Provenance And Tectonic Setting Of Metamorphosed Sandstones Of The Wutai Group In North China Craton

Detailed research on tectonic setting and evolution history of the Neoarchean Wutai Group in the north central portion of the North China Craton can provided important clues to gain new insights into the crust evolution history of the Central Orogenic Belt in the North China Craton.Combined previous results,the author systematically analysed sedimentary timing,material source,weathering characteristics and tectonic setting of the lenticular metamorphosed sandstones of the Wutai Group,providing new insights into the crust evolution history form late Neoarchean to Paleoproterozoic in the region.Several results and conclusions are derived as follows:(1)Analysis of detrital zircon age characteristics of sandstone and determination of sedimentary age:LA-ICP-MS laser denudation method is used to analyze detrital zircon U-Pb age,and the intersection age 2538±6 Ma on inconsistent line and the peak age of 2529 Ma are obtained,which indicate that the time of sandstones deposition in this paper should be later than 2.54 Ga;According to the sedimentary time limit of the Hutuo group and the history of deformation and metamorphism of the Wutai Group,it can be inferred that the upper limit of the Wutai Group is2.5?2.45 Ga.(2)Sedimentary geochemistry and zircon trace element comparison method were used to trace the material source:The whole rock major elements Si O₂,Al₂O₃,Fe₂O₃^T,K₂O and rare earth elements La,Yb,?REE are used to distinguish the protolith.The protolith of metamorphosed sandstone of the Wutai Group is greywacke,which is consistent with the results of field investigation and petrographic observation.Based on the comparative analysis of the inactive trace element ratios Th/La,Sm/La,Zr/Hf,Y/Ho between the metamorphic sedimentary rocks of the Wutai Group and the potential source areas(metamorphic volcanic rocks and granitic rocks in the Wutai Area),it is found that the materials are mainly deposited by the weathering materials of the metamorphic acidic volcanic rocks.On the basis of extensive collection of previous age data,the comparison of zircon U-Pb age and zircon Th/U ratio between metamorphic sedimentary rocks of Wutai Group and potential source area also supports this conclusion.The chemical composition of metamorphosed sandstones is reliable in the analysis of rock provenance and tectonic setting.(3)Analysis of weathering characteristics of the source area:Through whole rock geochemical analysis,the Si O₂ content of metamorphosed sandstone of the Wutai Group varies greatly(64.51%?71.80%),the maturity is low(Si O₂/Al₂O₃=3.98?4.73),the distribution trend of rare earth elements is very close to that of post Archean Australian shale(PAAS),and the Rb/Sr ratio is 0.40?1.45,which is mostly lower than that of PAAS(Rb/Sr=0.8).The weathering index(CIW)is 79?90(average 88),the chemical alteration index(CIA)is 70?80(average 76),the plagioclase alteration index(PIA)is 76?90(average 85),and the compositional variation index(ICV)is 0.4?0.7,which indicates that the metamorphosed sandstones are formed by rapid deposition of near source weathered clastic materials,and weathering has little influence on geochemistry of the sandstones.(4)Analysis of the tectonic setting of the metamorphosed sandstones:Using various discrimination diagrams of trace elements to construct,the results support that the metamorphosed sandstones are similar to the sandstones of the continental island arc.The provenance of metamorphosed sandstones mainly come from metamorphic acid volcanic rocks in the Wutai Area,which is consistent with the sedimentary material of continental island arc mainly comes from acid volcanic rocks.Based on the previous research results,this paper proposes that the tectonic setting of metamorphosed sandstones of the Neoarchean Wutai Group is continental island arc.