Temporal Geochemical Evolution of Neogene Magmatism in the Baguio Gold - Copper Mining District (Northern Luzon, Philippines)
Résumé
Baguio, in the Central Cordillera of Northern Luzon, is a district that displays porphyry copper and epithermal gold mineralization, associated with Early Miocene - Pliocene - Quaternary calc-alkaline and adakitic intrusions. Systematic sampling, K-Ar dating, major and trace elements, and Sr, Nd, Pb isotopic analyses of fresh magmatic rocks indicate three magmatic pulses: an Early Miocene phase (21.2 - 18.7 Ma), a Middle - Late Miocene phase (15.3 - 8 Ma) and fi nally a Pliocene - Quaternary event (3 - 1 Ma). The fi rst phase emplaced evolved calc-alkaline magmas, essentially within the Agno Batholith complex, and is thought to be related to the westward-dipping subduction of the West Philippine Basin. After a quiescence period during which the Kennon limestone was deposited, magmatic activity resumed at 15.3 Ma, in connection with the start of the subduction of the South China Sea along the Manila Trench. It emplaced fi rst petrogenetically related and relatively unradiogenic low-K calc-alkaline lavas and intermediate adakites. Temporal geochemical patterns observed from 15.3 to 1 Ma include progressive enrichment in K and other large ion lithophile elements, increase in radiogenic Sr and Pb and corresponding decrease in radiogenic Nd. These features are thought to refl ect the progressive addition to the Luzon arc mantle wedge of incompatible elements largely inherited from South China Sea sediments. The origin of the long quiescence period, from 8 to 3 Ma, remains problematic. It might represent a local consequence of the docking of the Zambales ophiolitic terrane to Northern Luzon. Then, magmatic activity resumed at 3 Ma, emplacing chemically diversifi ed rocks ranging from low K to high K and including a large proportion of adakites, especially during the Quaternary (dacitic plugs). The authors tentatively relate this diversity to the development of a slab tear linked with the subduction of the fossil South China Sea ridge beneath the Baguio area.