BEDROCK GEOLOGY AND IGNEOUS GEOCHEMISTRY IN THE NORTHERN HALF OF THE BOWDOINHAM 7.5’ QUADRANGLE, SOUTHWESTERN MAINE

The Bowdoinham 7.5’ quadrangle is located just north of the Casco Bay region in southwestern Maine and has not been previously mapped at the 1:24,000 or larger scale. The quadrangle is located along the boundary between two major lithotectonic belts – the Ordovician Liberty-Orrington belt and the Late Ordovician (?)-Early Devonian central Maine belt. The purpose of our work in this quadrangle is (1) to provide a detailed bedrock map, (2) to understand the Ordovician tectonic history of rocks in the Liberty-Orrington belt through field observations and amphibolite whole rock geochemistry, and (3) to structurally, petrologically and geochemically characterize a previously unrecognized deformed intrusive body found along the contact between the Liberty-Orrington and central Maine lithotectonic belts.

All stratified rocks in the field area have been penetratively deformed, folded and metamorphosed to upper amphibolite facies conditions. Migmatites and relatively small but mappable pegmatite bodies can be found throughout the quadrangle. Foliation and compositional layering are generally parallel and strike in a north-south direction with gentle to moderate dips to the east. Rocks in the study area can be grouped into four general units – from east to west these units include: (1) migmatitic biotite gneiss with minor amphibolite and rusty weathering biotite schist/gneiss (part of the Falmouth-Brunswick sequence), (2) a relatively thin and lithologically heterogeneous sequence of biotite-garnet gneiss, calc-silicate gneiss, garnet-bearing amphibolite and very rusty weathering sillimanite schist, (3) a deformed and recrystallized suite of intrusive igneous rocks – here termed the Hornbeam Hill Intrusive Suite, and (4) interlayered biotite granofels and calc-silicate gneisses of the Hutchins Corner Formation (central Maine sequence).

Preliminary geochemical analyses of amphibolites reveal tholeiitic basalt compositions with rare earth element patterns that are generally flat to slightly LREE enriched. Overall, the trace element geochemistry is consistent with formation in a back-arc basin environment. A new U-Pb (SHRIMP) zircon age of 385 ± 5 Ma from the Hornbeam Hill Intrusive Suite is interpreted to reflect the original crystallization age of the intrusion.