Regional tectonic setting and island arc magmatic rocks of the Philippine Islands

      The Philippine Islands, located at the easternmost edge of Southeast Asia, are formed by the merging and amalgamation of continental and oceanic island arcs. They are surrounded by still-active subduction zones, including the Philippine Sea Plate, the South China Sea Plate, the Sulu Sea Plate, and the Celebes Sea Plate, which subduct from the east and west sides of the Philippine Islands (Figure 1). The three marginal basins of the Celebes Sea, the South China Sea, and the Sulu Sea all formed within the Tethys tectonic domain, opening successively in the Middle Eocene (47 Ma), the Oligocene (33–15 Ma), and the Early Miocene (18 Ma), respectively. Their tectonic evolution is closely related to the closure of the Neo-Tethys Ocean and the subduction-collision process of the Indian Plate along the Sunda Trench. From 45 to 25 Ma, influenced by the northward movement of the Australian Plate, a northward subduction zone formed in the Philippine Islands-Hamahera island arc. This subduction continued until approximately 25 Ma, when the Australian Plate collided with the Philippine-Hamahera island arc in New Guinea, ending the northward subduction of the oceanic lithosphere north of Australia. Since the Cretaceous, magmatism in this region has been consistently active, reaching its peak in the Cenozoic era, forming a series of magmatic rock belts spanning the Cretaceous, Eocene, Oligocene, Miocene, Pliocene, and Quaternary periods. The vast majority of magmatic activity is related to subduction events.

     Figure 1. Map of tectonic-magmatic activity and distribution of major porphyry copper-gold deposits in the Philippines.

       Geotectonically, the Philippine Islands are located at the boundary between the Eurasian and West Philippine Sea plates, forming a terrane composed of ocean, continent, and island arcs. This region experiences intense seismic activity and numerous active volcanoes. Apart from the continental basement rocks originating from the Eurasian Plate in areas such as Palawan, Mindoro, Rampron, western Mindanao (Zamboanga), parts of Panay, and northwestern Luzon, the Philippine Islands and surrounding blocks are often referred to in academic circles as the "Philippine Active Zone," formed within an intra-oceanic island arc of the Philippine Sea/Pacific Plate. It is generally believed that this active zone formed an intra-oceanic island arc between the Philippine and Pacific Plates. The Philippine active zone is surrounded by several subduction zones in its eastern and western parts (Figure 1). The eastern island arc, located in the East Luzon Trough-Philippine Trench, is associated with westward subduction of pelagic sediments. The western island arc (Figure 1), located between the Manila, Sulu-Negros, and Cotabato Trenches, is associated with eastward subduction of the South China Sea (Early Oligocene-Early Miocene), Sulu Sea (Early Miocene), and Celebes Sea (Eocene) basins, which are covered by terrigenous sediments. Some scholars have proposed that in addition to the central Mindanao island arc, there may also be a northern extension of the Sangeehe island arc in southern Mindanao. The Philippine magmatic active zone is a complex assemblage dominated by exogenous terranes. Its main tectonic origin is in the South Pacific, likely stemming from a faulting of the Indo-Australian Plate during the Neogene, which subsequently merged to form the present tectonic structure. The Philippine active zone is longitudinally dissected by the Pliocene Philippine Fault Zone (Figure 1), a left-lateral strike-slip fault.

      The oldest metamorphic basement in the Philippine active zone originates from oceanic lithospheric fragments, including amphibolite, quartz-albite-mica schist, and serpentinite, with protoliths dating from the Jurassic to the Cretaceous periods. Most of these rocks exhibit Tethyan tectonic characteristics. Additionally, part of the active zone originates from the Indo-Australian Plate, but no ancient metamorphic basement predating the Jurassic (similar to the Australian Plate) has been found. Therefore, studying the age and properties of potentially existing ancient metamorphic basements in various regions of the Philippine active zone is of great significance, providing direct geological evidence for inverting their initial tectonic properties (whether they are merely island arcs within the Pacific Ocean or originated from the ancient Australian Plate).

      Adakistane andesite has been discovered in the Bataan region of the northern Luzon arc in the northern Philippines. Volcanic island chains distributed in the Mancayan and Baguio areas of the northern archipelago, as well as on the eastern and western sides of the central archipelago, also exhibit geochemical characteristics of adakites or adakistane rocks. These adakites are considered to be products of the subduction of the South China Sea basin along the Manila Trench into the depths of Luzon Island. The adakites discovered in the Negros Mountains region of central Negros Island are related to the subduction of the Sulu Basin along the Negros Trench. The adakites in southern Mindanao are slightly more complex, as the area is bordered by subduction zones on both the east and west sides: the Philippine Sea Plate subducts westward along the Philippine Trench, and the Celebes Sea Basin subducts northeastward along the Cotabato Trench. Furthermore, the area south of the Cotabato Fault Zone (including the present-day South Cotabato mineralization area, Figure 1) has been influenced by the westward subduction of the Moluccas Basin along the Sangeh Trench since approximately 10 Ma.

      The magmatic-mineralization activity in these regions was controlled by the subduction of the Celebes Sea, South China Sea, Sulu Sea, and Moluccas Sea basins formed within the Neo-Tethys tectonic domain. Previous studies have focused on the geological characteristics, chronology, and genesis of individual deposits and ore-bearing rocks in each region, but the sources of magmatic-ore-forming materials are rarely reported. Previous petrographic studies have mostly relied on whole-rock or potassium-bearing K-Ar dating. However, due to the low closure temperature of this dating system, the measured age may be lower than the actual age. Therefore, zircon u-Pb dating is more advantageous for accurately determining the formation age of various Cu-Au mineralization-related magmatic rocks.

      The southwestern ore cluster of Negros Island exposes Oligocene porphyry copper deposits and ore-bearing island arc rocks unrelated to the Sulu Sea subduction. In the Negros Mountains region of central Negros Island, some Holocene adakites related to the Sulu Sea subduction have been discovered. What are the differences in genesis and material origin between these two types of adakites? What type of subducting oceanic lithosphere do they correspond to? These questions also warrant further investigation. In-depth comparative studies will help understand the differences in crust-mantle composition and evolutionary history of different marginal basins on the western and southern sides of the Philippine active belt.