TECTONIC SETTINGS OF ORDOVICIAN AND DEVONIAN MAFIC ROCKS NEAR CHESUNCOOK LAKE, NORTHERN MAINE

Field relationships and geochemistry of the Bean Brook Gabbro and equivalents (Boom House Gabbro) indicate a correlation with the Ordovician Dry Way Volcanics at the Ripogenus Dam, northern Maine. The gabbros (K/Ar age 472.5 Ma) intrude the Hurricane Mountain Mélange and related Cambrian sedimentary strata, but are conspicuously absent above the Taconic unconformity at the base of the Siluro-Devonian. This temporal distribution of gabbros is similar to that seen in the Ordovician section of the Exploits Terrane and Dunnage Melange in Newfoundland. Geochemical tectonic discrimination and trace element patterns of Ordovician and Devonian basalts (Dry Way and West Branch), and Ordovician gabbros (Bean Brook and Boom House), indicate that all were geochemically influenced by subduction-related sources. Ordovician basalts and gabbros range from tholeiitic arc to calc-alkaline arc on Th/Ta/Hf/3 and Ta/Yb vs Th/Yb diagrams. Trace element diagrams of the Ordovician lavas and gabbros show low amounts of enrichment of the incompatible trace elements (relative to N-MORB) with a consistent Nb-anomaly suggesting that the Bean Brook and Boom House Gabbros are genetically related to the nearby Dry Way volcanics. REE patterns for the Bean Brook and Boom House gabbros and Ordovician Dry Way volcanics all show similar flat patterns, although at elevated concentrations (relative to chondrite), inconsistent with a direct MORB origin. These magmas could be the result of a ridge subduction event, or early stage arc formation.

The Devonian volcanics were erupted in a rapidly subsiding basin dominated by fine-grained mudrocks and subsequently overlain by the Acadian Seboomook Flysch. This suggests that these basalts may have been erupted on the outer trench slope of a subducting plate. The basalts show a significant Nb-anomaly but are strongly calc-alkaline-to-shoshonitic, consistent with either a volcanic arc or a within-plate origin. REE patterns are highly elevated (relative to chondrite) and show a negative slope also consistent with a volcanic arc or within-plate setting, or with melting of a slab-enriched lithosphere source during an early stage of the Acadian collision.