AN ANALYSIS OF METAMORPHISM ACROSS THE SENNEBEC POND FAULT, APPLETON, ME

We investigated the Sennebec Pond Fault in Appleton, ME. This site was discovered in a gravel pit during recent 1:24000-scale mapping (West et. al, 2006). The fault appears as a 40-m-wide, NE-trending depression separating outcrops of the Appleton Ridge Formation (ARF) to the NW and the Sennebec Pond Fault Complex (SPFC) to the SE. We sampled across the fault using a core drill and made thin sections from strike-parallel, foliation-perpendicular and strike-perpendicular, vertical cuts of each core. We evaluated mineralogy, metamorphic paragenesis, quartz recrystallization mechanisms, and shear sense in each section. Metapelites dominate both sides of the fault and record the prograde assemblage andalusite + muscovite + biotite + garnet that suggests high-Al bulk compositions and low-pressure metamorphism under upper-greenschist-facies conditions. Metapelites in the SPFC are identical to those assigned to the ARF on the NW side of the fault. The SPFC also contains an ~11-m-wide band of biotite-bearing quartzite and an outcrop of rusty-weathering biotite + plagioclase schist with calc-silicate layers. The former correlates well with the Bucksport Formation to the NW, and the latter correlates well with the Penobscot Formation, which is mapped directly SE of the SPFC. Chlorite coronae, replacement of biotite by chlorite, and pseudomorph replacement of andalusite by muscovite record retrograde metamorphism in metapelites. Retrograde minerals do not exhibit any deformation fabric, and quartz throughout the field area has equigranular texture with nearly straight extinction. Quartz grains also host many inclusion trails crossing grain boundaries that we interpret as healed fractures. These features indicate that retrograde metamorphism was driven by high fluid interaction under lower-greenschist-facies conditions during localized fault motion. Microfabrics throughout the site do not display a clear sense of motion. However, at the macro scale, andalusite porphyroblasts on subhorizontal faces in the ARF on the NW side of the fault indicate dextral motion. The inclusion of adjacent units with identical prograde assemblages in the SPFC and the lack of deformation associated with retrograde metamorphism both indicate that this portion of the Sennebec Pond fault is not a major terrane boundary.