Igneous geochemistry of mafic rocks in the Betts Cove ophiolite, Newfoundland

Abstract

The Betts Cove ophiolite, Newfoundland, consists of cumulate ultramafics, gabbro/clinopyroxenites, sheeted dikes and pillow lavas. The pillow lavas are divisible into three compositional groups: lower lavas ( 0.7 wt.% TiO2). The lower and intermediate lavas are very depleted in Ti, Zr, Y, P, and REE and have high Al2O3/TiO2 ratios relative to ‘normal’ oceanic tholeiite. The extreme depletion of these lavas and their dike equivalents (diabase and picrites) suggests they were derived by melting a severely depleted lherzolite. Conversely, the upper lavas, a volumetrically small part of the ophiolite, are compositionally similar to fractionated oceanic tholeiite and thus, their source material may be like that postulated for modern ocean floor basalts. Whereas the majority of basalts in the Betts Cove ophiolite are depleted in ‘incompatible’ elements, most dikes and lavas from the Blow-MeDown ophiolite, western Newfoundland, are not and have incompatible element concentrations similar to modern oceanic tholeiite. The chemical differences between the two ophiolite massifs are related to melting of ultramafic source materials which are in different states of depletion brought about by previous melting episodes.