The Age and Petrogenesis of a Sillimanite Gneiss within the Belchertown Igneous Complex, Massachusetts

Abstract

This study applies chemical dating of monazite following the methods of Williams et al. (2017) in samples from a septum of metapelite that lies within the Belchertown intrusion in central Massachusetts. This septum is identified as Partridge Formation sillimanite schist that is characterized by large (10 cm long) sillimanite porphyroblasts interpreted to be pseudomorphs after andalusite in previous studies. Three samples (BE17-2, PF-2, and MAR20-1) were collected from an outcrop on the southern end of the septum. Whole rock composition data for these samples was obtained from XRF analysis. Individual monazite grains were identified by compositional mapping of thin sections with the electron microprobe at UMass. Individual monazite grains were mapped for Y, Th, and other elements and chemical domains were identified. Complete chemical analysis of selected domains was conducted and an age was calculated following the methods of Williams et al. (2017). Pseudosections detailing P-T stability fields for these samples were generated using Theriak Domino and the whole rock composition data. The estimated pressure of stability for all three samples is greater than 5 kbar. The temperature stability range for one staurolite-containing sample is estimated to be 500-700°C, and for the other staurolite-lacking samples, the temperature range is greater than 650°C. The majority of monazite grains in all three samples give an age of ~370 Ma, which corresponds to the Neo-acadian orogeny and metamorphism in central Massachusetts (Robinson et al., 1998). A few grains from two samples give much younger dates of ~280 Ma, which dates to the Alleghanian orogeny. Highly irregular, patchy textures in Y composition maps of these grains are interpreted to be the result of a fluid-mediated alteration event. Fluid alteration would reset the U-Th-Pb system, leading us to conclude that the fluid alteration event would have occurred during the older Neo-acadian event.