IGNEOUS PETROGENESIS AND OVERPRINTING DEFORMATION AND METAMORPHISM OF THE EARLY DEVONIAN EDGECOMB GNEISS, MID-COASTAL MAINE

The Edgecomb Gneiss is a deformed and metamorphosed igneous intrusion exposed near Wiscasset in mid-coastal Maine. The gneiss is found in several sill-like bodies less than 200 meters wide adjacent to the Boothbay thrust, which juxtaposes two regionally extensive lithotectonic terranes: the Ordovician Casco Bay Group, and the Silurian Fredericton Trough. Structural, petrologic, and geochemical analysis of the Edgecomb Gneiss, in conjunction with new U-Pb geochronology, provide constraints on the tectonic setting of magma emplacement and subsequent timing of overprinting deformation and metamorphism.

The Edgecomb Gneiss contains large light-colored plagioclase augen set in a dark-colored strongly foliated matrix dominated by biotite and amphibole. Field work shows these rocks are contained entirely within the Fredericton Trough and do not cut the Boothbay thrust fault (D₁ deformational event). The gneiss has been overprinted by upright isoclinal folding that is consistent with D₂ regional deformation and has been subjected to regional amphibolite facies metamorphism. Whole rock geochemistry shows the Edgecomb Gneiss is intermediate in terms of SiO₂ content (55 to 62 wt.%) but has somewhat unusual trace element signatures. Specifically, it is relatively primitive with high concentrations of MgO, Ni, and Cr, but also relatively enriched in alkali elements. Chondrite-normalized REE plots show a pronounced enrichment in light REEs, and spider plots show a pronounced negative Nb anomaly. U-Pb SHRIMP-RG analysis of zircon cores indicate that the igneous protolith of the Edgecomb Gneiss crystallized at 414 ± 5 Ma (2σ). Rim ages cluster in two populations: 402 ± 8 Ma (2σ) and 373 ± 9 Ma (2σ) and are interpreted to reflect the timing of overprinting deformation and metamorphism.

The age and geochemical signature of the Edgecomb Gneiss is similar to the ultrapotassic Lincoln syenite and Turner Mountain syenite bodies exposed along strike to the northeast. This magmatism, exposed along a distance of over 150 km, was likely generated during latest Silurian to earliest Devonian decompression partial melting of a previously metasomatized mantle wedge that lay beneath the Fredericton Trough. Amphibolite facies metamorphism and upright isoclinal folding (D₂) post-dates the intrusion of the Edgecomb Gneiss protolith.