Geochemical Characteristics And Petrogenesis Of Basalts And Diabases From Yingan And Bachu Areas,Tarim Large Igneous Province

The Tarim large igneous province is the one discovered after the Emeishan large igneous province in China.The distribution surface area of mafic rocks is more than 250000 km².The geochemical characteristics and petrogenesis of the flood basalts and diabases are of great significance to understanding the magmatism process and the deep dynamic mechanism in the Tarim large igneous province.At present,it is generally believed that the evolution of basic magmatic rocks in Tarim did not experience obvious crustal contamination,and their isotopic compositions have been directly used to discuss the characteristics of their mantle source areas in many studies.However,the basalts and diabases in Tarim Basin have lower Mg^#(<50),and a large number of plagioclase and clinopyroxene phenocrysts are developed.These characteristics indicate that they have experienced significant fractional crystallization,and it seems difficult for mafic magmas not to be affected by crustal contamination in the process of crystallization differentiation.In this thesis,the petrographic observation and geochemical analyses of the basalts and diabases from Bachu and Yingan areas have been carried out in detail,including the whole rock and in-situ analyses of clinopyroxene phenocrysts and plagioclase phenocrysts of these samples,in order to further constrain their petrogenesis and associated geodynamic processes.The low Mg^#(24.8?54.5),Ni(11.9ppm?43.7ppm)and Cr(Cr₂O₃?0)contents and lack of obvious negative Eu anomaly(Eu/Eu^*=0.89?1.05)of these samples indicate that the fractional crystallization of olivine,clinopyroxene and Ti-Fe oxide may have occurred in the primary magma,while the fractional crystallization of plagioclase is limited.Plagioclase phenocrysts with zonal structure and resorption texture are observed in many samples,and clinopyroxene phenocrysts with zonal structure are also observed in a few samples.The core of these clinopyroxene phenocrysts has higher Mg#than their mantle,and the core of these plagioclase phenocrysts has higher An than their mantle.Both petrographic and geochemical characteristics indicate that these so-callled"phenocrysts"are actually xenocryts.The trace element characteristics of clinopyroxene phenocrysts reflect that their parental magma has similar composition and genesis with its host magma.The geochemical features of phenocrysts indicate that the deep magmatism process in the Tarim large igneous province is characterized by multiple stages of magmatic separation,crystallization and mixing.The samples from the Yingan area have relatively flat REE partition patterns(?LREE/?HREE=7.6?9.6),high Ce/Nb(2.5?3.2)and enriched Sr-Nd-Hf isotopic compositions(⁸⁷Sr/⁸⁶Sri=0.70490?0.70845,?Nd(t)=-5.4?2.0,?Hf(t)=-3.1?4.4).By contrast,the samples from the Bachu area have higher fractionation of REE(?LREE/?HREE=15.4?22.8),lower Ce/Nb(1.3?2.0)and more depleted Sr-Nd-Hf isotopic compositions(⁸⁷Sr/⁸⁶Sri=0.70464?0.7062345,?Nd(t)=0.1?2.2,?Hf(t)=1.9?4.6).The above characteristics reveals that compared with the mafic magmas in the Bachu area,the mafic magmas in the Yingan area may have been derived from the partial melting of a shallower and more enriched mantle source.The Si O₂ content of the Yingan basalts is negatively correlated with Nb/La,?Nd(t),positive correlated with ⁸⁷Sr/⁸⁶Sri,and their ⁸⁷Sr/⁸⁶Sri is negatively correlated with Nb/U.These geochemical characteristics indicate that the Yingan basalts have undergone significant crustal contamination.This finding is different from that of previous studies which generally believe that the large range of Sr isotopic composition of Group 1 basalts in the Yingan area reflects that the lithospheric mantle source region has undergone different re-enrichment processes to varying degrees.The samples from the Yingan region show the characteristics of the mixing of mantle plume,asthenosphere and crust.In conclusion,the mafic magmas in the Tarim large igneous province experienced complex fractional crystallization,magma mixing,and crustal assimilation before eruption.Careful,deeper and broader work is needed to exclude the influence of magmatic processes before constraining the source regions and related geodynamic processes.