Petrogenesis And Geodynamic Significance Of Upper Jurassic-Lower Cretaceous Sangxiu Formation Bimodal Volcanic Rocks In Southern Tibet

The Sangxiu Formation,a volcanic-sedimentary rock assemblage composed by alternate volcanic rocks and sedimentary rocks,widely distributes in the Tethys Himalaya of southern Tibet.The geological era of Sangxiu Formation is between Late Jurassic and Early Cretaceous according to the discovered paleontological fossils.The volcanic rocks of the Sangxiu Formation are main basalts and dacites with little or no andesites.There are significant Si O₂ interruption between the two kinds of volcanic rocks,indicating this is a suit of borad bimodal volcanic rock.For finding out the magmatic source,petrogenesis and geodynamic significance of the Sangxiu Formation volcanic rocks,this study deeply researched the petrological,geochemical and chronological chracteristics of Sangxiu Formation volcanic rock samples.The Sangxiu Formation basalts and dacites from different areas show similar geochemical characteristics.The basalts are peraluminous,alkaline or calc-alkaline and show general degrees of crystallization and fractionation,high REE contents,significant fractionation of LREE and HREE and weak Eu anomaly.These geochemical characteristics are similar with the OIB igneous rocks related to Kerguelen Mantle Plume.The dacites of Sangxiu formation show the characteristics of moderately to strongly peraluminous,calc-alkaline to alkaline and high degrees of crystallization and differentiation.They are also characterized by high REE contents,significant differentiation of LREE and HREE,and significantly negative anomaly of Eu.The REE patterns are"seagull type"and similar to the average value of the crust.Based on the chronological analyses of the dacite samples from study area and the basalt samples from the adjacent area,the crystallization ages of Sangxiu Formation dacite are 137.3Ma,the basalts are about 142Ma.The ages of bimodal volcanic rocks of Sangxiu formation are very close,they were formed during the Late Jurassic to Early Cretaceous.The geochemistry study shows that the basalts of the Sangxiu Formation are OIB-type,which are not contaminated by the crust and are contaminated by the lithospheric mantle materials.The magma may be derived from the enriched mantle.The basalts’geochemical characteristics are similar to the contemporaneous mafic intrusive rocks,they may be the interaction product of the Kerguelen Mantle Plume and the Lithospheric Mantle.The Sangxiu Formation dacites show the geochemical characteristics of A-type granite.Their geochemical characteristics and in-situ Lu-Hf isotopic compositions indicate that the magma was derived from the crust.The Sr-Nd isotopic compositions of dacites are close to the High Himalaya,which represents the continental crystalline basement of Gondwana,so the dacites may be the remelting product of continental crust.The bimodal volcanic rocks are multi-sourced and formed in the continental margin rift environment.Contact with the paleogeographic research of Tethys Himalaya,the basaltic magma may be formed by the interaction between Kerguelen mantle plume and lithospheric mantle.The underplating of basaltic magma during its rising process caused the remelting of aluminosilicate continental crust and the formation and eruption of acid magma.The bimodal volcanic rocks of the Sangxiu formation represent the strongly extensional tectonic background of the Tethys Himalaya during Late Jurassic to Early Cretaceous.This short and intense magmatic activity also indirectly triggered the breakup of Gondwana and the opening of the East Indian Ocean.