Characteristics And Forming Process Of The Sangxigou Granite,Yangxian County,Shaanxi Province

A series of Neoproterozoic layered mafic-ultramafic cumulative complexes occur on the northwestern margin of the Yangtze Block in the Beiba,Wangjiangshan,Chengshan and Bijigou regions from south to north,with a large number of granitic and granodioritic plutons.They mainly intrude in the Paleoproterozoic Houhe Group,Meso-Neoproterozoic Huodiya Group and the Neoproterozoic Xixiang Group.Previous studies revealed that the formation age of the basic complex in the Hannan Paleo-uplifted area is 830-780 Ma,but ages of felsic rocks is unknown.The granitic-granodioritic plutons associated with the layered mafic-ultramafic cumulative complexes crop out in the Bijigou V-Ti-magnetite-bearing deposit and the Sanxigou Town of Yangxian County,Shaanxi Province.Based on the field occurrence and rock assemblages,the plutonic rocks in the Sangxigou town can be divided into two rock suits,(1)the Bijigou Granodioritic Pluton(BGP)and(2)the Houjiagou(garnet-hosted,two-mica)Granitic Pluton(HGP).The BGP consists of banded,coarse-grained granodiorite and plagiogranite;they show coherent contact with the Bijigou gabbroic layers,thus implying closely paragenetic relationship.In contrast,the HGP is a large-scale batholith outcropping in the Houjiagou region and composed of two-mica granite and garnet-hosted white mica granite.through detailed field work,petrological observation,bulk-rock elemental analysis,in situ zircon U-Pb dating as well as Hf isotopic composition analysis for the BGP and HGP,this thesis draws the conclusions as follows:The BGP was formed at?822 Ma when the Yangtze Block were in the transitional stage(0.9-0.8 Ga)from the Sibao orogeny to intracontinental rifting.Previous studies suggest that the magmatism of?825 Ma may be in response to the plume-induced asthenosphere uplifting that lead to the breakup of the Rodinia Supercontinent.At this stage,the mantle heat flow accumulated at the bottom of the Yangtze Block and evoked the rapid heat conduction within the continental lithosphere,thus inducing high-extent partial melting of hydrous subcontinental lithospheric mantle(SCLM)in a relatively short time interval to generate magmas.In conclusion,the BGP is the product of the late-stage fractional crystallization of the tholeiitic magmas that formed at the beginning of Rodinia breakup and have the same origin with the Bijigou V-Ti-magnetite-bearing gabbroic orebody.The formation age of HGP is?784 Ma when the Yangtze Block is at the late stage of the peak of continental breakup(790?760 Ma).At such extension tectonic setting,due to the mantle plume upwelling,the thickness of continental lithosphere was further thinned.A large-scale partial melting occurred at the bottom of the lithosphere and the uppermost asthenosphere under the dual stack effect of both heat flow and material flow within the mantle plume.Comprehensive studies indicates that the HGP is a crust-remelting granitic pluton that formed in the low-pressure and high-temperature environment at the late stage of the Rodinia breakup.