Petrogenesis And Cooling History Of Granitoid Intrusions In The Feidong District,Southern Tan–Lu Fault Zone,China

The Feidong district is located in the southern segment of the Tan–Lu fault zone,belongs to the northern margin of the Yangtze Block.There are Neoproterozoic and late Mesozoic granitoid intrusions widely developed,which formed within the collision process between the Yangtze block and the Cathaysia Block in the Neoproterozoic,and the tectonic magmatic evolution of the Tan-Lu fault in the late Mesozoic respectively.Based on the detailed geological survey of the magmatic rocks in the Feidong area,we report zircon U-Pb geochronology and Hf isotope data,whole-rock geochemistry as well as zircon/apatite(U-Th)/He geochronology for the intrusions in the Feidong district.After combining with published geochemical data from the region,We discuss the genesis of the granitoid intrusions,which provide evidence of tectonism evolution and its cooling history of the Feidong district,southern Tan-Lu Fault Zone.Xishanyi complex pluton is located in the middle of the Feidong district.There are two different lithologies(granite gneiss and granodiorite)in this complex.Zircon U-Pb dating results show the emplacement ages of the Xishanyi granite gneiss are 809±5Ma,which belongs to the middle Neoproterozoic.The whole-rock geochemistry and of the Xishanyi granite gneiss demonstrate that it is peraluminous,high potassium calc-alkaline A2-type granites with enriched LILE,LREE,depleted HFSE,Eu.It is a typical feature of"arc".Highly enriched and homogeneous in-situ zircon?_Hf(t)values of Zircons(?_Hf(t)=-17.46?-13.56,avg.=-15.29;t_DM2=2545-2792Ma)and the Nb/Ta(?14.9)show the origin of Xishanyi granite gneiss was part melt of Archean crust.The above characteristics indicate that the Neoproterozoic granitoid intrusion in Feidong district are the products of arc magmatism.Zircon U-Pb dating results show that the emplacement ages of the Xishanyi granodiorite and Jianshan granite are 124±3 Ma and 130±1 Ma respectively,coeval with magmatic events linked to large-scale lithospheric thinning in eastern China.The whole-rock geochemistry of the Xishanyi and Jianshan intrusions demonstrate that they are peraluminous,high potassium calc-alkaline granites with adakitic characteristics.Both intrusions underwent weak assimilation during emplacement and fractional crystallization(apatite,ilmenite and sphene).The in situ zircon?_Hf(t)values of the Xishanyi granodiorites range from-26.4 to-21.8,with T_DM2 model ages of 2552 to 2841Ma.The in situ zircon?_Hf(t)values of the Jianshan granite are from-27.5 to-23.0 with T_DM2 model ages of 2632 to 2904 Ma.The peak age of inherited zircon grains from the Xishanyi granodiorite and the Jianshan granite were?2.07 Ga and?1.94 Ga,respectively.After compared with the regional magmatism,we suggest that both the late Mesozoic Xishanyi and Jianshan granitoid intrusions were derived from partial melting of the NCC lower crust.Zircon(U-Th)/He dating results of Xishanyi granite gneiss(elevation?230m)and Jianshan granite(elevation?80m)show 122±7Ma and 101±6Ma,respectively.Apatite(U-Th)/He dating results of Jianshan granite show 78±5Ma.Combined with the zircon U-Pb dating(?130Ma)results of Jianshan granite and different closed temperature of their isotopic system,we attained the cooling history of the Feidong district.The thermal history of the Feidong district can be divided into four stages:the first stage(?130Ma)was cooling rapidly,which is probably related to the emplacement of the magma;the second stage(123-102Ma)cooled slowly,with the uplift rate of 7.14m/My and the cooling rate of 0.21°C/My,the activity of the Tan-Lu fault zone was mainly horizontal in this period;the third stage(101-78Ma)cooled rapidly with the uplift rate of 166.7m/My and the cooling rate of 5°C/My,which was probably related to the low angle subduction of the Western Pacific plate toward NNW direction again;the fourth stage(78-0Ma)show the Feidong district became stable.