内蒙古兴阿钼铜矿区侵入岩锆石U-Pb年龄及Hf同位素组成

Zircon U-Pb ages and Hf isotopic compositions of the intrusive rocks in the Xing’a Mo-Cu deposit, Inner Mongolia

兴阿钼铜矿床为大兴安岭北段最新发现的又一大型斑岩矿床。钼铜矿化与二长花岗斑岩密切相关,矿体主要产于斑岩体及其外接触带。从岩体向外依次发育钾硅酸盐化、绢英岩化、青磐岩化等斑岩型蚀变分带。本文对矿区主要侵入岩开展了锆石LA-ICP-MSU-Pb年代学和Hf同位素研究,获得赋矿钾长花岗岩、成矿二长花岗斑岩和成矿后闪长玢岩脉的206Pb/238U加权平均年龄分别为131±1Ma、129±1Ma和124±1Ma,限定成矿作用发生于131～124Ma之间。获得三类侵入岩的锆石εHf(t)值分别为6.8～8.4、6.7～7.8和5.8～8.4,二阶段模式年龄(tDM2)分别为579～670Ma、616～680Ma和578～721Ma,揭示矿区早白垩世侵入岩源自新元古代晚期增生地壳的部分熔融。据此,认为兴阿矿床形成于大陆碰撞过程的地壳缩短-加厚向伸展-减薄转换的构造体制,大陆碰撞由蒙古-鄂霍茨克洋闭合引起,而这种后碰撞体制的地壳伸展-减薄作用又在早白垩世被由太平洋板块俯冲诱发的弧后伸展所叠加而加剧。

The Xing’a deposit is a recently discovered large porphyry Mo-Cu deposit in the northern Great Hinggan Range, associated with a multistage porphyry complex. The orebodies occur mainly in the porphyry and outer contact zones. The deposit shows porphyry-type alterations, ranging outwardly from potassic silication, through phyllic alteration, to propylitic alteration. We conducted a study of LA-ICP-MS zircon U-Pb dating combined with Hf isotope analysis on the intrusions. Zircon grains from the ore-bearing K-feldspar granite, ore-causative monzogranite porphyry and post-ore diorite porphyry yield weighted mean 206Pb/238U ages of 131±1Ma, 129±1Ma and 124±1Ma, respectively, bracketing the mineralization in a time span of 131Ma to 124Ma. Their εHf（t） values are 6.8~8.4, 6.7~7.8 and 5.8~8.4, with tDM2 values of 579~670Ma, 616~680Ma and 578~721Ma, respectively. These data imply that the magmas originated from partial melting of the juvenile crust sourced from a depleted mantle in late Neoproterozoic. The Xing’a Mo-Cu deposit was formed in a tectonic transition from collisional crust shortening-and-thickening to post-collisional extension-and-thinning, related to the suturing of the Mongol-Okhotsk Ocean. This post-collisional crust extension-and-thinning was followed and enhanced by the back-arc extension related to the northwestward subduction of the Pacific plate in Early Cretaceous.