Petrography of the Athol quadrangle, Massachusetts

Abstract

The Athol quadrangle of north-central Massachusetts is underlain by high-grade metamorphosed amphibolites, gneisses, schists, calc-silicate gneisses, quartzites, and orthogneisses. These include the Monson Gneiss of Middle Ordovician or older age, the Partridge Formation of Middle Ordovician age, and the Middle to Late Devonian Hardwick Granite of the New Hampshire Plutonic Series. The Monson Gneiss is a series of interlayered biotite and hornblende gneisses and amphibolites. They were originally felsic to mafic tuffs and minor flows that were deposited in shallow seas.- The light-gray gneisses are characterized by uniform bedding and minor magnetite and hornblende. The amphibolites generally show bedding and contain quartz, some of which is thought to have been introduced during deposition and consolidation in their marine environment. The Partridge Formation includes schists generally having graphite, sillimanite, cordierite, and a lack of potash feldspar; fine-grained, equigranular calc-silicate gneisses; minor quartzites; and minor amphibolites similar to those of the Monson Gneiss. The Hardwick Granite is an undifferentiated igneous complex which includes the Bethlehem. Gneiss, Kinsman Quartz Monzonite, Spaulding Quartz Diorite, and Concord Granite of the New Hampshire Plutonic Series. In general there are two distinctive rock types: granite and tonalite. The granite is characterized by red-brown biotite and zoned apatite crystals; the tonalite is finer grained, has dark-olive or green-brown biotite, and has relatively large amounts of sphene and allanite. A non-foliated, crosscutting, sulfidic mafic dike is thought to belong to the Late Devonian Plutonic Series of Page (in preparation).