Stratigraphic units overlying the Zambales Ophiolite Complex (ZOC) in Luzon, (Philippines): Tectonostratigraphic significance and regional implications

The Zambales Ophiolite Complex (ZOC) on the island of Luzon, Philippines is one of the most well-studied crust-mantle sequences in the region. Several massifs comprise the ZOC, one of which is the Coto Block overlain by clastic sedimentary units previously dated as Eocene. Geochronologic studies from diabase, granodiorites and other late-stage magmatic products similarly yielded the same age. Succeeding tectonic models have therefore all been grounded on the assumption that the entire ZOC is Eocene. Recent investigations, however, revealed the presence of chert blocks within the Early to Middle Miocene clastic formation overlying the Acoje Block in the northern part of the ophiolite complex. Radiolarians extracted from the cherts yielded a stratigraphic range that suggests a Late Jurassic to Early Cretaceous age. The recognition of a much older age than previously reported of the ZOC warrants a re-examination of its actual distribution and genesis. Correlating with other similarly-aged ophiolites, we suggest defining a western Mesozoic ophiolite belt, largely extending from the west-central portion of the archipelago to the northeastern tip of Luzon island. Tentatively, we attribute the Mesozoic ophiolitic and associated rocks in western Luzon to an arc-continent collision involving the Philippine Mobile Belt and the Palawan Microcontinental Block. In addition, differences in the clastic compositions of the Cenozoic sedimentary formations provide material not only for deciphering the ZOC's unroofing history but also for constraining the timing of province linkage. The intermittent appearance of lithic fragments and detrital minerals from the ophiolite in the units of the Middle Miocene Candelaria Limestone and the Late Miocene to Early Pliocene Sta. Cruz Formation indicates significant but geographically variable contributions from the ophiolite complex. In the northern Zambales Range, the Sta. Cruz Formation caps the Coto Block and the Acoje Block of the ZOC, providing a minimum age for their amalgamation.