The Petrological Characteristics And Tectonic Setting Of Mesozoic And Cenozoic Volcanic Rocks,Hainan Island

Hainan Island in the binding site of the Eurasian plate,Indian plate and the Pacific plate,with complicated geological tectonic evolution history since the Mesozoic era.This paper from the point of the volcanic rock,analyzed and discussed the petrographic,geochemical and Sr?Nd?Pb isotopic characteristics of Hainan Mesozoic and Cenozoic volcanic rocks in order to Clarify the volcanic rocks' types and rock series,analyzed and discussed the causes and tectonic setting of volcanic rocks.Combination of geodynamics and isotopic dating,the conversion mechanism and transformation period of Mesozoic-Cenozoic tectonic environment in Hainan Island are discussed and analyzed.Mainly the following results were obtained.1.The Mesozoic volcanic rocks are mainly rhyolite,andesite and a small amount of volcanic breccia,rock series is high in potassium calc-alkaline series;Cenozoic volcanic rocks are mainly basalt,a small amount of basalt,andesite,rock series are spot basalt series and some alkaline series.2.The rare earth elements show that the Mesozoic intermediate-acid volcanic rocks' light and heavy rare earth elements differentiation is obvious,light rare earth concentration,relatively weak Eu negative anomaly,trace elements show that the LILE relative enrichment(Rb,Th,U),the HFSE relatively depleted(Nb,Ta)and P,Ti is strongly depleted,this indicates that the characteristics of the Mesozoic volcanic rocks in continental crust.The Cenozoic basalts,its HREE and LREE differentiation is not obvious,the rare earth elements no negative Eu anomaly,Ba,Ta,Sr slight enrichment,Pb strongly enriched.U,Ce,Sm,a slight depleted,Ti is strongly depleted,the overall performance similar to the OIB trace element characteristics.Isotopes shows that Cenozoic volcanic rocks show Dupal anomaly,Sr,Nd isotopes show the depleted mantle(DMM)features,Pb isotope shows a type II EM mantle characteristics,This suggests that basaltic magma may come from the mixing of depleted mantle and crust deposits.3.Based on the relationship between trace elements and the trace element spider diagrams,the causes of Hainan Island Mesozoic volcanic rocks should be through partial melting of the lower crust;and may have the upper mantle materials participation.Mesozoic volcanic rocksformed by partial melting of mantle plume,and has experienced some degree of fractional crystallization and has not been contaminated by crustal materials.4.Mesozoic volcanic output environment for active margin squeeze volcanic arc environment,and Cenozoic volcanic rock is stable intraplate environment,which is formed with a continental rift related to extensional tectonic background.Mechanism of Mesozoic-Cenozoic tectonic environment transformation is due to the deep crust-mantle interaction,So that the environment in Hainan Island by the subduction squeeze to extensional continental rift conversion,and the tectonic transition occurs between the Paleocene and Eocene.