ANOTHER COASTAL NEW ENGLAND PLUTONIC SUITE: A POST-ACADIAN, EXTENSIONAL MAGMATIC PULSE AND ITS INFLUENCE ON THE PETROGENESIS OF LATE DEVONIAN – CARBONIFEROUS GRANITIC PLUTONS

Several Late Devonian to Carboniferous plutons and lavas across southeastern New Hampshire, coastal Maine and north-central Massachusetts have within-plate compositions that are distinct from the ~400 Ma, calc-alkaline plutons related to the Acadian Orogeny in the same region. These are the Appledore Island Diorite, Salamander Point Diorite, Rochester and Hardwick plutons. Additionally, the basalts of the Perry Formation and the mafic portions of composite dikes of the Deer Isle pluton of coastal Maine are of the same age and composition. The ages of these plutons and lavas range from 380 Ma (Salamander Point Diorite) to 360 Ma (Appledore Island and Hardwick) and are compositionally equivalent to within-plate tholeiites of the same age of Maritime Canada and the Narragansett Basin.

The petrogenesis of Late Devonian to Carboniferous peraluminous and metaluminous granitic plutons across central/coastal New England is problematic because the mafic plutons indicate that the region was undergoing extension during this time period. Comagmatic association of granitic rocks with these within-plate mafic rocks occur as composite dikes in the Deer Island pluton and intermingled dioritic enclaves in the granitic phase of the Appledore Island Diorite. If the two magma types were coeval, we can argue that extensional mafic magmas may have provided the heat flux for partial melting and account for the production of granitic magmatism in an extensional setting.