| The metamorphic amphibolites in the pre-Sinian period,which are widely distributed in the mid-south section of the Kangdian axis in the Panzhihua,Sichuan,and Mouding,Yunnan,are important part of the Kangding complex.The research is of great significance to ascertain the origin of the Kangding complex and the magmatic evolution of the Kangdian axis.In this parper,amphibolites in Panzhihua area and Mouding area is taken as the research object.Through field geological surveys,laboratory rock and mineral identification,main and trace element analysis test,zircon U-Pb-Lu-Hf isotopic analysis test,the petrological characteristics,geochemical characteristics of main trace elements and diagenetic age of the plagioclase amphibolites in Panzhihua and Muding areas were systematically studied.The diagenetic age,material source and tectonic enviroment of amphibolite in the Panzhihua and Muding areas were identified,and the following recognitions was obtained:(1)Field geological observation and borehole observations show that the amphibolites in the Panzhihua area are generally distributed in multiple layers,which have a thickness ranging from a few meters to tens of meters and the extension is relatively stable.The blastovesicular structure and amygdaloidal structure with volcanic characteristics can be seen.The rock and mineral identification shows that the rock types were mainly biotite amphibole schist and pyroxene plagioclase amphibolite.The amphibolites in Mouding area of the Julin Group was produced mainly as layered distribution in the field,and have a thickness of a few meters to more than ten meters.The rock types are mainly plagioclase amphibole schist,and amphibolites.(2)The characteristics of main and trace elements of the whole rock show that the protoliths of amphibolites in panzhihua and mouding areas are basic volcanic rocks.They are overall characteristized by low Si,high Ti,high Fe and Mg,similar to Hawaiian tholeiite.The primitive mantle-normalized trace elements spider diagram of the amphibolites in the two regions all show "uplift"-like featureand are characterized by enrichment of Th,Ta,Hf and LREE but depletion HREE,showing a source similar to those of OIB.The total REE contents(?REE)of amphibolites samples in panzhihua and mouding areas is relatively high,which is characterized by LREE-rich,similar to oceanic island basalt.(3)The zircon U-Pb isotope dating indicates that the diagenetic age of the amphibolite from Panzhihua area is 816 ± 15 Ma to 833.6 ± 9.0 Ma,which is the product of middle Neoproterozoic magmatic activity.And the diagenetic age of the amphibolite from Mouding area is 1161±11 Ma,which is the product of late Mesoproterozoic magma activity.The zircon Lu-Hf isotope indicates that the primary magma of zircons in the two regions should be similar,which may be from the mantle source area affected by the enriched components.(4)A comprehensive study of the trace element ratios of amphibolites in the two regions,such as ?Nb,Nb/U,Ce/Pb,Zr/Nb,etc.,and various discriminative diagrams,found that the primary magma originated from an enriched mantle source area similar to OIB.The amphibolites may formed in the continental rift environment caused by the breakup of Rodinia supercontinent rather than the island arc environment linked with plate subduction.(5)Previous studies believed that the convergence period of the Rodinia supercontinent was from the late Middle Proterozoic to the early Neoproterozoic.The breakup of the Rodinia supercontinent began in the middle Neoproterozoic.However,the results of this study show that the late Mesoproterozoic amphibolites in the Mouding area are homologous to the amphibolites formed in the mid-Neoproterozoic continental rift environment in the Datian area,Panzhihua,west of the Yangtze plate.Therefore,we believe that the breakup of the Rodinia supercontinent may begin in the late Mesoproterozoic. |
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