Compositions And Formation Of The Basement Metamorphic Rocks In The Xiangshan-Yuhuashan Area,Central Jiangxi Province,South China

The Xiangshan-Yuhuashan area in central Jiangxi Province is located near the middle section of the boundary between the Yangtze and Cathaysia Blocks,and important volcanic-hosted uranium deposits occur in the area.Thus,study on the compositions of the basement metamorphic rocks has great significance for understanding crustal evolution and genesis of uranium deposits.The Cretaceous Yuhuashan igneous complex contains abundant xenoliths of high grade metamorphic rocks,with the assemblage garnet ± hypersthene + biotite ± plagioclase + K-feldspar+ quartz.The biotite in these samples has high Ti02(>3.5%),indicating high-temperature metamorphism(623-778 ℃).P-T calculations for two felsic granulites indicate that the peak metamorphism took place at 880～887 ℃ and 0.64～0.70 GPa,in the low pressure/high temperature(LP-HT)granulite facies.Phase equilibrium modelling gives equilibrium conditions for the peak assemblage of a felsic granulite of>0.6 GPa and>840 ℃,consistent with the P-T calculations.Integrated with mineral assemblage and reaction relation of different minerals,an anticlockwise P-T-t path is inferred.LA-ICPMS U-Pb dating of metamorphic and detrital zircon from one xenolith reveals that the granulite facies metamorphism took place at 273.6±2.2 Ma,and the protolith was a sedimentary rock deposited later than 683 Ma.Late Paleozoic(Variscan)granulite-facies event is for first time identified in the South China Block(SCB).Coupled with other geologic observations,the LP-HT metamorphic conditions and anticlockwise P-T-t path suggest that Variscan metamorphism probably occurred in a post-orogenic or intraplate extensional tectonic setting associated with the input of external heat,related to the underplating of mantle-derived magma.Hence the discovery of these LP-HT granulites provides supporting evidence for a Late Paleozoic(Variscan)tectonothermal event in South China.Based on bulk geochemical and zircon U-Pb-Hf isotopic compositions,it is suggested that the basement under the Xiangshan-Yuhuashan area is made up of three layers:Neoproterozoic(822-750Ma)low-to medium-grade meta-sedimentary rocks in the shallow level,Neoproterozoic(761～683Ma)granulite-facies felsic meta-sedimentary rocks in the mid-lower crust,and high-grade Paleoproterozoic-Mesoproterozoic basement in the lower crust.There are distinctive composition differences among deep-seated xenoliths,surface rocks and drilled samples,which results in scatter of samples in the plot of SiO2/Al2O3 vs.K2O/Na2O,indicating that their protoliths probably were clastic sedimentary rocks with diverse provenances or suffered different degrees of weathering and leaching.The trace element characteristics of these meta-seedimentary rocks are similar to those of post-Archean Australian shales(PAAS),but obviously depleted in compatible elements(e.g.Cr,Ni,Ti),slightly depleted in large-ion lithophile elements(LILE,e.g.Ba,Cs)and strongly depleted in Sr,and weakly enriched in high field-strength elements(HFSE)with a large variation in U content.Uranium was activated during the granulite-facies metamorphism,and subsequent reduction and adsorption caused high content of uranium in the deep-seated xenoliths,considering that they contain many dark minerals such as garnet and biotite.All these samples show relatively high Th/Sc ratio(0.53～1.70)and low Cr/Zr ratio(0.13～0.78),suggesting that provenances of their protoliths contain more felsic components.Detrital zircon dating results show that the protoliths of deep-seated xenoliths are mainly composed of Neoproterozoic,Neoarchean and Paleoproterozoic constituents,quite distinct from those of surface rocks consisting of Neoproterozoic constituents with small amounts of Neoarchean and Paleoproterozoic fragments.Integrated with geochemical features,we consider that the sediments of deep-seated xenoliths come from Wuyi terrane in the Cathaysia Block,whereas the provenance of surface rocks in the study area is on the southern margin of Yangtze Block.Combining previous research data and geological observations,it’s confirmed that the deep-level boundary between the Yangtze and Cathaysia Blocks is located in the north of the study area,probably along the Shaoxing-Jiangshan-Pingxiang suture zone.The Caledonian Orogeny in the SCB led to the closing of the Nanhua Rift Basin during Silurian.Meanwhile,the protoliths of granulite xenoliths on the frontier edge of the fold-thrust belt were subducted to the deep level,and underwent Caleodonian metamorphism and subsequent Variscan metamorphism.On the other hand,shallow metamorphic rocks in the Jiangnan Orogen in the southern Yangtze Block were overthrust southward onto the basement of the Xiangshan-Yuhuashan area during the Indosinian to Yanshanian Orogeny.Consequently,a special structural framework was formed in the study area.During the granulite-facies metamorphism,the Th,U,K and Rb contents of these deep-seated xenoliths in this study are not decreased,and K/Rb and Rb/Cs ratios are not abnormally increased.Considering higher U contents of deep-seated xenoliths than surface metamorphic rocks and drilled samples,even higher than average U contents of the Middle Crust and Upper Crust,it can be inferred that Cathaysia basement rocks have higher U background values,and they supply abundant U metallogenic materials for the parental magma of the Xiangshan-Yuhuashan volcanic rocks.The Yuhuashan complex is mainly composed of sub-volcanic rocks,and the erosion depth is deeper than that of the Xiangshan complex.It’s inferred that most volcanic-hosted uranium deposits may have been eroded in the Yuhuashan area.Consequently,uranium prospection can only be targeted in the area where residual volcanic rocks are exposed.