BEDROCK GEOLOGY OF THE WINCHENDON 1:25,000 QUADRANGLE (MA-NH) AND IMPLICATIONS FOR THE BRONSON HILL-CENTRAL MAINE BOUNDARY ZONE

Most terrane maps of New England show the Winchendon quad of north-central MA and southern NH lying east of the Bronson Hill-Central Maine boundary zone (BHCMBZ). The bedrock geologic map of MA (Zen et al., 1983) shows the quad as underlain by a folded sequence of Ord. and Dev. strata abutting the eastern margin of elongate Dev. plutons. New 1:25000 bedrock mapping reveals strong constrictional strains in metasedimentary and metaigneous units across the entire quad. In the western portion, a 2-3 km-wide belt of deformation consists of highly strained Coys Hill and Hardwick plutons and Miss. Fitzwilliam granite interfolded with Sil. Rangeley metasediments. High-strain features include increased flattening of a NNE-SSW striking foliation and grain-size reduction at the margins of the plutons. This is accompanied by a consistent SSW plunging Sil stretching lineation in Rangeley metasediments. Included within the zone is the “Templeton Tectonic Mélange” (TTM) consisting of tightly and recumbently interfolded Hardwick and Rangeley gneisses and leucogranites, S-C fabrics, and 1-30 cm diameter pegmatite and tonalite clasts that result from NNE directed shearing. East of the high-strain zone, deformation is recorded by structures that illustrate strong flattening strains, accompanied by sillimanite and quartz rod stretching fabrics in the Rangeley metasediments with minor Paxton calc-silicates. Constrictional structures include strong foliations, tight isoclinal folding with hinge lines parallel to SSW-plunging Sil lineations, S-C fabrics and pinch-and-swell structures. Rangeley schists often enclose 1-10 meter scale pegmatite or granite lenses that exhibit foliated margins and are boudinaged. Rangeley metasediments reached upper amphibolites facies metamorphism (Sil + Grt + Musc + Bio + Plag). However, foliation, S-C and C' fabrics are typically defined by greenschist facies assemblages, (fabric-forming, aligned chlorite and fine-grained muscovite). Constrictional strains found within the Winchendon quad are interpreted to be the result of regional dextral transpression. The BHCMBZ is a high-strain zone based on the differences of deformation styles on opposing sides and outside the zone; it correlates along strike with the Conant Brook shear zone, which is the BHCMBZ in southern MA/northern CT.