豫西南地区北秦岭地体新元古代花岗岩类岩石成因及其地质意义

摘要

北秦岭地体中新元古代花岗岩类岩石是秦岭造山带的重要组成部分,记录了造山带基底前寒武纪地壳形成和演化历史.本文报道方庄和德河花岗岩岩体的锆石U-Pb年龄和O同位素组成、全岩元素和Sr-Nd同位素地球化学组成,探讨其岩石成因和地壳演化意义.结果表明,方庄花岗质糜棱岩的锆石结晶年龄为933.4±9.2Ma,δ18O值8.3‰～ 11.9‰,初始87Sr/86 Sr比值0.72455,初始εNd值-6.0,Nd模式年龄2.09Ga (tDM2)；德河黑云斜长片麻岩的锆石结晶年龄为948.1±8.9Ma,初始87 Sr/86 Sr比值变化较大,初始εNd值-5.0,Nd模式年龄2.02Ga.结合已报道的新元古代花岗岩类岩体的年龄和地球化学数据,北秦岭地体新元古代岩浆作用可以划分为980 ～870Ma挤压碰撞作用和～ 844Ma伸展裂解作用两大阶段,包括～940Ma强烈变形S型同碰撞花岗岩、～ 880Ma弱-无变形后碰撞Ⅰ型花岗岩和～844Ma板内A型碱性岩三类花岗岩体.地球化学组成显示,这些花岗岩类岩石可能源自不同时期形成的秦岭群基底杂岩的部分熔融,但在后碰撞阶段幔源物质或年轻地壳物质的加入明显增加.北秦岭地体中新元古代岩浆活动与Rodinia超大陆演化基本同时代,可能记录超大陆形成过程中的地壳响应.在新元古代之前,北秦岭地体或许具有不同于华北陆块和华南陆块的演化历史.%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 δ18O values of 8.3‰～ 11.9‰ and whole-rock yields initial εNd value of--6.0,Nd model age of 2.09Ga (tDM2) and initial 87 Sr/86Sr value of 0.72455,while zircons of the Dehe granite pluton crystallized in 948.1 ± 8.9Ma and whole-rock has initial εNd value of -5.0,corresponding to Nd model age of 2.02Ga (tDM2) 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.