Unraveling the Origin of Mesozoic Ophiolites in the Western Pacific : Clues from Paleomagnetic and Radiolarian Biostratigraphic Data from Northern Luzon

Karlo L. Queaño

Mapua Institue of Technology Manila, Philippines
Mines and Geosciences Bureau
Quezon City, Philipines

Abstract

Radiolarian biostratigraphic results from the Ilocos region and the Casiguran Ophiolite provide evidence for the existence of an oceanic substratum upon which Luzon and neighboring regions within the Philippine archipelago were likely built. Most chert samples from Ilocos Norte contain Upper Jurassic (Tithonian) assemblages as well as distinct Lower Cretaceous (upper Barremian to lower Aptian) radiolarian assemblage. Key taxa present include Eucyrtidiellum pyramis, Hiscocapsa acuta, Protunuma japonicus, Archeodictyomitra chalilovi, Hiscocapsa asseni, Cryptamphorella conara, Pseudodictyomitra carpatica and Xitus clava. Similar radiolarian biostratigraphic results are obtained from the cherts overlying the Casiguran Ophiolite in northern Sierra Madre, the stratigraphic range being upper Barremian-Aptian/Albian. These results closely resemble those obtained from the Mesozoic “Philippine Sea Plate” ophiolite basement scattered in other regions. These include eastern Halmahera, Gag, Gebe, Waigeo, Obi and other islands along the Sorong Fault Zone in Indonesia as well as Huatung Basin situated north of Luzon.

Tectonic reconstruction for the western Pacific prior to the Cenozoic has always been problematical, with much of the pre-Cenozoic tectonic elements in the region destroyed. In the Philippines, obtaining paleomagnetic data from Mesozoic ophiolitic rocks for tectonic reconstruction have met with only limited success. Previously acquired paleomagnetic data from these units are not only sparse but also are unreliable owing to wide scatter of the paleomagnetic plots. In this research, the most reliable data come from the Cretaceous Chico River pillow basalts (Dec = 159.3°, Inc = -12.5; a95=6.0o, k= 162.5) in the Central Cordillera suggesting formation at subequatorial region (6.3°N ±3.1°). Interestingly, this result is closely similar to that gathered previously from a Mesozoic ophiolite on Obi Island in eastern Indonesia. The similarity in the age, paleomagnetic signature, and to a certain extent, geochemistry of the ophiolite rocks in the western Pacific region suggests that these rocks might have a common, subequatorial provenance or basin origin.