GEOLOGICAL SOCIETY OF THE PHILIPPINES
Arc Magmatism in the Northern Sierra Madre Range, Northeastern Luzon, Philippines: The Benham Rise Connection
Rocks that are products of arc magmatism related to subduction are widespread in the Philippines, especially those that were emplaced during the Neogene. Volcanic flows and igneous intrusions related to subduction during Cretaceous – Paleogene are less widespread but still significant in terms of occurrence and extent. Arc magmatism in the eastern strip of the Philippines, from northern Luzon down to southern Mindanao is manifested by rock units that date back to the Eocene. These are products of subduction along a zone associated with a proto Philippine Trench and the present Philippine Trench. From southern Luzon through eastern Visayas (Leyte and Samar) to eastern Mindanao (Pacific Cordillera), the sequence of products of magmatism seems to have continued from Paleogene to Late Neogene. However, arc magmatism in northern Sierra Madre is only confined to the Paleogene. A review of igneous rocks related to subduction in the Northern Sierra Madre Range in northeastern Luzon reveals peculiarities that are not shared with other rocks related to subduction along this eastern strip.
Whereas subduction and consequent arc magmatism from Eocene to Late Neogene is a widespread occurrence in various areas in the Philippines, the Northern Sierra Madre Range does not harbor igneous rocks that could be construed as products of Neogene arc magmatism. The cessation of arc magmatism in this area is most likely related to the cessation of seafloor spreading from the Central Basin Spreading Center that is presently located east of Northern Sierra Madre Range, attendant to the collision of Benham Rise with northern Luzon – northern Bicol in earliest Miocene time (20 – 23 Ma). What remains as a vestige of the Paleogene east-facing arc is an ancient subduction zone that is presently designated as the East Luzon Trough.
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