苏鲁地体超高压和退变质时代的厘定:来自片麻岩锆石微区SHRIMP U-Pb定年的证据

Determination of UHP and Retrograde Metamorphic Ages of the Sulu Terrane: Evidence from SHRIMP U-Pb Dating on Zircons of Gneissic Rocks

锆石微区矿物包体的激光拉曼和阴极发光测试以及相应的SHRIMP U-Pb定年结果表明,苏鲁地体片麻岩锆石微区记录了十分复杂的年代学信息。其中副片麻岩锆石核部记录了345～743 Ma的继承性锆石年龄,标志着原岩碎屑锆石来源的复杂性;含柯石英的锆石微区记录了220～234 Ma的超高压变质年龄;而含石英包体的边部则记录了202～219 Ma的退变质年龄。正片麻岩继承性锆石核部所记录的年龄为574～680 Ma,表明原岩锆石曾经历了部分Pb丢失,原岩的形成年龄应大于680 Ma;含柯石英锆石微区所记录的超高压变质年龄为224～242 Ma;而锆石边部所记录的退变质年龄为209～219 Ma。两类片麻岩锆石微区所隐藏的超高压变质和退变质年龄信息十分相近,平均值分别为229±4Ma和211±4Ma,标志着苏鲁地体超高压变质时代应为印支期,相应的构造抬升速率约5.6 km/Ma。该项成果不仅确定了苏鲁地体超高压变质和退变质时代,而且对于深入探讨苏鲁地体快速折返过程中的动力学机制有着重要的科学意义。

Combined study of Laser Raman, cathodoluminescence image and ion probe U-Pb dating reveals that zircons separated from gneissic rocks in the Sulu terrane recorded complicated geochronological traces. Some zircons from paragneiss retain inherited cores with inherited ages of 345~743 Ma, indicating the complexity of the detrital zircons in protoliths. Coesite-bearing domains of zircons recorded 220 ~ 234 Ma for UHP metamorphic conditions, and quartz-bearing rims of zircons recorded 202 ~ 219 Ma for the late-stage amphibolite facies overprint. The cores of zircons from orthogneiss recorded 574-680 Ma for inherited zircons, implying partial loss of Pb from zircon in the protolith, so the protolith age should be older than 680 Ma. Coesite-bearing domains of zircons recorded 224~242 Ma for UHP metamorphic conditions, and free-mineral inclusion rims of zircons recorded 209--219 Ma for the late-stage amphibolite facies. These data indicate that UHP metamorphism in the Sulu terrane occurred in the Indosinian time. The exhumation rate of the Sulu terrane is approximately 5. 6 km/Ma calculated based on the SHRIMP data and relative P-T conditions. The large exhumation rate of the massif is in the same range as other UHP metamorphic terranes in the world, and suggests that slab decoupling may be a driving force for the exhumation of UHP metamorphic rocks from mantle depths.