Geochemistry Of The Feidong Complex In The Northern Margin Of The Yangtze Craton

Changes in different layers of the Earth during the period from late Archean to early Paleoproterozoic are signicant,including but not limited to the composition and growth rate of the continental crust,the atmospheric oxygen level,mantle potential temperature.This period is also the key stage for the formation and evolution of craton.The Yangtze Craton is one of the main Precambrian blocks in East Asia.Compared with the North China Craton,the cover of the Yangtze Craton is thicker and the Precambrian rocks are only sporadically cropped out at the north and southwest margin.Cumulated age and isotopic data reported in recent years indicate that Archean-Paleoproterozoic crystalline basement is widespread beneath the Yangtze Craton.However,many essential problems about the formation and evolution of the Precambrian basement of the Yangtze Craton are still open,especially whether the basement of the Yangtze craton is coherent or composed of multiple blocks.A geochronological and geochemical study was carried out for the felsic gneiss,amphibolite and intrusive granite from the Feidong Complex in the Zhangbaling uplift at the northern margin of the Yangtze Craton.The results reveal the existence of Archean-Paleoproterozoic basement in this area and provide an orppotunity to constrain the nature of the early Precambrian basement beneath the Yangtze Carton via comparing with other Archean-Paleoproterozoic crystalline basements.Zircon U-Pb dating on the felsic gneisses gives similar protolith ages of 2449±17 Ma,2444±15 Ma,2495±23 Ma and 2478±24 Ma.The ages of the amphibolite and k-feldspar granite are 2032±35 Ma and 747±4 Ma,respectively.This indicates that the late Archean-Paleoproterozoic crystalline basement of the Yangtze Craton is cropped out in this area.The zircon ?Hf(t)values of the ca.2.45-2.50 Ga gneisses range from-6.6 to 0.7 with an average value of-3.6±0.5 and the corresponding two-stage Hf model ages(TDM2)peaks at 3.3-3.2 Ga with an average of 3185±31 Ma.They also display enriched whole-rock Sr-Nd-Hf isotope compositions,with high initial 87Sr/86Sr ratios of 0.707968 to 0.708619,negativ ?Nd(t)values of-5.7 to-2.0 and negative ?Hf(t)values of-4.2 to-0.9,which show that the 2.45-2.50 Ga magmatic event is mainly the reworking of Paleo-Mesoarchean crust.The Paleoproterozoic amphibolite is characterized by negative Nb-Ta anomaly and positive Pb anormaly.The zircon ?Hf(t)values range from-8.3 to-1.3 with an average of-5.8±1.1.The occurrence of magmatic rocks with similar age and Hf isotope compositions in other terranes,including the Yudongzi group and Douling complex in the northern Yangtze Craton,as well as detrital zircons and volcanic xenocrystals with similar ages widespread in the Yangtze Craton,suggests that the basement is coherent throughout the whole Yangtze Craton.The Neoproterozoic k-feldspar granites which intrude into the Feidong complex display the characteristics of high silicon,alkali-rich,high potassium,low calcium and magnesium,high FeOT/(FeOT+MgO)value and slight peraluminous and belong to A2 type granite,indicates that the Yangtze craton was in a post-collisional extension tectonic setting during this period.