Geochemistry and tectonic setting of the Luzon arc, Philippines

Abstract

Miocene (10 Ma) to Recent volcanism is associated with eastward subduction along the Manila Trench for some 1,200 km from the Coastal Range in Taiwan south to Min-doro. We suggest calling this the arc.. There are five distinct segments along this arc: Mindoro, Bataan, Northern Luzon, Babuyan, and Taiwan. The Bataan and Mindoro segments in the south are separated by a northeast-southwest zone of volcanoes. The Northern Luzon segment consists of Pliocene to Pleistocene necks and plugs that run along the entire western section of Northern Luzon. The Babuyan segment (10 Ma to Recent) is made up of a larger number of active volcanoes than are other segments of the Luzon arc. The Taiwan segment is composed entirely of extinct volcanoes in the Coastal Range and on the islands of Lanhsu and Lutao (late Miocene to 1.8 Ma). The nature of volcanism throughout the Luzon are is clearly subduction related with calc-alkaline affinities; high Th/La ratios; and negative Nb, Ta, and Ti anomalies. The earliest phase of volcanism so far recorded (10 Ma), however, is tholeiitic (island of Calayan in the Babuyan segment) with mid-oceanic-ridge basalt-like (MORB-like) tight-rare-earth element and large-ion lithophile element concentrations and low Th/U ratios. There are clear differences in Sr and Nd isotopic ratios among the samples from the Luzon are. Samples from the southern segments (Mindoro through Bataan) fall within the mantle array, with higher Sr and lower Nd isotopic values than those of MORB. Pb isotope data indicate sediment involvement. The isotopic values have been interpreted to result from source contamination by slab-derived fluids (Bataan segment) and by subducted crustal material from collision of the Mindoro-Palawan terrane (Mindoro segment) with the Manila Trench. In contrast, Babuyan segment samples (north) have unusual island-arc Nd and Sr isotopic ratios that fall below the mantle array; they appear to be the result of sediment source contaminants or a unique mantle composition. Incorporation of sediments analyzed from eastern China and Taiwan into a MORB-type mantle can explain the Nd and Sr isotopic ratios of the Babuyan segment.