Petrogenesis Of Early Jurassic Igneous Rocks At Quni In The Southern Margin Of Gangdese And Its Constraint On The Early Stage Of Neo-Tethyan Subduction

The subduction of the Neo-Tethyan Ocean yielded typical subduction-related geological units in the southern Tibetan Plateau.These units include the Yarlung Zangbo Ophiolite,the Xigaze forearc basin and the Gangdese arc.The Gangdese arc is located in the southern margin of the Tibetan Plateau and in the north of the Yarlung Zangbo Ophiolite.The east-west distributed Gongdese arc is composited of Triassic to Miocene igneous rocks.These magmatic rocks record thesubduction history of the the Neo-Tethyan ocean and the collision processes between India and Asia.The understanding of the early subduction processes and the subduction-related rock assemblages of the Neo-Tethysalong the Asian continental margin remains unclear.In this paper,detailed field geological observations,petrological analysis of microscopy,geochemical analysis of minerals and whole-rocks,and zircon U-Pb ages are conductedfor the Quni igneous rocks at the Quni of Xigaze,central part of the Gangdese arc.The petrographic associations of the Quni igneous rocks are relatively complex,including cumulate hornblende gabbro and quartz diorite and monzonite granite and diabase dykes intruding into the quartz diorite.The cumulate hornblende gabbro,quartz diorite and monzonitic granite show relatively continuous composition variations in their major and trace elements,indicating that they may have a unified magmatic source area and have undergone a magmatic evolution process.The negative correlation between?Dy/Yb?_N and SiO₂ shows that quartz diorite and monzonite granite have undergone a differentiation process,and hornblende is an important separated crystalline phase.The zircon U-Pb dating results indicate that the quartz diorite and monzonite granite have an emplacement age of about 180 Ma,while the zircon Hf isotope indicates that they are all from the depleted mantle.The hornblende geobarometer from the cumulate hornblende gabbro shows that the cumulate hornblende gabbro was formed at 25-30 km depth,which further confirming that it is mainly formed in the crystallization process of mantle-source basaltic magma in the deeper magma chamber.The above three types of rocks all show obvious negative anomalies of Nb and Ta and positive anomalies of Pb,suggesting that they are mainly formed by percolation of subduction fluid to induce the partial melting of mantle wedge to form basaltic magma.The other melt differentiation crystals form quartz diorite,and finally the high degree of crystallization differentiation forms monzonite granite and intrudes into the quartz diorite.As for the diabase intruding into the quartz diorite,the correlation diagram of the main and trace elements in the whole rock is inconsistent with other rocks,and the REE and trace element patterns diagrams show similar geochemical characteristics with E-MORB.It shows that it was formed in different tectonic setting.The age of diabase was not obtained in this paper,but it was inferred that it was formed post-180Ma according to the field relationship.In this paper,the formation mechanism of diabase is proposed as follows:the melt formed by the low partial melting of the subducting oceanic crust penetrated into the upper mantle wedge,and mantle wedge undergoes a high degree of partial melting to generate the E-MORB diabase dykes.The characteristics of Quni igneous assemblages in the southern margin of the Gangdese arc and their mechanisms indicate the early Jurassic subduction in the Neo-Tethys Ocean,and the characteristics of rocks assemblage indicate that this stage has gone through the process from cumulate to differentiation crystallization.However,the late E-MORB diabase dykes indicate that the arc magma of Gangdese arc has the complexity of multi-stage and genetic mechanism.