Petrogenesis And Tectonic Implications Of Eocene Composite Granite Plutons In The Middle Segment Of The Gangdese Belt

The magmatic rocks developed along the Gangdese belt has recorded the whole geological process including Neo-Tethys subduction,the continential collision between India and Asia,and the uplift of the Tibetan Plateau.Therefore,research on these magmatic rocks has important significances in revealing the formation and evolution of the Tibetan Plateau,and in exploring the accretion and reworking process of the continental crust.In this paper,the author selected two typical Eocene composite granite plutons(i.e.,the Daju and the Sangsang plutons)outcropped in the middle segment of the Gangdese belt as examples,and conducted an integrated study including field geology,petrography,zircon U-Pb geochronology,whole-rock elemental and Sr-Nd and zircon Lu-Hf isotopic geochemistry.Based on these studies,this paper has obtained the precise emplacement ages for these two plutons,and comprehensively revealed their geological and geochemical characteristics,and discussed their genetic types and the deep dynamic crust-mantle interaction process involved in their petrogenesis,and finally explored their implications for the Neo-Tethys subduction and the continental collision between India and Asia.Therefore,this study has enriched the understanding of granitic magmatism and provided further petrological evidence in unravelling the tectonic evolution along the middle segment of the Gangdese belt.The Daju pluton is geographically located in the western of the XietongmenCounty and tectonically lies to the north of Yalung Zangbo suture in the middle segment of the Gangdese belt.According to the lithological and petrographic characteristics,this pluton can be divided into two units,i.e.,the Changna(CN)unit and the Sangga(SG)unit.The CN unit consists mainly of granodiorites with abundant dioritic enclaves,while the SG unit is composed of monzogranites with generally homogenerous lithology.Zircon LA-ICP-MS U-Pb dating yields coeval ages of 52.7±0.7Ma for the granodiorites and of 52.3±0.6Ma for the enclosed dioritic enclaves in the CN unit,while the monzogranites of the SG unit were emplaced at 45.3±0.7Ma.Geochemically,all rocks of the two units show sub-alkaline and metaluminous signatures,with agpaitic index(AKI)values mostly less than 0.9 and aluminum saturation index(A/CNK)values less than 1.0,thus can be grouped into metaluminous calc-akaline I-type granites.They are also enriched in LREEs ad LILEs(e.g.,Rb,Th,Pb),and depleted in HFSEs(e.g.,Nb,Ta,Ti),showing the geochemical characteristics of arc magma rocks.Compared with granodiorites in the CN unit,the monzogranites in SG unit are rich in silicon and alkali,have higher Rb/Sr and Rb/Ba ratios,and display more pronounced negative europium anomalies,indicating that the SG unit has experienced higher degrees of differentiation.The granites of Daju pluton generally show depeleted Sr-Nd-Hf isotpic compositions(Isr=0.7047?0.7059,?Nd(t)=-0.82?+6.10,?Hf(t)=-1.0?+11.76),suggesting that they were generated from partial melting of depleted mantle or juvenile crust.The Sangsang pluton is situated in the west of the Daju pluton and can also be divided into two units,i.e.,the Amuxiong(AMX)unit and the Dajia(DJ)unit.The AMX unit mainly consists of hornblende monzogranite,while the DJ unit is composed of monzogranite.Zircon U-Pb dating results show that the Sangsang pluton were emplaced at 54?42Ma,with detailed ages of 54?49Ma for AMX unit and 42Ma for DJ unit.Chemically,all the rocks are subalkaline,metaluminous and poor in phosphorus(A/CNK<1.10,P2O5<0.20%),thus can be grouped into calc-alkaline I-type granites.They are also enriched in Cs,Rb,Ba,Th,Pb and LREE,and depleted in Nb,Ta,Ti,showing the geochemical characteristics of arc magmatic rocks.In comparison with the AMX unit,rocks of the DJ unit are more enriched in Rb,Th,U,and have higher Rb/Sr and Rb/Ba ratios,and display more pronounced negative europium anomalies,indicating that they have experienced higher degrees of differentiation.The AMX unit has lower initial 87Sr/86Sr ratios(Isr)and depleted Nd isotopic composition(Isr=0.7054?0.7062,?Nd(t)=+5.62?-+5.84),while the DJ unit has enriched Sr-Nd isotopic coposition(Isr=0.7078,?Nd(t)=-6.50),implying that ancient crustal materials likely have contributed to their generation of monzonites in the middle Eocene.The Daju pluton and the Sangsang pluton show similar geological-geochemical characteristics.Both plutons have two episodes of granites with ages of Early Eocene(54-49Ma)for the main phase and of Middle Eocene(45-42Ma)for the complement phase.The integrated geochronological,geochemical,and Sr-Nd-Hf isotopic data indicate that Early Eocene granites and the enclosed mafic microgranular enclaves(MMEs)were generated by mixing of depleted mantle-derived magma and its induced juvenile crustal-melting magma.The Neo-Tethys slab break-off triggered the upwelling of asthenosphere mantle and induced the partial melting of the juvenile crust,thus generated the main phase granites.The granites of the complement phase have common magmatic source with the main phase granites.Due to the continuate underplating of mantle-derived magmas along the southern margin of the Gangdese belt during Ecocene,which kept the magma chamber in the deep crust cooling slowly,thus led the magmas experienced high degrees of differentiation.Meanwhile,together with the on-going of the India plate subducted beneath the Asia plate,the subducted ancient India continental crust materials kept involved in the generation of the granites.Therefore,the highly differentiated I-type granites of the complement phase were likely generated via a joint process including highly differentiation and involvement of ancient India continental crust.