大陆碰撞造山带榴辉岩相的熔体活动:来自南阿尔金基性麻粒岩中长英质脉体的证据

本文通过对南阿尔金巴什瓦克基性麻粒岩中长英质脉体的岩相学、锆石U-Pb年代学、Lu-Hf同位素及全岩主微量地球化学的综合研究,首次限定了该区基性麻粒岩中长英质脉体的形成时代为491±2 Ma (MSWD=0.91),此年龄与寄主麻粒岩高压—超高温阶段(榴辉岩相)的变质时代在误差范围内近一致,表明长英质脉体形成于榴辉岩相的变质阶段。由于长英质脉体中锆石明显不同于寄主基性麻粒岩中变质锆石的形态特征,基本排除了长英质脉体中的锆石为捕获锆石的可能性。此外,该长英质脉体中的锆石具有少量的长石、石英包裹体和较低的Th/U比值(0.05-0.41),锆石稀土元素配分模式图显示其具有重稀土富集和Eu负异常的特征,这些证据表明长英质脉体中的锆石可能结晶于变质熔体之中。长英质脉体的全岩地球化学特征显示其该脉体具有相对较低的K2O值,基本上排除了其来源于围岩长英质麻粒岩的可能,而且这些锆石的Hf同位素特征与寄主岩石的锆石Hf同位素特征具有相似性,表明形成长英质脉体的熔体为内部来源。结合全岩地球化学特征,推测其可能为基性麻粒岩发生部分熔融所致。

Eclogite- facies melt activity in the continental collisional orogen: evidence from the felsic vein in mafic granulite, South Altyn Tagh

This paper reports a felsic vein in host mafic granulite from the Bashiwake area, South Altyn Tagh. Zircon grains yield a weighted mean 206 Pb/ 238 U age 491±2 Ma (MSWD=0.91). The characteristics of zircon morphology, mineral inclusions, low Th/U values, HREE enrichment, and obviously negative Eu anomalies indicate that these zircons from felsic veins were formed from the metamorphic melts. The U- Pb age of 491±2 Ma is consistent with the zircon U- Pb age of host mafic granulite, which was interpreted to be the peak metamorphic age. Thus, the felsic veins from the host mafic granulite may be formed at the peak metamorphic event. The similarity of Hf isotope features between the felsic vein and host mafic granulite indicate the melt may be an internal source, on the basis of whole- rock geochemistry, the felsic vein was generated by the partial melting of the host mafic granulite.