U–Pb Geochronology of the Baguio Area, a Major Mining District in Northern Luzon (Philippines)

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.