Geochemistry and plate-tectonic significance of the volcanic rocks of the Summerford Group, north-central Newfoundland.

Petrography and field evidence led to the assumption that Ordovician volcanic rocks of the Summerford group were calc- alkaline basalts generated in an island arc now located in the Dunnage zone. However, chemical analyses, in particular trace-element abundances, show that these rocks are tholeiitic and alkali basalts and were generated either at an ocean island or at a mid-ocean ridge. The long time span reported for the Summerford group and the associated carbonate and siliceous sediments indicates that these basalts may form a chain of sea-mounts that reached above the calcium compensation depth. It is suggested that the Summerford group is the youngest sea-floor yet reported for the NE Dunnage and that the volcanic rocks may have been underthrust into an accretionary wedge that was active in early Silurian time. -L.di H.