Mesoproterozoic Mafic Magmatism And Its Geological Significances On Hainan Island

Precambrian is an important period for understanding the early stages of formation,evolution and dynamic mechanisms of the Earth.The Columbia/Nuna supercontinent formed in Proterozoic is an important supercontinent in the evolution of the Earth.Previous studies have shown that the North China Craton,Yangtze Block and Tarim Block record numerous evidence of amalgamation and breakup of Columbia/Nuna supercontinent.However,the study of Mesoproterozoic mafic magmatic activity in the Cathaysia Block has paid less attention,and thus hinders us in understanding the Precambrian tectonic evolution history of the block and its relationship with the Columbia/Nuna supercontinent.Proterozoic strata and Precambrian magmatic activity are well preserved on Hainan Island,which provide an ideal window for us to study the characteristics of Proterozoic tectonic evolution of the Cathaysia Block and its relationship with the amalgamation and breakup of the Columbia/Nuna supercontinent.This thesis identified a set of Proterozoic（～1.4 Ga）metabasites from the Baoban Group on the Hainan Island.Based on this,detailed field geological survey,zircon U-Pb geochronology,mineralogy,petrology,and elemental and isotopic geochemistry have been carried out on the metabasites in order to reveal their petrogenesis and tectonic setting.This provides new evidence for understanding the Precambrian tectonic evolution of the Hainan Island and its relationship with the Columbia/Nuna supercontinent.The following achievements have been obtained.1.Zircon U-Pb geochronology of the mafic rocks from the Baoban Group on the Hainan Island shows that two mafic rocks were formed at 1420±19 Ma and 1435±20 Ma,respectively,indicating the development of middle Proterozoic mafic magmatism on the Hainan Island.2.The major and trace element analysis results of the mafic rocks show that their Si O₂contents is 47.7%～49.0%,similar to those of tholeiite.According to their different geochemical characteristics,the samples can be divided into two groups.Group 1 has Mg^#of56.7～57.2,Cr and Ni of 180～227μg/g and 32.7～40.2μg/g,respectively.They show obvious negative Nb and Ta anomalies.The total contents of rare earth elements（REE）are 43.8～47.0μg/g.They show no obvious Eu anomalies（δEu=0.99～1.05）,indicating a relatively flat REE distribution patterns.Group 2 is characterized by lower Mg^#（41.6～42.9）,Cr（10.0～17.0μg/g）and Ni（7.96～8.05μg/g）contents than the Group 1 samples.They show high Th and U,and REE contents（74.9～77.2μg/g）,and not obvious Eu anomalies（δEu=0.88～0.92）.These geochemical characteristics are similar to those of intraplate basalt.3.Zircon Hf isotopic compositions show that theε_Hf（t）values of the rocks range from+0.41 to+11.05,and the corresponding two-stage model ages(t _DM2)range from 2.00 to 1.48Ga,indicating that the rocks have depleted Hf isotopic compositions.4.Comprehensive analysis shows that the two groups of Proterozoic metabasites of the Baoban Group on Hainan Island were soured from different degrees of partial melting of spinel lherzolite mantle,and were contaminated by some crust materials.Crystallization differentiation of olivine,pyroxene,ilmenite and apatite were also play a role druing the magma evolution.Combined with other sedimentological and regional geological data,we propose that the metabasites were formed in an extensional environment which was resulted by the breakup of the Columbia/Nuna supercontinent.The Hainan Island shows close relationship with the Laurene continent during the Mesoproterozoic.Its paleogeographic location was likely close to the western margin of the Laurene continent,and thus can receive the detrital materials from the western margin of the Laurene continent.This is the reason why Hainan Island and the blocks from western margin of the Laurene continent show similar detrital zircon Hf isotopic compositions and magmatic rock assemblages.