东昆仑造山带海德乌拉辉绿岩成因及其地质意义

Petrogenesis of Haidewula Diabase,Eastern Kunlun Orogenic Belt and Its Geological Implications

古特提斯洋在东昆仑造山带的闭合时间仍存在争议.对东昆仑东段海德乌拉地区产出的辉绿岩开展了系统的研究工作.LA-ICP-MS锆石U-Pb同位素定年的结果显示,海德乌拉辉绿岩形成于238±2 Ma.在地球化学组成上,该辉绿岩具有较高的TiO_(2)(1.75%~2.46%)、Fe_(2)O_(3)^(T)(8.88%~12.30%)含量和较低的Mg O含量(2.76%~6.34%);富集不相容元素,相对亏损Nb、Ta、Sr、Ti;Sr-Nd同位素组成较为富集且均一,(^(87)Sr/^(86)Sr)_(i)为0.711 61~0.712 95,ε_(Nd)(t)为-3.2~-2.8.上述特征表明海德乌拉辉绿岩形成于板片俯冲环境,其源区为由俯冲板片释放的流体交代所形成的富集地幔.结合前人的研究成果,认为古特提斯阿尼玛卿洋的北向俯冲至少持续到中三叠世末期;随后,洋盆在晚三叠世早期闭合;在不晚于228 Ma时,东昆仑东段地区进入后碰撞伸展的环境.

The timing of the closure of the A’nyemaqen ocean,which was a branch of Paleo-Tethys ocean at the East Kunlun orogenic belt(EKOB),is still a subject of debate.In this study,it presents zircon U-Pb geochronology,whole-rock elemental geochemistry and Sr-Nd isotopic data on the Haidewula diabase in the eastern section of the EKOB.Zircon LA-ICP-MS U-Pb isotope dating reveals that the Haidewula diabase formed at 238±2 Ma.The diabase is characterized by low MgO(2.76%-6.34%)contents,as well as relatively high TiO_(2)(1.75%-2.46%)and Fe_(2)O_(3)^(T)(8.88%-12.30%)contents.The diabase shows enrichment in incompatible elements,but is relatively depleted in Nb,Ta,Sr,and Ti.All these geochemical characteristics imply that the diabase is a production of island arc magmatic activity.Additionally,the diabase displays enriched and uniform Sr-Nd isotopic compositions((^(87)Sr/^(86)Sr)i=0.71161-0.71295,ε_(Nd)(t)=-3.2 to-2.8).Based on the geochemical characteristics and isotopic compositions,it suggests that the Haidewula diabase was derived from partial melting of an enriched mantle,which had undergone metasomatism induced by the fluid released from the subducting slab,and had experienced some degree of crustal contamination during migration through continental crust.Combing our results and previous studies on Middle Permian-Triassic basic igneous rocks in the EKOB,it proposes that the northward subduction of the A’nyemaqen ocean lasted to the end of the Middle Triassic(238 Ma);the closure of the A’nyemaqen ocean happened during early Late Triassic;then,the tectonic environment of the eastern section of the EKOB transited into post-collisional extension no later than 228 Ma.