High-pressure and -temperature subophiolitic kyanite - garnet amphibolites generated during initiation of mid-tertiary subduction, Palawan, Philippines

J.P. Encarnación, E.J. Essene, S.B. Mukasa, C.H. Hall

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46 Scopus citations

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 4O0-500°C. This represents an early prograde clockwise path consistent with underthrusting. Two amphibolite samples yield hornblende 39Ar/40Ar-36Ar/Ar40Ar isochron ages of 34{dot operator}0±0{dot operator}6 Ma (2σ) (early Oligocene), which are indistinguishable from a 34{dot operator}3±0{dot operator}2 Ma isochron age of muscovite from a kyanitechlorite-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. © 1995 Oxford University Press.
Original languageEnglish (US)
Pages (from-to)1481-1503
Number of pages23
JournalJournal of Petrology
Volume36
Issue number6
StatePublished - 1995

Bibliographical note

Cited By :28

Export Date: 3 November 2016

Keywords

  • Garnet amphibolite
  • Ophiolite
  • Subduction
  • Thermobarometry

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