Geochemistry and tectonic significance of the mafic volcanic blocks of the Dunnage melange, north central Newfoundland.

Abundant volcanic blocks are present in the Dunnage melange. These mafic volcanic rocks consist predominantly of pillow lava, tuff breccia, isolated pillow-tuff breccia, and minor amounts of ropey lava. Major- and trace-element compositions of the basalts show that these volcanic rocks do not resemble calc-alkaline or low-K island-arc suites. The majority of the samples are enriched-type ocean-floor tholeiites, whereas some show alkali basalt affinities. Discrimination diagrams suggest that these basalts may have been erupted as within-plate basalts, but the chemical composition of the volcanic blocks is most similar to that of basalts generated at bathymetric highs located astride (or slightly off) mid-ocean ridges. The geochemistry of the Dunnage melange is similar to that of the mafic volcanic rocks from the nearby Summerford group and the Lawrence Head formation. This correlation is supported by sedimentary and petrographic evidence and by partial age equivalency. Major- and trace-element analyses (13) of volcanic blocks in the Dunnage melange are compared with average chemical composition of basalts from various tectonic environments, and averages of major- and trace-element concentrations in basalts from modern oceanic islands, seamounts and ridges are compared with averages of the Dunnage, Summerford and Lawrence Head mafic volcanic rocks. -P.Br.