OCEANIC-ARC MAGMATIC PROCESSES AS RECORDED IN GABBROIC ROCKS FROM TOBAGO, WEST INDIES

On Tobago, West Indies, an Albian intraoceanic-arc plutonic suite ranging from ultramafic to tonalitic intruded the cogenetic Tobago Volcanic Group rocks (TVG) and older greenschist-facies rocks of the North Coast Schist. The plutonic suite includes abundant cumulate rocks ranging from dunite, wehrlite, olivine (+ hornblende +/- plagioclase) clinopyroxenite, hornblendite, and various gabbroic rocks. The gabbroic samples can be divided into three mineralogical-textural groups: (1) olivine melagabbro-gabbro, (2) hornblende+/-augite gabbro, and (3) porphyritic gabbronorite. Group 3 forms small intrusive bodies in the other gabbroic rocks and locally intruded the TVG. The two-pyroxene rocks are distinct from other ultramafic-mafic rocks on Tobago in that orthopyroxene is an important igneous phase in the crystallization history of these rocks. Phenocrystic augite and hornblende from the porphyritic gabbronorite are compositionally indistinguishable from those minerals in the hornblende+/-augite gabbro, and plagioclase (phenocrysts and matrix grains) exhibits strong oscillatory zoning. With the exception of a narrow sodic rim on plagioclase phenocrysts in the porphyritic gabbronorites, plagioclase phenocryst compositions are also indistinguishable between groups 2 and 3. These observations suggest that all of these minerals crystallized from compositionally similar magmas. The orthopyroxene, fine-grained matrix, and compositional zoning in plagioclase in the porphyritic gabbronorites are most probably the products of a two-stage crystallization history of gabbroic magma: early crystallization of comparatively water-rich magmas at depth first produced olivine + clinopyroxene + plagioclase, with later reaction of the olivine with melt to produce hornblende. This crystallization sequence was interrupted when crystallizing magmas were abruptly transported to shallower depth. This decompression event resulted in some dehydration, a change in crystallization path, and probably some degree of supercooling of the magma. Decompression resulted in the crystallization of more sodic plagioclase preserved on the rims of the phenocrysts and in the plagioclase crystals of the matrix.