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Geochemistry and SHRIMP zircon U-Pb data of Neoproterozoic meta-magmatic rocks in the Suizhou-Zaoyang area,northern margin of the Yangtze Craton,Central China

2011; Science Press; Linguagem: Inglês

2095-8927

Xue Huai,

High-pressure geophysics and materials

The Suizhou-Zaoyang area (northern margin of the Yangtze craton),where is the least affected area of the whole Qinling-Tongbaishan-Dabieshan-Sulu orogen and their foreland by the deep subduction of the Yangtze Craton beneath to the North China Craton and HP-UHP metamorphism during Triassic,relative well preserved Precambrian basement of the northern Yangtze Craton.These low-rank metamorphosed Precambrian rocks not only provide an ideal opportunity to study the palaeo-tectonic setting of the northern Yangtze Craton and nature of the lithosphere,but also are rare-references for resuming protolithes of the metamorphic complex in the orogen and examining the element mobility during HP-UHP metamorphism.The Precambrian rocks exposed here include Neoproterozoic volcanic-sedimentary rock series (Suizhou Group) and voluminous Neoproterozoic ultramafic-mafic sill swarm.The Suizhou Group is composed of meta-acidic volcanic rocks,meta-basic volcanic rocks,and meta-sedimentary rocks.The ultramafic-mafic sill swarm is composed predominantly of two-pyroxene troctolites,with minor gabbronorites and pyroxenites.In this work,we used zircon SHRIMP U-Pb technique accurately dating the rhyodacitic tuff and meta-trachyandesite of the Suizhou Group,as well as two-pyroxene troctolitic sill swarm,yielding an emplacement ages of 763±7Ma,741±7Ma and 632±6Ma,respectively.The two former ages are similar to the emplacement ages of protolithes of the HP-UHP metamorphic rocks outcropped in the orogen,but it is still not sure whether there are any emplacement ages of protolith of the eclogite in the orogen are similar to the emplacement ages of the troctolitic sill swarm (632±6Ma).Geochemically,the acidic volcanic rocks exhibit obvious depletion in Sr and high field strong elements (HFSEs) such as Nb and Ta,enrichment in strong incompatible elements such as Rb,Ba,Th,U and K as well as light rare earth elements (LREEs).This type of rocks also show characteristics of strong enrichment in LREE over the HREE ((La/Yb)N=10.44),and weak negative Eu anomalies (δEu=0.75),similar to the biotite plagiogneisses widely distributed in the Dabie-Sulu orogen and acidic end-member of bimodal volcanic rocks developed in rift setting.The meta-trachyandesite of the Suizhou Group show no fractionation between LREE and HREE,no negative HFSEs anomalies such as Nb and Ta,similar geochemically to most eclogites in the orogen.The troctolitic sill swarm is characterized by high Al2O3 and MgO contents,low TiO2 and alkaline contents,strong enrichment of LREE over HREE ((La/Yb)N=5.49),particular for strong positive Ba,Sr and Eu anomalies (δEu=1.22),indicating plagioclase crystallization/accumulation.No HFSEs such as Nb,Ta,U,Th,Zr and Hf anomalies for the troctolitic sill swarm show little contamination of crustal material during evolution of the magmas,and that the parent magma could be derived from partial melting of an enriched subcontinental lithospheric mantle.