| The magmatic activities of the northern margin of the Yangtze block in the Neoproterozoic are strong,forming a large area of intrusive complex,and many scholars have different explains about the rock geochemical data forming at 780-830 Ma,and the study of Chengshan rocks adjacent to Bijiogou rocks is deficient.This study provide more constraints on the evolution process of Hannan massif through exploring its age,geochemistry and petrology of Chengshan gabbro,and combining with geological structure background to confirm its tectonic environment,and as a supplement to the different genesis of Hannan complex.The paper comes the following conclusions:1.The Chengshan rock consists of basilite gabbro,gabbro,mica-gabbro,and a small amount of olivine gabbro.Compared with the Bijigou rock area,it has a relatively stable of rocks type,the composition of the rhythm layer is not obvious,and hypersthene,biotite show a weak orientation,leading a change in the size of the mineral structure of the rhythm layer.The granitic diorite surrounding gabbro mainly composed of plagioclase,clinopyroxene and amphibole,which are arranged in a striped structure.2.Mineralogy studies have shown that the Ca content of plagioclase in Chengshan gabbro varies greatly,An: 38-70,mainly andesine,small mounts of albite and bytownite;pyroxene are mainly augite and hypersthene.Compared with the mineral composition of the V-Ti bearing magnetite deposits in Bijigou,it is shown that the mineral types are consistent,it is reflected that the Chengshan gabbro area is far from the magma recharge area,near the edge of the magma chamber,and the crystal differentiation is not developed.3.According to the geochemical study of rock,spider diagram shows Chengshan gabbro is enrich Ba,K,Sr and other large ion lithophile elements(LILE),the relative depletion of Nb,Ta,Zr,Hf and Th and other high field strength elements(HFSE),and some gabbro shows that the weak Ti negative anomaly;in the chondritic meteorite diagram,the Chengshan gabbro shows a LREE-enriched right-leaning pattern,and some rocks show Eu anomalies,which indicate that the diagenetic processes are accompanied by the crystals of the plagioclase,and the Eu anomalies are related to Zr,Hf anomalies show a positive correlation,that is,Eu more enrichment,Zr,Hf loss is weaker.Combined with the main elements of the elements along the trend of the evolution of the characteristics of the line,but significantly enriched alkali metal,showing its diagenetic mother magma alkaline basalt fused.4.The LA-ICP-MS zircon U-Pb dating analysis of Chengshan gabbro indicate that Chengshan gabbro was 783 Ma,and the 822 Ma age of sample 15BJ-120 indicated the formation of Chengshan gabbro may have experienced two magmatic activities.5.The oxygen isotopic composition of zircons in the sample(15BJ-125)shows that the oxygen isotope ?18O(‰)of the gabbro mineral is between 4.52-7.14 and the mean value ?18O(‰)is 5.53,indicating that the diagenetic original magma was not contaminated by crustal,and the mantle-derived basaltic pulp rose to the lower crust and crystallized in a relatively stable and closed environment.6.The study shows that the Chengshan gabbro is a product of a magmatic activity in 782 Ma after the large-scale invasion of the basaltic magma of the northern margin of the Yangtze.The magmatic composition of this period has the characteristics of calc-alkaline rocks,which contain relatively high K2 O + Na2 O,and LREE and large ion lithophile elements,which reflects that Chengshan gabbro is part of the melting of the resulting acid magmatic activity with the Rodinia supercontinent pyrolysis. |
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