GEOCHEMICAL SIGNATURES OF EARLY TO LATE DEVONIAN PLUTONS OF THE PISCATAQUIS MAGMATIC BELT, CENTRAL MAINE

Two distinct magmatic provinces are located in Maine – the Coastal Maine Magmatic Province situated in the coastal lithotectonic block, and the Piscataquis Magmatic Belt (PMB) located in an orogen parallel zone across central Maine. Both contain a broad diversity of igneous rocks and are temporally concentrated into a Siluro-Devonian phase (420-400My) and a Late Devonian phase (380–360My) with a distinct magmatic hiatus. This synthesis is an examination of the geochemical signatures of the PMB plutons to elucidate any trends and contrasts between the early and later magmatic phases.

The 40 plutons that constitute the PMB are a compositionally diverse suite ranging from gabbros to leucocratic granites emplaced at various levels in the assembling crust of the northern Appalachians. Some plutons outcrop as thin tabular sheets, e.g. the Mooselookmeguntic pluton, often with an irregular distribution of petrographic variants. Others, e.g. the Onawa and Shirley-Blanchard, demonstrate more regular compositional zoning. Their petrography and geochemistry suggest they are the products of fractionation in situ and/or at depth.

Many of the PMB plutons have typical metaluminous mineralogy with I-type signatures whereas others have peraluminous mineralogies with S-type compositions. Trace element discrimination diagrams (Pearce et al. 1984 and Harris, 1990) show the PMB plutons to have volcanic arc signatures though some have syn-collisional/upper crustal melt chemistries. On the multi-variant R1-R2 diagram (De LaRoche et al., 1980) most PMB granitoids fall in the Late Orogenic/Post Collisional Uplift field similar to the Donegal granites of Ireland (Atherton and Ghani, 2000) although some plot in the Pre-plate Collision field. These geochemical signatures suggest multiple sourcing and collection of magmas at differing crustal locations throughout the Acadian orogeny, sometimes overprinted by varying magmatic processes.