CONTACT RELATIONSHIPS BETWEEN GOSHEN DOME ROCKS AND SURROUNDING SCHISTS, WEST-CENTRAL MASSACHUSETTS, USA

Involving undergraduates in your petrological research may mean traveling to a distant field area, but it is well worth it. This study is an undergraduate project that is part of a larger study of a small (1 – 2 km) structural dome in West-Central Massachusetts. The dome was mapped by Hatch and Warren (1981; USGS map GQ-1561) as the Collinsville Formation (Oco), a felsic biotite gneiss with layers of foliated hornblende-plagioclase amphibolite, in the core. A ring of Cobble Mountain Formation (Ocb), interlayered feldspar-mica schist with quartz-feldspar-biotite gneiss, is mapped along the western to southern to eastern boundaries, but not on the northern boundary. The surrounding rocks are a gray carbonaceous quartz-mica schist grading in beds to quartz-mica-garnet-staurolite-plagioclase schist mapped as the Goshen Formation (Dg). The whole structure is informally known as the Goshen Dome. Outcrops of Ocb on the western boundary of the dome have yielded 2- 3 cm diameter garnets containing possible evidence of ultra-high-pressure (UHP) metamorphism (Snoeyenbos & Koziol (2008) EOS transactions AGU 89 n. 53 Abstract # V31E-07).

We explored rocks to the north of the above-mentioned garnet outcrops, to possibly find more UHP rocks, and to determine contact relations between the Dg schist and the dome rocks of Oco and Ocb. Mapping of a large outcrop on the northwest boundary of the dome was an appropriate undergraduate project. Our mapping shows Dg directly in contact with Oco, verifying the map of Hatch and Warren (1981). Results from this one outcrop lead to broader implications. Hatch and Warren (1981) suggest that Ocb is structurally above Oco, and that there is a structural decollement between these Ordovician gneisses and the Dg formation. If so, why doesn’t Ocb occur along the entire boundary of the dome? There is limited outcrop in this area but we were able to map the disappearance of Ocb along the northwestern boundary of the dome. The available aeromagnetic data (USGS Geophysical Investigations Map GP-626, 1968) used by Hatch and Warren (1981) supports their map of the extent of the Ocb unit, but questions remain. The presence of potential slivers of UHP rocks in dome rocks adds to the complexity of the structural history.