Rolando E. Peña’s research while affiliated with University of the Philippines System and other places

The Baguio District, located in northern Luzon (the Philippines), is one of the major gold and copper mining districts in the Philippines and SE Asia. U–Pb zircon ages for igneous rocks and for detrital zircons extracted from sediment samples of the principal formations of the Baguio District reveal major magmatic pulses at ca. 1, 3, 16, and 20 Ma. The oldest zircon dated in this study has an age of 27.3 ± 1.6 (2σ) indicating earlier activity which has, however, left only a small imprint on the zircon inventory of the sediments. The absence of older zircons suggests that northern Luzon formed as an intra‐oceanic arc. Zircon Hf isotope data suggests that the magmas, from which the zircons crystallized, were derived from time‐integrated highly depleted mantle sources, compatible with an intra‐oceanic origin of the island. The results of the U–Pb dating of zircons mostly confirm previous estimates of the ages of the rock units. Mid‐Miocene ages obtained for samples thought to represent the Pugo Formation, the lowermost unit, which is thought to be Cretaceous‐Eocene in age, most likely date magmatic rocks that intruded the Pugo Formation.

The Philippine Archipelago comprises two types of crusts: volcanic island arc and continent. Sandstones from the Philippines contain important information to reconstruct the development history of the Philippine Arc. Sandstone composition reflects the geology of the source area when the rock was deposited. The sandstones from the Middle Eocene Cabog Formation of the Philippine Mobile Belt (PMB) which belong to a group of the oldest sandstones in the PMB are composed of clasts of plagioclase, pyroxene, basaltic rock fragment with very poor quartz. The sandstones from the upper Oligocene to lower Miocene Zigzag Formation, middle Miocene Klondyke Formation and upper Miocene to Pliocene Amlang Formatin of the PMB have basaltic to andesitic composition. The sandstone compositions of the PMB suggest that the PMB was formed in Eocene as a primitive basaltic volcanic arc and developed basaltic to andesitic volcanic arc in Oligocene to Pliocene time. Sandstones from the upper Cretaceous succession in the Palawan Continental Block (PCB) are rich in clasts of quartz and felsic volcanic rock fragments and contain K-feldspar. It suggests that the PCB was part of a continent and supports the idea that the PCB collided with the PMB and formed the Philippine Archipelago.

The Palawan Continental Terrane (PCT) is a fragment of the margin of SE China that drifted south as a result of the Cenozoic opening of the South China Sea. This fragment is of great interest as it may contain a record of the early history of the continental margin of SE China that is not exposed on the mainland. The age and potential correlations of meta-sediments exposed on Palawan Island, Philippines, with those exposed on the other islands of Philippine Archipelago and Asia are a long standing problem of the geology of this island. Given the presence of non-metamorphic sedimentary sequences of Permian and Triassic age in the northern part of the island, a Paleozoic age was inferred for the metamorphics but recently also a younger age for these rocks was considered possible. U/Pb dating of detrital zircons by the laserablation ICP-MS method reveals the presence of 80-98 Ma old zircons in all of the usually distinguished units, hence despite significant differences in degree of metamorphism and tectonic deformation, all meta-sediments appear to be of late Cretaceous or younger age. Thus, in principle, these clastic rocks could be overly derived from the older sequences. However, the relatively higher degree of metamorphism of the younger rocks suggests a tectonic contact with the older sequences. (C) 2012 Elsevier Ltd. All rights reserved.

The Philippine Mobile Belt (PMB) is a zone of intense deformation and active seismicity between convergent zones bounding the Philippine Archipelago. This zone was first defined by Gervasio (1967) to distinguish the seismically active portion of the Philippine Archipelago from the southwestern region of the Philippines. According to Gervasio, the “mobile belt”, includes North Luzon, South Luzon, West Visayas, Northwest Mindanao, Bicol Region, East Visayas, Zamboanga, Cotabato and the rest of Mindanao with Catanduanes Island representing the east outer zone of the PMB. The aseismic region, on the other hand, is constituted by Palawan and Mindoro. In the plate tectonics framework, the PMB represents a zone of deformation between surrounding major plates, namely: the Philippine Sea, Eurasian (Sunda Block) and Indo-Australian Plates. Only Palawan and Mindoro, the aseismic regions of the Philippines are part of the Eurasian Plate. Based on recent Global Positioning System, gravity, and seismicity data, we reinterpret Philippine Plate boundaries that define the extent of the Philippine Mobile Belt. The eastern section of North Luzon, heretofore described as part of the PMB, moves in a northwest direction with similar velocity as the Philippine Sea Plate. This suggests that the northwestward motion of the Philippine Sea Plate at the eastern margin of Luzon is mainly decoupled along the left-lateral Polilio-Philippine fault zone. The Polilio-Philippine fault is a zone of intense seismicity and is herein regarded as the crustal tear generated by the ongoing collision of the Benham Rise with Luzon starting from 20 Ma. This interpretation considers the eastern section of North Luzon as part of the Philippine Sea Plate and has major implications to the delineation of plate boundaries of the Philippines.