Bedrock geology of the Winchendon quadrangle is divided into two zones based on structures and lithology. The eastern zone consists of tightly folded Silurian Rangeley and Paxton metasediments and pegmatites. Planar, NNE-SSW striking, W dipping foliations are characterized by alternating phyllosilicate-rich and leucosome layers. Fold axes of tight to isoclinal upright and recumbent folding are parallel to SSW-NNE shallowly plunging sillimanite, quartz and muscovite lineations. In the western zone Rangeley schists were intruded by Devonian Hardwick and Coys Hill and Fitzwilliam plutons. Planar NNE-SSW striking foliations dip shallowly to steeply west. Fold axes of tight-to-isoclinal asymmetric to recumbent folds plunge shallowly to steeply SSW, parallel to elongate quartz and feldspar lineations. Fabrics in both portions developed in by greenschist facies mineral assemblages Asymmetric porphyroclasts and S-C/C’ fabrics from both areas display dextral asymmetry. Lateral extrusion as the result of pure-shear dominated E-W shortening and N-S extension is recorded by structures that exhibit strong contractional strains, accompanied by stretching fabrics. A 2-3 kilometer wide belt of deformation named the Bronson Hill-Central Maine Boundary Zone (BHCMBZ) correlates with the Conant Brook Shear Zone. This creates a zone of deformation that widens to the north and separates Ordovician plutons of the Bronson Hill zone from units of the Central Maine zone. North of Winchendon, retrograde deformation is absent and Acadian metamorphism and structures are preserved. This creates an inconsistency in along-strike deformation the length of the inferred terrane boundary. A larger terrane, composed of the Bronson Hill and Central Maine zones is proposed to account for the discontinuous deformation along strike.

The Belchertown Intrusive Complex is a ~164 km2 Devonian pluton that intruded Paleozoic metasedimentary and metavolcanic rocks in west-central Massachusetts. Intrusion of the pluton was synchronous with Acadian deformation (Ashwal, 1974). The complex is concentrically zoned, with a core of orthopyroxene-biotite monzodiorite, a middle zone of clinopyroxene-hornblende-biotite granodiorite, and an outermost zone of hornblende-biotite granodiorite. Zoning from a more to less hydrous mineral suite from the outside to the inside of the pluton led Ashwal (1974) to suggest that metamorphic hydration most strongly affected the outermost zones of the complex. Basaltic inclusions occur most commonly near the edges of the pluton. Many of these inclusions preserve textures suggestive of mafic-felsic magma interaction. The abundance of basaltic enclaves on the edges of the complex may suggest a bowl-shaped structure, with lower more mafic-dominated rocks exposed on the edges of the complex. Bulk rock major element analyses of granitoids, basalts and gabbros, and diorites from all zones of the complex show that intermediate samples, regardless of zone, plot on a mixing line between mafic and felsic end members, supporting a model in which mafic and felsic magmas may have mingled and mixed. Trace element analyses of intermediate-composition samples are similar across the entire complex, with enrichment in large ion lithophile elements and a pronounced Nb trough. In contrast, trace element concentrations in both the mafic rocks (basaltic enclaves and gabbroic inclusions), and in the granitoids, show considerable variation. The diversity of composition within both the mafic and felsic end-members suggests that either fractionation or differing degrees of partial melting of source rocks may account for these compositional ranges. Sanukitoids comprise a majority of the inner zone, and point to the mixing of parental melts at depth in a subduction tectonic regime as a likely model of petrogenesis for the complex.