Abstract

Ordovician volcanic rocks of the Summerford Group were assumed to be calc-alkaline basalts generated in an island arc now located in the Dunnage zone. However, chemical analyses show that these volcanic rocks are tholeiitic and alkalic basalts. Trace-element and REE abundances indicate that the basalts were generated either at an ocean island (off-axis basalt) or at a midocean ridge. Associated carbonates and siliceous sediments indicate that these basalts formed a seamount(s) that reached above the calcium compensation depth (CCD). The long time span reported for the Summerford Group (Tremadocian-Llandeilian/Caradocian) might suggest that they represent more than one seamount, perhaps a chain of seamounts. We suggest that the Summerford Group is the youngest seafloor yet reported for the northeastern Dunnage zone and that the volcanic rocks may have been underthrust into an accretionary wedge that was active in Early Silurian time.

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