Northeastern Luzon is composed of two principal mountain ranges: the Northem Siena Madre
(NSM) and the Caraballo Mountain (CM). In the NSM region, the so-called basement complex is exposed. This is composed of three distinct rock assemblages: 1) a Jurassic (?) to Early Cretaceous metamorphic and non-metamorphic ophiolitic units - the Dibut Bay Meta-ophiolite and the Casiguran Ophiolitic Belt, respectively; 2) a Late Cretaceous metamorphosed volcanic arc — the Lubingan Formation and the Dalugan Metamorphics and 3) a late Late Cretaceous volcanic complex — the Dibuakag Volcanic Complex. Small outcrops of isolated peridotites and schists are likewise exposed on the northern portions of the Caraballo Mountain. Its proximity to the pre-Late Eocene Pingkian Ophiolite makes it more likely correlatable to this metamorphic unit, rather than with those ultramaiic rocks exposed in the NSM.

In the Polillo Group of Islands (PGI), the oldest rock unit is represented by ultramafics and related metamorphic rocks found in Buhang Point, northwest of Polillo Island and in Jomalig Island. The ultramafic rocks in Buhang Point are composed of a small outcrop of serpentinized pyroxenite, metamorphosed massive (pegmatitic) and hightly foliated layered gabbro, while the mafic rocks crop out as small portions of pillow basalt. In this study we prefer to name this rock assemblage as the Buhang point Meta-ophiolite. Small exposures of isolated ultramafic rocks were also reported east-southeast of Barrio Sabang, south of Polillo town cropping out near metamorphic rocks belonging to the Quidadanom Schist. Regionally, the meta-ophiolitic rocks in the PGI are correlated with the Dibut Bay Meta-ophiolite found in northeastern Luzon and the Lagonoy Meta-ophiolite found in Caramoan Peninsula dated as Late Jumssic to Early Cretaceous. These meta-ophiolitic sequences are thought to represent the metamorphic equivalent of the Casiguran Ophiolitic Belt. The above-mentioned ophiolitic units lie in fault contact with the Eocene magmatic arc and both are in turn unconformably overlain by Late Oligocene to Middle Miocene sedimentary clastic deposits.

The presence of these ophiolitic rocks together with island arc rock sequences and associated sedimentation gives evidence for the juxtaposition of allochtonous terranes. The tectonic accretion of the ophiolites with the island arc rock sequences is a result of the dynamic structuring of the Philippine Mobile Belt during the pre-Tertiary time.