POLYPHASE SUBDUCTION AND EXHUMATION OF THE SULU HIGH-PRESSURE-ULTRAHIGH- PRESSURE METAMORPHIC TERRANE

Abstract

Four structural-metamorphic zones separated by ductile shear zones comprise the Sulu high- to ultrahigh-pressure metamorphic terrane. From south to north, they are the southern high-pressure zone (0.2-0.4 GPa, 350-400 °C), the central very highpressure zone (1.5-2.5 GPa, 500-600 °C), the northern ultrahigh-pressure (UHP) zone(>2.8 GPa, 650-800 °C) dominated by paragneiss, and the northern UHP zone dominated by granitic gneiss. Exhumation was associated with retrograde amphibolite- to greenschist-facies metamorphism and oblique southeastward ductile extrusion within the subduction channel. Geologic observations and seismic reflection profiles suggest an eroded asymmetric antiformal slab intruded by numerous Mesozoic granitic plutons. Zircon U/Pb ages indicate that peak metamorphism in the UHP zone occurred at 240-220 Ma, and 40Ar/39Ar thermochronology documents later exhumation at 220- 200 Ma. In contrast, in the high-pressure zone, peak metamorphism and exhumation ages are older than 253 Ma and 253-240 Ma, respectively, by the same mineral. Together with previous results, these new data suggest that subduction and exhumation of different parts of the Yangtze slab occurred at different times, depending upon vertical variations in the physical properties of the subducted continental crust. © 2006 Geological Society of America.