Geochronology of multi-stage metamorphic events: Constraints on episodic zircon growth from the UHP eclogite in the South Altyn, NW China

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• 作者：Liang Liu^a ; ^{Liuliang@nwu.edu.cn} ; Chao Wang^a ; ^b ; Yu-Ting Cao^a ; Dan-Ling Chen^a ; Lei Kang^a ; Wen-Qiang Yang^a ; Xiao-Hui Zhu^a
• 关键词：South Altyn ; UHP eclogite ; Zircon ; Multi-stage metamorphism ; P&ndash ; T&ndash ; t path
• 刊名：Lithos
• 出版年：2012
• 期刊代码：54_00244937
• 类别：ear
• 出版时间：April, 2012
• 卷：136-139
• 期：Complete
• 页码：10-26
• 文件大小：4290 K

摘要

Petrography, mineral chemistry and pressure-temperature (P-T) estimates were carried out for the eclogite from the South Altyn in NW China. The results suggest three stages of metamorphism: an ultra-high pressure (UHP) eclogite-facies metamorphism at 717-871 掳C and 鈮?#xA0;2.8 GPa, a high pressure (HP) granulite-facies metamorphism at 624-789 掳C and 1.42-1.52 GPa, and an amphibolite-facies metamorphism at 597-728 掳C and 0.99-1.17 GPa. Cathodoluminescence investigation revealed that zircons from the retrograde eclogite display a distinct core-rim structure. Cores are grey-white luminescent and contain mineral inclusions of Garnet + Omphacite + Rutile + Quartz, suggesting eclogite-facies metamorphic origin. The rims are dark grey luminescent and contain Garnet + Clinopyroxene + Pagioclase inclusions, forming at HP granulite-facies conditions. A few residual zircon grains with mottled internal structure also occur as the metamorphic cores. LA-ICPMS zircon U-Pb dating yielded three discrete age groups: (1) a Neoproterozoic protolith age of 752 卤 7 Ma for the residual grains, (2) an eclogite-facies metamorphic age of 500 卤 7 Ma for the metamorphic cores, and (3) a HP granulite-facies retrograde age of 455 卤 2 Ma for the rims. These ages indicate that the protolith of the Altyn eclogite probably formed in response to breakup of the Rodinia supercontinent during the Neoproterozoic; it was subjected to continental deep subduction and UHP metamorphism during early Paleozoic (~ 500 Ma) and subsequently underwent two stages of retrograde metamorphism during exhumation. The petrological and geochronological data suggest a clockwise P-T-t path for the UHP eclogite. According to pressures and ages for the peak UHP eclogite-facies and the retrograde HP granulite-facies metamorphism, an exhumation rate of 1.2 mm/yr was estimated for the eclogite, which is considerably slower than that of some UHP rocks from other UHP terranes (> 5 mm/yr). While the peak metamorphic age of 500 Ma is consistent with previous dates of 480-504 Ma, it is 40-60 Myr older than the HP/UHP metamorphic ages of 420-461 Ma for UHP eclogites in North Qaidam. The retrograde metamorphic age is 455 卤 2 Ma for the Altyn eclogite, which is 30-55 Myr older than ~ 400-425 Ma for the North Qaidam eclogites. These age differences suggest that the South Altyn and North Qaidam eclogites do not belong to the same HP/UHP metamorphic zone.