New constraints for the tectonic development of the western Pacific margin since the Mesozoic: comprehensive SHRIMP zircon U-Pb dating of the Philippine ophiolite belts

Friday, 19 December 2014: 12:05 PM
Kenichiro Tani1, Jillian Aira Sibal Gabo2, Kenji Horie3, Osamu Ishizuka4, Jenielyn Padrones5, Betchaida D. Payot6, Maria Luisa G Tejada7, Decibel V. Faustino-Eslava8, Akira Imai5, Shoji Arai9, Graciano P Yumul Jr10 and Carla B. Dimalanta6, (1)National Museum of Nature and Science, Ibaraki, Japan, (2)Kyushu University, Fukuoka-Shi, Japan, (3)NIPR National Institute of Polar Research, Tokyo, Japan, (4)Geological Survey of Japan, Tsukuba, Ibaraki, Japan, (5)Akita University, Akita, Japan, (6)National Institute of Geological Sciences, University of the Philippines, Quezon City, Philippines, (7)JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, (8)University of the Philippines Los Baños, Laguna, Philippines, (9)Kanazawa University, Kanagawa, Japan, (10)University of the Philippines, Quezon City, Philippines
The post-Mesozoic tectonic history of the western Pacific margin is critical in understanding the major global tectonic events that occurred in the Eocene, such as the westward change in Pacific Plate motion and the simultaneous subduction initiation of the Izu-Bonin-Mariana (IBM) Arc (Ishizuka et al., 2011 EPSL).

The present location of the Philippine Islands continuously serves as a major tectonic boundary between the southeastern Eurasian and the Pacific Plates, accommodating the large left-lateral movement associated with the clockwise rotation of the Philippine Sea Plate (PSP) from Eocene to Miocene and now the subduction of the PSP from the east at the Philippine Trench - East Luzon Trough.

The basement rocks of the Philippine Islands are characterized by the presence of ophiolitic complexes exposed among the islands. Yumul (2007, Island Arc) defined four belts in the Philippine ophiolites and proposed that they progressively become younger towards west, from Early – Late Cretaceous at the easternmost belt to Eocene - Oligocene in the west. However, most of the ophiolitic complexes have been dated by radiolarians and foraminifera in the overlying sediments.

To precisely determine the igneous ages of the Philippine ophiolites, we have conducted SHRIMP zircon U-Pb dating of the gabbroic and leucocratic rocks collected from the ophiolitic complexes in the Philippine Islands, including those from Luzon (Zambales and Isabela ophiolites), Masbate (Balud ophiolite), Tablas (Sibuyan Ophiolite), and Cebu. New zircon ages show that all of the ages obtained so far from the eastern ophiolite belts are Eocene in age, from 52 Ma to 41 Ma. These ages coincide well with the opening of the West Philippine Basin (49 – 33 Ma, Taylor and Goodliffe, 2004 JGR), which is a backarc basin formed behind the incipient IBM Arc. Furthermore, geochemical data available from the igneous rocks in the eastern ophiolite belts show backarc basin basalt-like geochemical affinities (e.g. Yumul, 2007), suggesting that these ophiolites are genetically associated with the West Philippine Basin. On the contrary, gabbroic and leucocratic rocks that are associated with the ophiolitic complex in Cebu are Late Cretaceous (~90 Ma) in age, significantly older than those in the eastern ophiolite belts, suggesting a different origin.