ZIRCON U-PB GEOCHRONOLOGY OF THE ZAMBALES AND ANGAT OPHIOLITES, LUZON, PHILIPPINES - EVIDENCE FOR AN EOCENE ARC-BACK ARC PAIR

Two basement terranes, the Zambales ophiolite in the west, and the Ang at ophiolite in the east, am exposed on the island of Luzon, separated by a circa 10 km thick and circa 100 km wide sedimentary basin. The s tructural and age relationships between the two ophiolitic blocks are central to understanding the geologic and tectonic development of the northern Philippines and evaluating models of terrane evolution propos ed for this area of the western Pacific. We analyzed zircons from the Zambales and Angat terranes to better constrain their origin. Two zirc on fractions froin tonalite in the Acoje block of the Zambales ophioli te give concordant U-Pb ages at 44.2 (+/-0.9) Ma. Two zircon fractions from plagiogranite and one fraction from diorite in the Coto block of the Zambales ophiolite give concordant U-Pb ages of 45.1 (+/-0.6) Ma. These results provide a Middle Eocene age for the Zambales ophiolite, in agreement with the minimum Late Eocene age of the overlying Aksite ro Formation. No age difference is discerned between the arc-like Acoj e block and MORB-like Coto block of the Zambales ophiolite. Four zirco n fractions from two sample sites in the Angat ophiolite give concorda nt ages of 48.1 (+/-0.5) Ma. This age is considerably younger than the Late Cretaceous age based on radiolarian fauna derived from a sheeted dike-pillow lava-sediment sequence south-southeast of the main ophiol ite. The small age difference between the Zambales and Angat ophiolite s suggests a common origin and obviates the need for a major structura l discontinuity west of the Southern Sierra Madre beneath the Central Valley of Luzon. The Cretaceous biostratigraphic ages of the ophioliti c rocks southeast of the Eocene Angat ophiolite implies that there are two ophiolitic basements exposed in the Southern Sierra Madre. The re lationship between the two ophiolites is constrained by the overlying stratigraphic relations which indicate that an Eocene volcanic arc and associated volcaniclastic apron was built on both the Eocene and Cret aceous ophiolitic basement. This suggests that the Zambales-Angat ophi olite represents a preserved Eocene back-arc basin that opened behind an Eocene arc that developed within Cretaceous oceanic basement. In th is model, the Zambales-Angat ophiolites are therefore not allochthonou s terranes but part of a single plate, generated in situ, forming part of the autochthonous basement of Luzon.