滇川西部金沙江石炭纪蛇绿岩SHRIMP测年:古特提斯洋壳演化的同位素年代学制约

SHRIMP Dating of Carbpniferous Jinshajiang Ophiolite in Western Yunnan and Sichuan: Geochronological Constraints on the Evolution of the Paleo-Tethys Oceanic Crust

本文应用SHRIMP方法精确测定了金沙江蛇绿岩带中蛇绿岩的辉长岩和斜长岩、呈脉状产于辉长岩和变质橄橄岩中的斜长花岗岩、以及呈岩株状侵入蛇绿岩中的花岗闪长岩的锆石U-Pb年龄,提供了古特提斯洋壳演化的年代学制约。滇西之用层状角闪辉长岩的年龄为328±8Ma,书松斜长岩为329±7Ma,白马雪山辉长岩为282～285Ma,它们可能反映了海底扩张不同阶段的时代。研究还表明,在这些辉长岩和斜长岩中,部分锆石记录了375～352Ma的略老的年龄,暗示蛇绿岩浆活动可能始于晚泥盆世。在金沙江蛇绿岩中,滇西娘九丁斜长花岗岩和川西雪堆斜长花岗岩,具有高硅低钾的成分特征。但是,这些岩石的REE总量高,LREE富集;Sr初始值高,达0.7058～0.7070;在其锆石组成中,存在继承锆石。这些证据表明斜长花岗岩中存在陆壳物质的混染,可能与洋壳俯冲消减有关。娘九丁斜长花岗岩的年龄为285±6Ma,雪堆斜长花岗岩为300±5Ma,记录了古特提斯洋壳俯冲消减事件。滇西吉义独花岗闪长岩,呈岩株状侵入蛇绿岩中,年龄为263±6Ma,限定了蛇绿混杂岩的年代上限。

SHRIMP (sensitive high resolution ion microprobe) zircon U-Pb ages of the Jinshajiang ophiolite, China, have resolved important geological problems. Dating of six samples of gabbro, anorthosite, granodiorite and plagiogranite occurring in the ophiolite along the Jinshajiang River provides geochronological constraints on the evolution of the Paleo-Tethys oceanic crust. In western Yunnan Province, the Zhiyong layered amphibole gabbro is dated at 328±8 Ma,the Susong anorthosite,329±7 Ma, and the Baimaxueshan gabbro, 282~285 Ma, which might reflect formation ages of the oceanic crust at spreading centers. The study also reveals a few older zircons of c. 375 to 352 Ma from the dated ophiolitic gabbros and anorthosite, indicating that the magmatic activity in this part of Paleo-Tethys ocean started in the late Devonian. In the Jinshajiang ophiolite, the Niangjiuding plagiogranite from Yunnan and the Xuetui plagiogranite from Sichuang are both characterized by high SiO2 and low K2O. However, the rocks have high total REE content, enriched in LREE with high initial 87Sr/86Sr about 0. 7058 ~ 0. 7070. Inherited zircons also indicate the presence of an older crust component. This evidence suggests that continental contamination was involved in the magma generation and evolution, probably related to the subduction of the ocean crust. The magmatic age of the Niangjiuding plagiogranite is 285 ± 6 Ma and that of the Xuetui plagiogranite is 300 ± 5 Ma, which recorded the subduction-related events. The Jiyidu granodiorite from western Yunnan Province intruded into the ophiolite at 263 ?6 Ma, which gives the youngest possible age of the ophiolitic complex.