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ABSTRACT: Supra-Subduction Zone Volcanism: Evidence for Evolving Mantle Source, Degree of Partial Melting and Crystallization

Graciano P. Yumul, Jr., Raymundo S. Punongbayan

Petrological studies of the residual peridotites-ultramafic/mafic cumulates and volcanic rocks of the Zambales Ophiolite Complex, Luzon, Philippines, reveal its transitional island arc-MORB geochemical affinity (supra-subduction zone signature). This ophiolite complex is characterized by three volcanic rock units: the Coto volcanics, Acoje volcanics, and Coto dikes intruded into the Coto residual harzburgites. These three volcanic suites are characterized by differing major, trace, and mineral chemistries. The Coto volcanics (TiO2, 0.50-1.50 wt %; Zr, 31-75.8 ppm; Y, 13.5-30.70 ppm) have higher Ti, Zr, and Y than the Acoje volcanics (TiO2, 0.26-0.86 wt %; Zr, 10.9-45.4 ppm; Y, 10-23.2 ppm) and the Coto microgabbro and diabase dikes (TiO2, .52-0.94 wt %; Zr, 15.1-54.8 ppm; Y, 9.627.1 ppm) signifying either origin from different sources or differences in the degree of partial melting if they came from the same source. Olivine, plagioclase, pyroxenes, and, to a limited extent, spinel were the fractionating phases involved. The respective sources of the Coto volcanics, Coto dikes, and Acoje volcanics have undergone increasing degrees of partial melting from the former to the latter. The chemistry and petrographic characteristics of these rock units also suggest low pressure fractionation, low oxygen fugacity, and limited participation of magma mixing.

Several mechanisms are responsible for the formation and preservation of such mineral deposits. High degree of partial melting, expansion of the olivine-spinel phase volumes caused by the presence of water, incongruent melting of clinopyroxene to spinel plus liquid, changes in f02, temperature, pressure, and magma mixing can explain the chromite deposits in SSZ ophiolites. The presence of sediments and volcanics overlying massive sulfide deposits and the tectonics of emplacement are believed to be critical in the preservation of SSZ ophiolite-hosted massive sulfides since marginal basins (= SSZ ophiolites) are easier to emplace compared to the large, open sea oceanic basins that are usually subducted. Lastly, PGEs are believed to be derived from multi-stage melti g events that basically characterize SSZ ophiolites.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990