PETROLOGY OF THE ROLLSTONE PEGMATITE, FITCHBURG, MA

The Rollstone Hill pegmatite (RHP) has not been well studied since Hitchens' work in 1935. The units consist of an irregular granite unit, the Fitchburg granodiorite, and the late pegmatite of this study. This work presents petrographic microscopic analysis, geochemical analysis using ICP-MS, and XRF, and electron microprobe data. Pegmatites of this area, based on mineral assemblages and lack of rare earth elements, would be classified as a muscovite pegmatite based on London's classification. These pegmatites are distinct in their large tourmalines, some measuring centimeters in length and highly zoned. Additionally they also consist of albite, orthoclase, muscovite, biotite, quartz, apatite, illmenite and garnet. Hence, the pegmatite is a binary or subsolvus granite. The granitic host rock contains similar assemblages but is absent of garnet and contains an abundance of allanite. The RHP was emplaced during the Acadian Orogeny, but its precise timing is unknown. The pegmatite and the Fitchburg granitic host rock are not metamorphosed. The granite of the RHP was dated by Maczuga (1981) to be 382 Ma, but is not well constrained. The granodiorite and related granite have high quantities of rare earth elements largely hosted in allanite. The pegmatites also contain large amounts of uranium, an average of about 15 ppm, while the host rock is depleted in uranium.