Felsic veins within UHP eclogite at xitieshan in North Qaidam, NW China: Partial melting during exhumation
Felsic veins within UHP eclogite at xitieshan in North Qaidam, NW China: Partial melting during exhumation
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DOI：
10.1016/j.lithos.2011.11.006
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发表时间：
2012-04
期刊：
影响因子：
3.5
通讯作者：
Dan Chen;Liang Liu;Yong-gang Sun;Wei-dong Sun;Xiao-hui Zhu;Xiaoming Liu;Cai-Lian Guo
中科院分区：
文献类型：
--
作者：
Dan Chen;Liang Liu;Yong-gang Sun;Wei-dong Sun;Xiao-hui Zhu;Xiaoming Liu;Cai-Lian Guo
柴北缘锡铁山超高压榴辉岩中的长英质脉体宽度从1cm以下到几分米不等，呈粗粒状结构，矿物组合分布不均匀。它们包围或切割变形的次透镜状榴辉岩体。一些细脉延伸到榴辉岩中并终止于榴辉岩中，与围岩没有明显的联系。它们主要由长石和石英组成，并含有少量的白云母、黑云母和角闪石。为明确长英质脉体的形成时代、成脉流体/熔体的条件和来源，以及脉体与寄主超高压榴辉岩的关系，对长英质脉体及其寄主榴辉岩进行了锆石U-Pb定年和矿物包裹体联合研究。长英质脉中的锆石颗粒为完全自形，具分带性，Th/U比值很低，重稀土配分模式陡峭，无Eu异常。这些锆石中含有斜长石、白云母、石榴石和石英等微包裹体，除石榴子石是从寄主榴辉岩中捕获的外，其他矿物组合与脉岩矿物组合一致，表明这些锆石是在脉岩形成过程中结晶形成的。LA-ICPMS U-Pb法测得206 Pb/238 U加权平均年龄为428± 2 Ma，代表了脉体的形成年龄。寄主榴辉岩中的锆石颗粒呈椭圆形至圆形，内部结构均匀或浑浊，局部可见残留核。变质域中含有榴辉岩相石榴子石、绿辉石、金红石和蓝晶石等微包裹体。LA-ICPMS U-Pb定年结果为458± 2 Ma。地球化学分析表明，长英质脉体为英云闪长岩，具有高Na/K比值、高Sr含量、低Y和Yb含量，且LREE富集，与高压下榴辉岩中绿辉石的部分熔融相一致。因此，长英质脉很可能是由寄主超高压榴辉岩部分熔融形成的。超高压榴辉岩相变质作用与长英质脉状变质作用的锆石U-Pb年龄相差约3000万年，表明部分熔融发生在超高压地体的折返阶段。
Felsic veins within the Xitieshan UHP eclogite in North Qaidam, NW China vary from 1cm or less to several decimetres in width, exhibiting coarse granular in texture and heterogeneous distribution in mineral assemblages. They encircle or cut the deformed sub-lentoid eclogite body. Some fine veins extended into and ended in the eclogite, without visible link to their country rocks. They consist mainly of feldspar and quartz with minor amounts of muscovite, biotite and amphibole. To constrain their formation age, the conditions and source of vein-forming fluid/melt, and the relationship between the veins and host UHP eclogite, a combined study of zircon U–Pb dating and mineral inclusions was carried out on the felsic veins and their host eclogite. Zircon grains from the felsic veins are perfectly euhedral with zonations, characterized by very low Th/U ratios and steep HREE patterns without Eu anomalies. These zircons contain micro-inclusions of plagioclase, muscovite, garnet and quartz, which are consistent with mineral assemblage of the veins except for garnet captured from the host eclogite, indicating that these zircons crystallized during the veining. LA-ICPMS U–Pb dating yielded a weighted mean206Pb/238U age of 428±2Ma, representing the formation age of veins. Zircon grains from the host eclogite are oval to round with homogeneous and/or cloudy internal-structure; residual core can be observed locally. They contain eclogite–facies micro-inclusions of garnet, omphacite, rutile and kyanite in metamorphic domains. LA-ICPMS U–Pb dating gave a metamorphic age of 458±2Ma. Geochemical analyses show that the felsic veins are tonalitic with high Na/K ratios, high Sr contents but low Y and Yb contents, and LREE-enriched patterns, consistent with partial melting of omphacite in eclogite at elevated pressures. Therefore, the felsic veins were most likely derived from partial melting of the host UHP eclogite. There is a considerable difference of about 30 Myr in zircon U–Pb age between the UHP eclogite–facies metamorphism and the felsic veining, indicating that the partial melting took place in the exhumation stage of the UHP terrain.