新疆中天山干沟一带晚石炭世花岗岩的岩石成因及其地质意义

Petrogenesis and geological significance of the Late Carboniferous granites from Gangou,Central Tianshan Mountains,Xinjiang

中天山干沟一带花岗岩广泛分布,前人对北部的眼球状花岗岩和细粒花岗岩进行了精确的SHRIMP定年研究,其侵位年龄分别为428Ma和361-368Ma,笔者对干沟南部的晚石炭世花岗岩进行了锆石LA-ICP-MSU-Pb定年研究,其侵位年龄为（301.2±1.0）Ma.干沟晚石炭世花岗岩属于高钾钙碱性系列,其σ值为2.6-3.22,显示钙碱性与碱性过渡的特点,富集轻稀土元素及Rb、K、Th、La及高场强元素Zr和Hf,亏损Ba、Sr、Nb、Ta、P和Ti,显示I型花岗岩及A型花岗岩过渡的特征.与干沟北部较老的花岗岩相比,晚石炭世花岗岩具有较低的（87Sr/86Sr）i（0.704771-0.705451）及较高的εNd（t）（2.14-2.51）,显示后造山阶段有较多的新的亏损地幔源区物质的加入.文章认为晚石炭世花岗岩的形成同后造山阶段幔源岩浆底侵有关,同时其形成为该区显生宙存在地壳垂向增生提供了新的证据.

Granitoid plutons are widespread in Gangou of Central Tianshan orogen. Previous studies of the augen granite and thefine- grained granite from north Gangou revealed that their SHRIMP U- Pb dating ages were 428 Ma and 361- 368 Ma,respectively. Comparatively, zircon LA-ICP-MS U-Pb dating of the south granitoid pluton in this study yielded the 206Pb/238U ageof 301±1.0 Ma. Major and trace elements show that the granites belong to high-K calc-alkaline series, and theirσ =2.67-3.22. Inaddition, they are enriched in LREE, Rb, K, Th, La, Zr and Hf, and depleted in Ba, Sr, Nb, Ta, P and Ti. They show a transitional features from I-type to A-type. Compared with the Silurian-Devonian granites, late-Carboniferous granits are characterized bylower ISr values （0.704771-0.705451）, higher εNd（t） （2.14-2.51） and younger TDM2 （861-891 Ma）, suggesting that more new depletedmantel-derived materials contributed to the generation of the granites during post orogen. It is therefore held that the petrogenesis ofthe late- Carboniferous granites was related to the underplating of the mantle- derived magmas. This study also provides newevidence for Phanerozoic vertical continental crustal growth in the Central Asian orogenic belt.