Mineral Geochemical Characteristics And Significance Of The Bijigou Layered Basic Complex On Northern Margin Of The Yangtze

Numerous Neoproterozoic granitic and mafic-ultramafic intrusions(850-620Ma)are widely distributed in the northern margin of the Yangtze Block.However,the petrogenesis and tectonic setting of these intrusions are still in debate.In this study,we reported a comprehensive study of petrography,in situ mineral and bulk-rock elemental geochemistry on the layered basic complex with aims to(1)characterize and compare the rock-forming minerals in different cumulative layers;(2)reveal the variations of geochemical features among different rocks types formed by fractional crystallization and cumulation of magma;(3)decipher the discrimination conditions of diagenetic tectonic setting.The main conclusions are listed below:(1)The field geological observations show that the Bijigou mafic layered complex can be divided into three lithology belts from bottom to top,including:the troctolite layer,the gabbro layer and the ore-bearing gabbro layer.The troctolite layer is mainly composed of dark-colored troctolite,troctolite,light-colored troctolite and minor Cpx-hosted peridotite.The remarkable rhythmic layering,formed by the changes of relative contents between olivine and plagioclase,shows that the plagioclase crystallized prior to pyroxene in the early stage of fractional crystallization of Bijigou layered basic complex.The gabbro layer is composed of olivine gabbro,light-colored gabbro and gabbro.The remarkable rhythmic layering is presented by the differences of mineral grain size.The ore-bearing gabbro layer is mainly composed of norite gabbro,fine-grained gabbro,coarse-grained gabbro,banded gabbro and V-Ti-magnetite-hosted gabbro,in which the rhythmic layering of compositional variation and grain size change are both well-developed.(2)The Bijigou mafic-ultramafic complex is the products of FC of the tholeiitic basaltic magma,characterized by a overall flat REE pattern.The ? REE concentrations rise as the decreasing basicity of magma,showing obvious characteristics of FC and cumulation.The data of in situ trace elements(LA-ICP-MS)for the main rock-forming minerals in different lithologies indicate that the trace elemental concentrations in the whole rock are closely related to the mineral assemblages,and the trace element contents of the minerals are controlled by the distribution coefficients between olivine,plagioclase and clinopyroxene and the matrix of tholeiitic magma.Therefore,the elemental geochemical characteristics of rock from single lithology belt of layered basic complex can-not be used to discriminate their diagenetic tectonic environment.(3)The MgO content of the parent magma in the Bijigou layered mafic-ultramafic complex calculated by the olivine contents is 9.63 wt%,which is high Mg basaltic magma.The crystallization temperature of olivine and clinopyroxene is 1227?-1295? and 1128?-1152?,respectively,indicating that the temperature of the parent magma is relatively higher.In conclusion,the parent magma of the Bijigou layered mafic-ultramafic complex is plume-related tholeiitic magma with high temperature and enriched Ti content,which is closely related to the magmatism associated with the Rodinia breakup.