湘东丫江桥岩体同位素年代学、地球化学及其构造意义

江南造山带中段湘东丫江桥岩体主要岩性为二长花岗岩,侵入于新元古代和晚古生代地层中。本文对该岩体进行了系统的锆石LA-ICP-MS U-Pb定年,元素地球化学、全岩Sr-Nd同位素测试,探讨了其成岩年龄、岩石成因及地球动力学背景。花岗岩体的锆石U-Pb年龄为223.2±1.3~224.3±1.8 Ma,代表侵位时代为晚三叠世,属印支晚期花岗岩。元素地球化学表现为高硅、富铝,富集轻稀土元素、大离子亲石元素(Rb、K、Th、U)和Pb,相对亏损重稀土元素、高场强元素(Nb、Ta、Ti)和(Sr、Ba)。花岗岩具有低的Nd初始比值(εNd(t)=-8.78^-7.19)和老的Nd模式年龄(TDM2=1.58~1.71 Ga)。元素地球化学及Sr-Nd同位素组成显示,丫江桥岩体形成于后碰撞环境,为印支主碰撞期后应力松弛阶段地壳伸展、减压快速隆升背景下地壳部分熔融的产物,也表明湘东地区在220 Ma左右正式进入后碰撞阶段。

Geochemical,Geochronological and Its Tectonic Significance of Yajiangqiao Granitiod in Eastern Hunan Province

The Yajiangqiao batholith in the middle part of the Jiangnan Orogenic Belt is composed of mainly monzonitic granite and intruded into the Neoproterozoic and Late Paleozoic strata. A combined study of whole-rock major-trace elements and Sr-Nd isotopes as well as zircon LA-ICP-MS U-Pb dating was carried out. The zircon U-Pb ages of the granitoid are from 223.2±1.3 Ma to 224.3±1.8 Ma, representing the late Triassic emplacement and belonging to the late Indosinian granite. Element compositions with high Si and Al content, exhibiting general enrichments of LREE and LILE(Rb, K, Th, U) and Pb, but depletion of HREE and HFSE(Nb, Ta, Ti) and(Sr, Ba). They have a low initial ratio of Nd(εNd(t)=-8.78 ^-7.19) and an old Nd model age(TDM2=1.58 ~ 1.71 Ga). The whole-rock major-trace elements and Sr-Nd isotopes characteristics suggest that the Yajiangqiao granitiod were emplaced at post-collisional background after the main Indosinian collision. The emplacement mechanism is partial melting of the low continental crust in an extensional tectonic setting. These results indicate that the Eastern Hunnan region is under post orogenic stage after 220 Ma.