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 Eoceneophiolite in central Palawan, Philippines, preserve a recordof the P-T-t conditions under which the ophiolite was detachedand emplaced onto the rifted southeast margin of China. Garnetamphibolites in contact with mantle harzburgite preserve peakmetamorphic temperatures of 700–760°C and minimumpressures of 9 kbar, consistent with the presence of kyanite.Garnet grains which preserve strong prograde zoning (Pyp₁₁ incores to Pyp₄₃ in rims) contain inclusion assemblages that constrainpressures above 5–6 kbar at 4O0–500°C. Thisrepresents an early prograde clockwise path consistent withunderthrusting. Two amphibolite samples yield hornblende ³⁹Ar/⁴⁰Ar–³⁶Ar/Ar⁴⁰Arisochron ages of 34•0±0•6 Ma (2) (early Oligocene),which are indistinguishable from a 34•3±0•2Ma isochron age of muscovite from a kyanitechlorite-muscoviteschist that is interlayered with the amphibolites. The highpressures (equivalent to a depth of >30 km in oceanic lithosphere)and temperatures, and the age difference between ophiolite crystallizationand metamorphism of the sole, are incompatible with the rockshaving formed in a mature subduction zone or oceanic spreadingcenter. Instead, the timing, P-T conditions, and regional geologysuggest metamorphism in a nascent subduction zone. Rapid coolingand exhumation of the metamorphic rocks followed peak metamorphicconditions in the earliest Oligocene, perhaps by the rapid thickeningof the accretionary complex that is preserved beneath the ophiolite.The amphibolites formed during the inception of southward subductionwithin proto-South China Sea oceanic lithosphere south of theEurasian margin. This led to the formation of an arc-trenchsystem that accommodated extension along southeast China andthe opening of the South China Sea. KEY WORDS: ophiolite; thermobarometry; garnet amphibolite; subduction