BEDROCK GEOLOGIC MAPPING AND STRUCTURAL ANALYSIS OF THE WESTERN HALF OF THE PETERSHAM QUADRANGLE, CENTRAL MASSACHUSETTS: FURTHER TESTS OF THE MODEL FOR MIDDLE TO LATE PALEOZOIC DUCTILE TRANSPRESSION, VERTICAL EXTRUSION, AND LATERAL ESCAPE IN THE NORTHERN APPALACHIANS

Abstract

Bedrock mapping, structural analysis, and geochronology reveal the distribution of lithologies and timing of metamorphism and deformation in the western half of the Petersham 7.5’ quadrangle, western Massachusetts. Underlying lithologies are: (from west to east) the Ordovician Monson granitic orthogneiss, Silurian Rangeley migmatitic paragneiss, and Late Devonian (357 Ma) Hardwick tonalitic orthogneiss. Their tightly folded contacts strike north to south. The 361 Ma, unfoliated, strike-parallel Nichewaug quartz-diorite (10-100 m wide) intrusion spans the map area within the Rangeley. Evidence for vertical and lateral extrusion/escape of the Monson orthogneiss, as observed in the Palmer MA area, is absent. Instead, petrofabrics (foliations and lineations) indicate E-W shortening and N-S stretching concentrated within the Rangeley Fm. and orthogneiss margins. Asymmetric structures at the Rangeley-Hardwick contact indicate localized sinistral displacement parallel to unit boundaries and tectonic fabrics. U-Th-Pb chemical age dating of Rangeley monazite revealed three precise age populations (1 = 344 Ma; 3 = 377 Ma; 4 = 405 Ma) and one broad population divided into two subpopulations: 2b (~370 Ma) and 2a (~ 360 Ma). The similarity in age between tonalitic/dioritic magmatism and monazite growth in the Rangeley suggests regional metamorphism was driven by magmatic heat input from latest Devonian/earliest Carboniferous plutonism