Genesis and geological significances of Neoproterozoic granitoids in the North Qinling terrain,SW Henan,China

Neoproterozoic granitoids exposed in the North Qinling terrain belong to an important part of the Qinling orogenic belt and have recorded Precambrian crustal formation and evolution of the basement of this orogenic belt. In this study,we report zircon U-Pb ages and oxygen isotopic data and whole-rock geochemical and Sr-Nd isotopic composition for the Fangzhuang and Dehe granitoid plutons,emplaced into the Qinling Group in the North Qinling terrain,and discuss petrogenesis and geological significances for crustal evolution of the basement. Analytical results show that zircons of the Fangzhuang granite pluton formed in 933. 4 ± 9. 2Ma,zircons have δ 18 O values of 8. 3‰ ～ 11. 9‰ and whole-rock yields initial e Nd value of - 6. 0,Nd model age of 2. 09Ga ( t DM2 ) and initial 87 Sr / 86 Sr value of 0. 72455,while zircons of the Dehe granite pluton crystallized in 948. 1 ± 8. 9Ma and whole-rock has initial e Nd value of - 5. 0,corresponding to Nd model age of 2. 02Ga ( t DM2 ) and variable initial 87 Sr / 86 Sr value. Integrating with isotopic ages of fifteen Neoproterozoic granitoid plutons exposed in the North Qinling terrain previously reported,the Neoproterozoic magmatism in this terrain can be subdivided into two major stages resulted from 980 ～ 870Ma compressional and ～ 844Ma extensional tectonic settings,mainly including ～ 940Ma syn-collisional,～ 880Ma post-collisional and ～ 844Ma intraplate extensional granitoids. Geochemical and isotopic features of these granitoids indicate that they probably originated from partial melting mainly of crustal material similar to the Qinling Group,but contribution of mantle or juvenile material to the magmas became significant after ～ 900Ma in the post-collisional processes. Neoproterozoic magmatic activities in the North Qinling terrain broadly simultaneous with the formation of supercontinent Rodinia and hence,they record response of the basement in this terrain. The crustal evolution in North Qinling terrain prior to the Neoproterozoic was likely different from those in the North China and South China blocks.