A REVIEW OF THE CASCO BAY GROUP AND FALMOUTH-BRUNSWICK SEQUENCES OF SOUTHERN MAINE: REGIONAL SIGNIFICANCE OF AN EXTENSIVE MIDDLE TO LATE ORDOVICIAN PERI-GONDWANAN VOLCANIC ARC TERRANE

Metamorphosed Middle to Late Ordovician volcanic and volcanogenic sedimentary rocks of the Casco Bay Group and Falmouth-Brunswick sequence are exposed in an approximately 170 km long, relatively narrow (< 25 km), northeast-trending belt in southern Maine. Lying along the eastern margin of a huge expanse of Silurian turbidites (Central Maine belt), these rocks occupy an important temporal and spatial position in the northern Appalachians and provide a window into the pre-Acadian tectonic history of the region.

The Casco Bay Group is dominated by meta-volcanic rocks that display arc and back-arc geochemical signatures, and associated meta-volcanogenic sedimentary rocks. Horizons of rusty-weathering, carbonaceous schist and rare metamorphosed Fe- and Mn-rich rocks are useful for correlative purposes and provide insight into the Middle Ordovician depositional setting. The Falmouth-Brunswick (F-B) sequence is exposed immediately northwest of the Casco Bay Group and the length of the contact between the two belts has been complicated by late faulting. The F-B sequence contains a higher proportion of meta-volcanogenic sedimentary rocks, but available whole rock geochemistry from meta-volcanic rocks is consistent with a magmatic arc tectonic setting. Available radiometric ages from the F-B sequence protoliths are more variable, but overlap with those from the Casco Bay Group.

Detrital zircon ages and Nd isotopic compositions from rocks of the Casco Bay Group are both consistent with a peri-Gondwanan, Gander terrane tectonic affinity. Protoliths of both the Casco Bay Group and F-B sequences are consistent with formation in an evolving volcanic arc to back-arc tectonic setting in Middle to Late Ordovician time. Correlations with the Bathurst Supergroup in the Miramichi belt of northern New Brunswick suggest elements of the Tetagouche-Exploits back-arc basin can be confidently traced into southern Maine. The Rye Complex in coastal New Hampshire represents a likely continuation of this major Ordovician tectonic province to the south. The Nashoba terrane of southern New England, while containing Cambrian rocks, also exhibits characteristics consistent with formation in a back-arc setting and may allow for the continuation of the Tetagouche-Exploits back-arc basin intermittently for a distance of over 800 kms.