PETROGENETIC HISTORY AND GEOTHERMAL POTENTIAL OF THE ANDOVER GRANITE, NORTHEASTERN MASSACHUSETTS

Northeastern Massachusetts can be described as a series of fault-bounded, northeast-striking accretionary terranes associated with the Acadian orogeny. One of these, the Nashoba Terrane, is located northwest of the Clinton-Newbury fault zone and southeast of the Bloody Bluff fault zone and includes the felsic, heterogeneous Andover granite. The Andover granite exhibits a variable gneissic fabric and localized pegmatites and aplitic dikes. Initial geochemical studies of samples from the Andover granite classify it is a peraluminous granite. Plots of Rb and Y compared to Nb suggest a syn-tectonic/volcanic arc affinity, in agreement with previous interpretations. This study has two aims: 1) obtain petrographic and geochemical data to understand the petrogenesis of the Andover granite in the context of Acadian orogenesis; and 2) evaluate the geothermal potential of the Andover granite. The second goal is part of a larger, statewide project focused on evaluating the geothermal potential of granitoids in Massachusetts. Geochemical analyses include the heat producing elements U, Th, and K. Heat source candidacy requires temperatures that significantly exceed those of the average continental geotherm. A simplified heat production equation, including geologic constraints, has been applied to geochemical data from the Andover granite. Modeling parameters assume uniform 2.65 kg/m³density, thermal conductivity of 2.9 W/m°C and that sample compositions represent the granite at depth. Sample analyses obtained to date indicate variable U, Th, and K₂O concentrations of 0.4 to 20.1 ppm, 0.3 to12.1 ppm and 0.73 to 8.03 Wt. %, respectively. Preliminary temperature modeling calculations indicate that Andover granite lacks required elemental abundances for local energy source production. However, one sample yields a maximum temperature of 129.5 °C at depths of 7.5km (123.9°C ≈ geotherm) and minimum temperature of 20.1°C at 0.5km depths (19.1°C ≈ geotherm). This sample, located in the pegmatitic phase of the granitoid, indicates a possible presence of local hot phases, in the Andover granite. Future work will focus on petrographic studies and additional whole-rock geochemistry to refine the current understanding of the emplacement and development of the Andover granite and the geothermal potential of this granite body.