic,late Cretacous and Eocene magmatic rocks in the Gangdese belt show an intense crust-mantle interaction,indicating that the Gangdese belt(South Lhasa subterrane)was prabbaly an ancient microcontinent with Proterozoic crystalline basement,rather than a new island-arc terrane.The Gangdese belt may have experienced multistage crustal growth and reworking during this time.