AN EXAMPLE OF ISLAND-ARC PETROGENESIS - GEOCHEMISTRY AND PETROLOGY OF THE SOUTHERN LUZON ARC, PHILIPPINES

Volcanism throughout the Luzon arc is associated with eastward subduction of the South China Sea floor along the Manila Trench. The southern section of the arc, the focus of this study, extends from the Lingayen-Dingalan fault to the small islands just south of Luzon. Two segments appear to exist along this section of the arc: the northern Bataan and southern Mindoro segments which are separated by the Macolod Corridor. The volcanic rocks have typical arc phenocryst mineralogies: olivine, clinopyroxene, plagioclase, and titanomagnetite in the most mafic rocks and clinopyroxene, plagioclase, orthopyroxene, titanomagnetite, +/- amphibole in the more felsic samples. Complex zoning, sieve textures, and decoupling of incompatible trace elements suggest that processes such as assimilation have taken place. The rocks from the study area range from basalts to rhyolites and show typical calc-alkaline features. The rocks of the Macolod Corridor and Mindoro segment are particularly enriched in large ion lithophile elements (LILE), light rare earth elements (LREE), and radiogenic Sr compared with the Bataan segment. The data fall within the mantle array on Sr-Nd isotopic diagrams and grade toward higher Sr and lower Nd isotopic values from north to south. A likely source for the volcanics of this study is either a mid-ocean ridge basalt (MORB)-type mantle that undergoes higher degrees of partial melting than regions involved in MORB generation or a previously depleted source. We suggest that the high field strength element (HFSE) anomalies have been derived through differential element partitioning during fluid transport from the subducted lithosphere to the mantle wedge. Continental crustal material seems to play a significant role particularly in the Macolod Corridor and the Mindoro segment, based on the high LILE, La/Sm ratios, radiogenic Sr isotopes, and delta-O-18 values. The Macolod Corridor and the Mindoro segment have undergone source contamination by crustal material from the North Palawan-Mindoro crustal block either during the collision of this block with the Manila Trench or by subduction of sediments rich in this crustal material. A similar component has also been detected in the Bataan segment but in minor amounts. The trace element and isotopic differences between the northern and southern sections of the arc are interpreted in terms of variable composition (i.e., variable amounts of a crustal component introduced from the Palawan-Mindoro crustal terrain) of the metasomatic fluids released into the source.