SHRIMP U-Pb ages of ultrahigh-pressure and retrograde metamorphism of gneisses, south-western Sulu terrane, eastern China

摘要

Laser Raman spectroscopy and cathodoluminescence (CL) images reveal that most zircon separated from paragneiss and orthogneiss in drillhole CCSD-PP2 at Donghai, south-western Sulu terrane, retain low-P mineral-bearing inherited cores, ultrahigh-pressure (UHP) mineral-bearing mantles and low-P mineral-bearing (e.g. quartz) rims. SHRIMP U-Pb analyses of these zoned zircon identify three discrete and meaningful age groups: Proterozoic protolith ages (> 680 Ma) are recorded in the inherited cores, the UHP metamorphic event in the coesite-bearing mantles occurred at 231 +/- 4 Ma, and the late amphibolite facies retrogressive overprint in the quartz-bearing rims was at 211 +/- 4 Ma. Thus, Neoproterozoic supracrustal protoliths of the Sulu UHP rocks were subducted to mantle depths in the Middle Triassic, and exhumed to mid-crustal levels in the Late Triassic. The exhumation rate deduced from the SHRIMP data and metamorphic P-T conditions is 5.0 km Ma(-1). Exhumation of the Sulu UHP terrane may have resulted from buoyancy forces after slab break-off at mantle depths.