Identification of UHP and Non-UHP Orthogneisses in the Sulu UHP Terrane, Eastern China: Evidence from SHRIMP U-Pb Dating of Mineral Inclusion-Bearing Zircons

Abstract

Mineral inclusions in zoned zircons reveal that both ultrahigh-pressure (UHP) and non-UHP orthogneisses occur in cores from drill hole CCSD-PP1 at Donghai, southwestern Sulu terrane. Zircons from the UHP orthogneiss have inherited magmatic cores containing low-P mineral inclusions, narrow mantles with UHP mineral inclusions, and rims that contain low-P minerals or are inclusion free. In contrast, zircons from non-UHP orthogneiss have only magmatic cores and metamorphic rims, both of which are characterized by low-P mineral inclusions, or are inclusion free. SHRIMP U-Pb analyses of zircons from the UHP orthogneiss identify three discrete and meaningful age groups: Proterozoic magmatic protolith ages >753 Ma in the cores, a UHP metamorphic event at 227 ± 8 Ma in the coesite-bearing mantles, and an amphibolite-facies overprint at 213 ± 7 Ma in the overgrowth rims. Zircons from non-UHP orthogneiss record only two discrete age groups, inherited magmatic cores with Proterozoic protolith ages >748 Ma, and metamorphic rims with an age of about 211 ± 6 Ma. The metamorphic rims have the same Triassic age as the late amphibolite-facies overprint of the UHP orthogneiss within analytical uncertainty. These data, together with previous studies, suggest that Neoproterozoic supracrustal protoliths of the Yangtze craton experienced Late Triassic (227 ± 8 Ma) subduction to depths of at least 120 km, and were then rapidly exhumed to mid-crustal depths where both UHP rocks and a few non-UHP Neoproterozic granitic intrusives were subjected to coeval amphibolite-facies retrograde metamorphism at 211 ± 6 Ma. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P-T data is estimated to be 6.3-7.1 km/m.y. This rapid exhumation suggests that the Sulu UHP metamorphic rocks returned to crustal depths as part of a buoyant sliver, possibly produced by slab breakoff.