The oceanic substratum of Northern Luzon: Evidence from xenoliths within Monglo adakite (the Philippines)

Abstract

Abstract A 8.65 Ma adakitic intrusive sheet exposed near Monglo village in the Baguio District of Northern Luzon contains a suite of ultramafic and mafic xenoliths including in order of abundance: spinel dunites showing typical mantle‐related textures, mineral and bulk rock compositions, and serpentinites derived from them; amphibole‐rich gabbros displaying incompatible element patterns similar to those of flat or moderately enriched back‐arc basin basalt magmas; and amphibolites derived from metabasalts and/or metagabbros of identical affinity. A single quartz diorite xenolith carrying a similar subduction‐related geochemical signature has also been sampled. One amphibolite xenolith provided a whole‐rock K–Ar age of 115.6 Ma (Barremian). We attribute the origin of this suite to the sampling by ascending adakitic magmas of a Lower Cretaceous ophiolitic complex located at a depth within the 30–35 km thick Luzon crust. It could represent an equivalent of the Isabela‐Aurora and Pugo‐Lepanto ophiolitic massifs exposed in Northern Luzon.