The Petrogenesis And Mineralization Potential Of Gaojiacun Complex,in Western Margin Of The Yangtze Block,China

The northern margin and western margin of the Yangtze block entered a large-scale continental cracking period during the late middle Proterozoic to the early Neoproterozoic.The Neoproterozoic was an important period of the crustal growth and reworking in the Yangtze block.Relying on geochemical data and lacking of basic geological observation,the tectonic background of the region has long been controversial,and formed four models for the petrogenesis:Plume model?Arc model?Rifting model and Subduction-extension model.The essence of the debate is the different understanding of the dynamic mechanism and structural background of magma origin.If we only focus on the geochemical data of the piled rock,it is difficult to resolve this dispute.Objectively,we need comprehensive study on geology and mineralogy in order to understand petrogenesis correctly.Gaojiacun mafic-ultramafic intrusions is the largest one distributed in the western margin of the Yangtze block,China.It intruded into the Neoproterozoic Yanbian Group?⁸10Ma?and was concentric.The mafic-ultramafic intrusions can be divided into the inner lithofacies belt and the outer lithofacies belt.The inner zone?the main object of this study,referred as the Gaojiacun complex in the text?is mainly exposed dunite,clinopyroxene peridotite,troctolite and olivine gabbro.The outer zone mainly include hornbeam gabbro and amphibolites.In this paper,a detailed field geological survey was carried out on the exposed rocks of the Gaojiacun complex,and rock geochemical analysis tests were carried out on the collected rocks and ore samples.The comprehensive research shows that:?1?The Gaojiacun complex is mainly composed of dunite,clinopyroxene peridotite,troctolite and olivine gabbro,localized Ni-Cu mineralization,and intruded by diabase veins.The main rock assemblage is igneous mounds of crystal rocks.The main rock combination isapileofcrystallites.Themainmineralformationsequenceof olivine-plagioclase-monoclinic clinopyroxene?orthopyroxene?indicates that the magma originated in an anhydrous or water-deficient environment.?2?The REEs distribution model of rock samples shows a right-dip type,light rare earth element?LREE?enrichment,high field strength element?Nb-Ta,Zr-Hf?depletion.The analytical results of the whole rocks are as follows:(⁸⁷Sr/⁸⁶Sr)_i=0.704375⁰.705259,?143Nd/144Nd?_i=0.511428⁰.511777,?Nd?t?=–0.58⁺3.68.Slightly enriched Sr-Nd isotope composition and Ba,Pb,Sr characteristics show obvious“crust”imprint that mainly inherited from the mantle source,not crustal contamination.It is possible that the the ancient depleted lithosphere mantle was metamorphosed by preexisting subduction fluids/melts.?3?The geochemical characteristics of the Gaojiacun complex,the newly revealed troctolite?>300m?,combined with the magmatic rocks distributed along the Kangdian rift in the same period in the region,indicate that the Gaojiacun complex?⁸10Ma?is the product of magmatism in the continental rift within plate.?4?According to the comprehensive study,the Yanbian area experienced complex tectonic evolution in the Neoproterozoic:>⁸80Ma magmatism was arc volcanic activity,extension formed back-arc basin at latest⁸60Ma,then the amalgamation related to subordinate continental terranes in Yangtze block had occurred during 860⁸20Ma and formed the unified ancient Yangtze continental terranes,finally 820⁷40Ma developed Kangdian rift.magmatic activity during 820⁷40Ma is intraplate magmatism,but magama formation mechanism is not clear that needs further research?mantle plume?Rift?Like Cenozoic magmatism in eastern China??.?5?The parental magma composition of the Gaojiacun complex is high-magnesium basalt and the partial melting degree of the mantle is¹4%.The primitive magma is enrichment of Cu?Ni and PGE.?6?The parent magma have experienced a low degree of sulfide separation and depleted in PGE.Limited addition of crustal materials during magmatic evolution suggest that the metallogenic potential of the Gaojiacun complex is limited and separated sulfide deep-seated mineralization in somewhere needs further research..