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High Pressure and Temperature Subophiolitic Kyanite-Garnet Amphibolites Generated During Initiation of Mid-Tertiary Subduction, Palawan, Philippines by J. P. ENCARNACIÓN, E. J. ESSENE, S. B. MUKASA AND C. HALL
ABSTRACT Metamorphic rocks exposed near the base of a pre-middle Eocene ophiolite in central Palawan, Philippines, preserve a record of the P-T-t conditions under which the ophiolite was detached and emplaced onto the rifted southeast margin of China. Garnet amphibolites in contact with mantle harzburgite preserve peak metamorphic temperatures of 700-760 C and minimum pressures of 9 kbar, consistent with the presence of kyanite. Garnet grains which preserve strong prograde zoning (Pyp11 in cores to Pyp43 in rims), contain inclusion assemblages that constrain pressures above 5-6 kbar at 400-500 C. This represents an early prograde clockwise path consistent with underthrusting. Two amphibolite samples yield hornblende 39Ar/40Ar-36Ar/40Ar isochron ages of 34.0 ± 0.6 Ma (2-sigma) (early Oligocene), which are indistinguishable from a 34.3 ± 0.2 Ma isochron age of muscovite from a kyanite-chlorite-muscovite schist that is interlayered with the amphibolites. The high pressures (equivalent to a depth of >30 km in oceanic lithosphere) and temperatures, and the age difference between ophiolite crystallization and metamorphism of the sole, are incompatible with the rocks having formed in a mature subduction zone or oceanic spreading center. Instead, the timing, P-T conditions, and regional geology suggest metamorphism in a nascent subduction zone. Rapid cooling and exhumation of the metamorphic rocks followed peak metamorphic conditions in the earliest Oligocene, perhaps by the rapid thickening of the accretionary complex that is preserved beneath the ophiolite. The amphibolites formed during the inception of southward subduction within proto-South China Sea oceanic lithosphere south of the Eurasian margin. This led to the formation of an arc-trench system that accommodated extension along southeast China and the opening of the South China Sea. 
Journal of Petrology, 1995, vol. 36, pp.1481-1503
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