PETROLOGY AND GEOCHEMISTRY OF FAULT AND SHEAR ZONE - RELATED ROCKS, SOUTH CENTRAL MAINE

Mylonites of the Whitten Hill mylonite zone and Sunny Side fault contain rocks which have equilibrated in the lower to lower – middle amphibolite facies. The Whitten Hill mylonite zone has a maximum width of approximately 600 m. Within this zone is a several meter wide band of hornblende bearing, lineated and banded striped gneiss. The overall mineralogy suggests that this rock equilibrated in the lower to middle amphibolite facies. This striped gneiss may have had a Nehumkeag Pond Formation protolith. The Sunny Side fault which crops out to the southeast of the Whitten Hill zone, is commonly less than 100 m wide and ranges from phyllonite to ultramylonite. Well developed garnet - biotite coticules are locally present. The Sunny Side fault everywhere displays mylonitic foliation and lineation. Textures, mineralogy and geochemistry suggest that the coticules were derived from a metasedimentary protolith, and equilibrated in lower amphibolite facies. Rocks between the Whitten Hill and Sunny Side mylonites are extensively sheared and exhibit mineralogies consistent with middle and upper -middle amphibolite facies (staurolite/andalusite and sillimanite zones). In the area between the Whitten Hill and Sunny Side faults, units are assigned to metasedimentary formations consistent with the Casco Bay Group formations. In these rocks, andalusite commonly occurs as shear - modified porphyroblasts in the presence of fine – grained sillimanite.

Southeast of the Sunny Side fault is a wide zone of ductile shear. This zone contains abundant pelitic migmatite with equilibrium assemblages consistent with upper amphibolite facies (sillimanite - K feldspar zone). The Mixer Pond map unit is an elongated belt of heterogeneous gneisses included within the ductile sheared rocks. The Mixer Pond is a heterogeneous unit with diverse mineralogy and chemistry. These rocks range from hornblende - rich amphibolites and hornblende bearing granite to microcline granite gneisses with minor accessory biotite. Field and textural evidence for the Mixer Pond, together with trace element geochemistry, suggests a plutonic origin. Tectonic discrimination diagrams suggest a complex origin from within plate and magmatic arc sources.