High level and low level plagiogranites from the nicoya ophiolite complex, Costa Rica, Central America

Abstract

Two suites of plagiogranitic rocks within the oceanic crust of the Jurassic-Cretaceous ophiolitic Nicoya complex are distinguished. High level plagiogranites (HLP) crop out at the top of the cumulate sequence in the northwestern part of the Nicoya peninsula. They are associated with iron-rich dolerites and noncumulus gabbros. Based on field relationships and geochemical characteristics (e.g. enrichment in REE and HFS elements) the origin of these rocks is explained in terms of a two stage model of fractional crystallization and filter pressing. The first stage comprises the formation of tholeiitic Fe-rich melt and a highly differentiated crystal-melt mush. The second stage is characterized by filter pressing of plagiogranitic interstitial magma. Fe-dolerites form the residual rocks. Crystal fractionation of this plagiogranitic magma is due to the formation of a rather complete series of siliceous rocks.