Sr isotopic and trace element variations in Oligocene to recent igneous rocks from the Philippine island arc: evidence for recent enrichment in the sub-Philippine mantle

Abstract

Oligocene-Miocene plutonic rocks from the Philippine island arc are characterized by uniform 87Sr/ 86Sr isotopic ratios of 0.7035–0.7039. In contrast, volcanics erupted in the past 5 Ma in Central Luzon, Marinduque, and Mindoro have higher radiogenic Sr (0.7042–0.7054) and show regional variations in their Sr isotopic compositions. Volcanics of the Pliocene-Recent Bicol volcanic chain of Southern Luzon have 87Sr/ 86Sr in the same range (0.7034–0.7040) as the Mid-Tertiary plutonics. Increases in radiogenic Sr are coupled with increases in Rb, Sr, Ba, and Th abundances in the Central Luzon-Mindoro-Marinduque Volcanic Belt, while no such correlation exists for the Mid-Tertiary plutonics. We consider the low values observed for Mid-Tertiary plutonics and Bicol volcanics to be controlled by fluids released from the subducting Philippine Sea plate basalt that have 87Sr/ 86Sr of the same range. Sr isotopic compositions of the volcanics in the northern part of the Central Luzon Volcanic Belt may be controlled by fluids generated from the subducted South China Sea crust. The highest values of radiogenic Sr (0.7051–0.7054) and highest LIL abundances, which are observed in the southern part of the Central Luzon volcanic arc, appear to be the result of source contamination by slivers of the continental crust of the North Palawan-Mindoro terrane, that collided with the Philippine arc in the Miocene.