Multiple Intrusions of Barren and Fertile Magmas in a Complex Island Arc System: Insights from Igneous Rocks in the Suyoc Area, Southern Mankayan Mineral District, Philippines

The Baguio-Mankayan Mineral District (BMMD) in Northern Luzon, Philippines, is renowned for its world-class auriferous porphyry-Cu and epithermal deposits, which are associated with arc magmatism during the Pliocene-Pleistocene. One of the less studied areas in this region is in southern Mankayan, where the Suyoc epithermal prospect is located. New U-Pb zircon ages, combined with whole-rock and in-situ geochemical analyses of igneous rocks in the Suyoc area, revealed a shift from barren to fertile magmatic episodes in the BMMD. 1) Late Eocene (36.93-37.20 Ma) barren magmas generated in a thin, nascent Luzon arc, 2) Fertile Middle Pliocene (2.87-3.4 Ma) magmatism attributed to the mixing of primitive mafic and silicic crustal melts due to ridge subduction, and 3) Early Pleistocene (1.73 Ma) fertile magmatism associated with more hydrous melts due to progressive crustal thickening. The Late Eocene barren magmatism enhanced both source and intra-crustal conditions for the onset of younger fertile magmatic events via sub-arc modification and crustal addition/ thickening. Furthermore, we consider the fertile Pliocene and Pleistocene magmatism in the BMMD as two separate events with different petrogenesis. Mafic-silicic interaction in the Pliocene was documented through core-rim analysis of amphiboles from the Pliocene Suyoc Diorite. On the other hand, the Pleistocene fertile magmatism, represented by the Bato Dacite Porphyry, showed geochemical signatures consistent with a thick-ened crust and also reflected higher magmatic H2O contents ranging from 6.3 to 7.8 wt%.