南阿尔金巴什瓦克榴辉岩的变质演化:对南阿尔金超高压变质作用的启示

Metamorphic evolution of the eclogite in the Bashiwake area:implications for the ultrahigh pressure metamorphism in South Altyn Tagh

高压/超高压变质带中的榴辉岩对于揭示造山带的热演化历史及造山过程具有重要的研究意义。本文通过对南阿尔金巴什瓦克地区榴辉岩的详细岩相学研究,认为该榴辉岩经历了榴辉岩相的峰期变质阶段(M1,946~1026℃和2.9~3.2 GPa)、峰后退变质阶段(M2,硬玉+石英=钠长石反应线附近)以及晚期麻粒岩相-角闪岩相的变质阶段(M3,~0.8 GPa, 789~841℃)。此外,锆石U-Pb年代学结果表明该榴辉岩的变质时代为492.8±2.8 Ma (MSWD=0.77),结合锆石中的石榴子石、绿辉石、金红石和石英包裹体及前人所获得的年代学数据,认为该变质时代记录了早古生代榴辉岩相的变质事件,进而恢复了南阿尔金榴辉岩所记录的顺时针P-T-t轨迹。本研究为南阿尔金巴什瓦克地区超高压变质作用提供了有效的矿物学与岩石学证据,同时为探讨南阿尔金动力学演化过程提供了重要的约束条件。

Eclogite, a volumetrically minor component within orogenic belts, witnessed the geodynamic evolution of subduction, collision, and exhumation. Petrographic observations suggested that the eclogite in the Bashiwake area, South Altyn Tagh, experienced at least three metamorphic stages, including the eclogite-facies metamorphism(M1)(946~1026℃, 2.9~3.2 GPa), the early retrograde stage(M2) characterized by an intergrowth of clinopyroxene with albite after omphacite, and the later retrograde stage(M3) represented by the occurrence of kelyphitic rims of amphibole and plagioclase around garnet(~0.8 GPa, 789~841℃). Zircon U-Pb dating yielded the metamorphic age at 492.8±2.8 Ma, representing the eclogite-facies stage based on geochronological data and the inclusions of garnet, omphacite, rutile, and quartz in zircon grains. These results suggest a clockwise P-T-t path for the eclogite in the Bashiwake area, South Altyn Tagh. The P-T conditions of eclogite provide the mineralogical and petrological evidences for the issue of the South Altyn experienced the ultrahigh pressure metamorphism and the dynamic evolution process of South Altyn Tagh.