柴北缘绿梁山高压基性麻粒岩的变质演化历史:岩石学及锆石SHRIMP年代学证据  

Metamorphic history recorded in high pressure mafic granulites in the Luliangshan Mountains to the north of Qaidam Basin, northwest China: evidence from petrology and zircon SHRIMP geochronology

作　　者：张建新[1] 孟繁聪[1] 于胜尧[1] 戚学祥[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《地学前缘》2007年第1期85-97,共13页Earth Science Frontiers

基　　金：国家自然科学基金项目(40472102;40272095);中国地质调查局地质调查项目(1212010611811)

摘　　要：高压基性麻粒岩出露在柴北缘HP／UHP变质带的绿梁山地区，它主要呈透镜体状分布在石榴蓝晶（夕线）黑云片麻岩中。岩石学和矿物学数据显示高压基性麻粒岩经历了多阶段变质历史，早期可能经历了榴辉岩相变质作用（声〉15kbar），以石榴子石中保留的少量绿辉石为特征；高压麻粒岩组合（Grt-Cpx-PI-Qtz±Amp±Rt-Ilm）为退变质作用产物，其形成的变质条件为p=9．6～13．5kbar，T=730-870℃。晚期的变质反应以围绕石榴子石和后成合晶生成斜方辉石的为特征，形成的少T条件为6．2～8．5kbar和720-860℃。高压基性麻粒岩中的锆石SHRIMP测定共获得两组年龄，分别为（448±3）Ma和（421±5）Ma。结合锆石阴极发光和矿物包体研究，前者代表高压麻粒岩阶段的变质年龄，后者代表晚期与斜方辉石形成有关的中低压麻粒岩阶段的变质年龄。这些年龄结果显示麻粒岩相变质作用持续了大约27Ma，这可能与早古生代祁连地块与柴达木地块碰撞作用所引起的地壳加厚和后来的热松驰有关。High pressure mafic granulites in the Luliangshan Mountains to the north of the Qaidam Basin, in the northwest China HP/UHP metamorphic belt, form lenses within garnet-kyanite （sillimanite）-biotite gneisss. Petrologic and mineralogical data indicate its complex multi-stage metamorphic history. An earlier ec- logite facies metamorphism （p〉15 kbar） is deduced from rare omphacite inclusions within garnet. The high pressure granulite assemblages （Grt-Cpx-PI-Qtz ± Amp ± Rt-Ilm） were formed during decompression under a p-T condition of 9. 6-13.7 kbar and 730-870 ℃. The later metamorphic reactions led to growth of orthopyroxene coronas on garnet or symplectlte domains, indicating a significant drop in pressure （6. 2-8. 5 kbar） for similar temperatures （720-860 ℃）. In conjunction with CL-imagery and mineral inclusions, SHRIMP U-Pb dating of zircons reveals two age groups： （1） （4485：3） Ma representing the age of high pressure granulite metamorphism and （2） （4215：5） Ma reflecting the time of middle--low pressure granulite facies metamorphism. These data suggest that the granulite facies metamorphism lasted for about 27 Ma and was related to crustal thickening associated with the Early Paleozoic continental collision between the Qilian and Qaidam blocks, and to subsequent thermal relaxation and exhumation.

关 键 词：柴北缘 高压基性麻粒岩 变质历史 SHRIMP U-PB定年 

分 类 号：P588.3[天文地球—岩石学] P597.3[天文地球—地质学]