HIGH-PRESSURE AND TEMPERATURE SUBOPHIOLITIC KYANITE-GARNET AMPHIBOLITES GENERATED DURING INITIATION OF MID-TERTIARY SUBDUCTION, PALAWAN, PHILIPPINES

Metamorphic rocks exposed near the base of a pre-middle Eocene ophioli te in central Palawan, Philippines preserve a record of the P-T-t cond itions under which the ophiolite was detached and emplaced onto the ri fted southeast margin of China. Garnet amphibolites in contact with ma ntle harzburgite preserve peak metamorphic temperatures of 700-760 deg rees C and minimum pressures of 9 kbar, consistent with the presence o f kyanite. Garnet grains which preserve strong prograde zoning (Pyp(11 ) in cores to Pyp(43) in rims) contain inclusion assemblages that cons train pressures above 5-6 kbar at 400-500 degrees C. This represents a n early prograde clockwise Path consistent with underthrusting. Two am phibolite samples yield hornblende Ar-39/Ar-40-Ar-36/Ar-40 isochron ag es of 34.0+/-0.6 Ma (2 sigma) (early Oligocene), which are indistingui shable from a 34.3+/-0.2 Ma isochron age of muscovite from a kyanite - chlorite - muscovite schist that is interlayered with the amphibolite s. The high pressures (equivalent to a depth of >30 km in oceanic lith osphere) and temperatures, and the age difference between ophiolite cr ystallization and metamorphism of the sole, are incompatible with the rocks having formed in a mature subduction zone or oceanic spreading c enter. Instead, the timing, P-T conditions, and regional geology sugge st metamorphism in a nascent subduction zone. Rapid cooling and exhuma tion of the metamorphic rocks followed peak metamorphic conditions in the earliest Oligocene, perhaps by the rapid thickening of the accreti onary complex that is preserved beneath the ophiolite. The amphibolite s formed during the inception of southward subduction within proto-Sou th China Sea oceanic lithosphere south of the Eurasian margin. This le d to the formation of an arc-trench system that accommodated extension along southeast China and the opening of the South China Sea.