Metamorphic basement rocks of Yap Islands, Western Pacific: possible oceanic crust beneath an island arc

Abstract

The basement rocks of the Yap Islands are composed principally of greenschist and amphibolite, unconformably overlain by a breccia bed with mid-Tertiary foraminifera and by a deeply weathered andesitic agglomerate. The metamorphic grade increases eastwards from the greenschist to the amphibolite facies, but the epidote amphibolite facies appears to be absent; the metamorphism is of low pressure type. Chemically the rocks of basic igneous origin are characterized by high MgO and low K 2O contents and a high ratio of Na 2O/K 2O, similar to oceanic tholeiitic basalts, but are distinguished from the Tertiary basalts of island arc tholeiitic series in Guam by lower K 2O and somewhat higher TiO 2 contents. The basic metamorphic rocks may represent part of a lower layer of oceanic crust or have formed as the earliest manifestation of island arc development.