Geochemical Characteristics And Tectonic Significance Of Jurassic Gabbro-syenite In Hainan Island

Gabbro-syenite suite is produced in the environment of the lithospheric thinning which relating to the geological background,this paper studyies the Jurassic gabbro-syenite formation in Hainan Island,which can provide some instructions for the Southern China in Late Mesozoic tectonic evolution of the long-standing dispute.Hainan Island Jurassic?early Yanshan period?gabbro syenite set distribution of Lingshui fault zone on the Qinsai area,which is about 1.5 km2,this suit is characterized by the large span of component evolution,combining with biotite gabbro,quartz syenite-monzonite,quartz syenite and other rocks as an evolutionary series.It can be divided into the medium grained biotite gabbro unit,the middle-and long-grained fine-grained rock unit and the giant quartz hornblende syenite unit.LA-ICP-MS zircon U-Pb dating method is used to obtain the age of 165.2±1.6Ma,The gabbro age is 167.1±2.2Ma,and the epoch of the formation is middle jurassic.The content of SiO₂ in the unit changed greatly,ranging from 40.02%-65.09%,the total alkali content changed variable,and most of the silicon-base maps fall into the alkaline series,the ratio of K2O/Na2 O is 0.55¹.32,which defined this suit is potassic alkaline basalt.REE patterns curve overall showed rightwards type medium slope,light rare earth differentiation medium is obvious,advanced unit of LREE fractionation becomes strong,and heavy rare earth elements in early stage of unit fractionation,the fractionation of light rare earth elements in late stage was stronger,while the heavy elements in the early stage were flat except for the early units.The initial Sr ratio of the rock is relatively low,which is 0.70406-0.70493,?Nd?t?is higher,ranging from 1.6-4.1,with the characteristics of OIB?oceanic island basalt?.The source of material is derived from the mixing product of depleted asthenosphere and enriched lithospheric mantle,they share the same isotopic compositions of Sr and Nd,suggesting that they have the same magma origin.The alkaline basalts are originate in the environment of passive continental margin rift,which originated from the metasomatism of the asthenosphere to the lithospheric mantle,The main component of the magma is derived from the partial melting of phlogopite or amphibole,and the melting depth is between 60-80 km.When the magma is rising,the mixing of the crustal material around the magma chamber firstly formed gabbro,and the primary magma in the magma chamber had crystallization differentiation,they invaded the crust and forming the late two and syenite rocks.