PETROGRAPHY, GEOCHEMISTRY AND CRYSTALLIZATION HISTORY OF LATE DEVONIAN GRANITIC PLUTONS OF THE PISCATAQUIS MAGMATIC BELT, CENTRAL MAINE

Granitoid plutons of the Piscataquis Magmatic belt, central Maine, span a wide spectrum of a) ages, from Early to Late Devonian, and b) composition, from metaluminous granodiorites and granites to two-mica peraluminous granites. They also vary in intrusive style – concentrically zoned intrusions, small stocks and flat-lying sill-like bodies. This study focuses on the group of Late Devonian metaluminous plutons and examines their petrography, geochemistry and crystallization history.

The plutons investigated intrude along the strike of the orogen and are, from SW to NE, the Songo granodiorite (382 Ma), the metaluminous phase of the Mooselookmeguntic pluton (389 Ma), the northern lobe of the Lexington pluton (365 Ma), the Dodling Hill phase of the Rome – Norridgewock pluton (undated), and the Beaver Cove pluton (372 Ma). All have typical I-type metaluminous mineralogies with bt ± hb ± sp and often contain zoned plagioclase feldspar. Primary epidote may also be present in some of these intrusions, but has not been confirmed. Geochemically they range in SiO₂ content from 65 – 72 wt. %, with A/CNK ratios ~ 1 and have a volcanic arc affinity on the granitic discrimination diagrams.

Collectively these plutons represent some of the youngest intrusions associated with the Acadian orogeny but their origin is problematic. Investigation of the mineral chemistry of these granites, particularly the possibility of magmatic epidote and application of the Al in hornblende geobarometer, may help constrain their origin.