VOLCANISM ABOVE A SUBDUCTED EXTINCT SPREADING CENTER - A RECONNAISSANCE STUDY OF THE NORTH LUZON SEGMENT OF THE TAIWAN-LUZON VOLCANIC ARC (PHILIPPINES)

The aseismic ridge system of the South China Sea, extinct since about 17 Ma, subducts below the central part of the Taiwan-Luzon volcanic ar e, North Luzon Segment (NLS). The buoyancy of the relatively young and warm crust of the ridge system may be responsible for the high elevat ion of the NLS. Volcanism in this part of the are is characterized by the eruption of small volumes of intermediate to felsic magma (57-65% SiO2). The eruption products from the volcanic front in the central pa rt of the Cordillera form. a low-K, calc-alkaline suite, characterized by high Sr and low HREE and Y contents. Some workers consider these c haracteristics to be the geochemical fingerprints of magmas generated by slab-melting. However, model calculations require the amount of res idual garnet in the source to be small (less than or equal to 10%), in contrast to experimental work that indicates high amounts of residual garnet (greater than or equal to 30%). Alternatively, the melts may h ave been generated by melting of amphibolitic lower crust. The magmas erupted in the eastern and southern parts of the segment are calc-alka line suites with moderate to high K contents and Sr and Y contents typ ical for are suites. The small volume and evolved composition of the e rupted magmas may be related to the uplift of the NLS with the crust a cting as a density filter, which prevents more mafic magmas from reach ing the surface.