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

Abstract

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°C and ≥2.8GPa, a high pressure (HP) granulite-facies metamorphism at 624–789°C and 1.42–1.52GPa, and an amphibolite-facies metamorphism at 597–728°C and 0.99–1.17GPa. 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±7Ma for the residual grains, (2) an eclogite-facies metamorphic age of 500±7Ma for the metamorphic cores, and (3) a HP granulite-facies retrograde age of 455±2Ma 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 (~500Ma) 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 1.2mm/yr was estimated for the eclogite, which is considerably slower than that of some UHP rocks from other UHP terranes (>5mm/yr). While the peak metamorphic age of 500Ma is consistent with previous dates of 480–504Ma, it is 40–60Myr older than the HP/UHP metamorphic ages of 420–461Ma for UHP eclogites in North Qaidam. The retrograde metamorphic age is 455±2Ma for the Altyn eclogite, which is 30–55Myr older than ~400–425Ma 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.