MULTIPLE EPISODES OF MAGMATISM, DEFORMATION, AND METAMORPHISM IN THE GREEN MOUNTAIN MASSIF AT STRATTON MOUNTAIN, VERMONT: A ONE-OUTCROP FIELD TRIP

A new road cut on Stratton Mountain, southwestern Vermont, displays a complex geologic history that is nonetheless clear and obvious enough to be examined and understood by secondary school teachers and students. This road cut is easily accessible, has low traffic, and is in a picturesque location. In the field this one-outcrop field trip provides excellent examples of metamorphic rocks with multiple episodes of magmatism and deformation. With photographs of rock sample thin sections one additional episode of metamorphism and deformation becomes clear on a microscopic scale. Lithologies in the outcrop include: highly-deformed Precambrian felsic gneiss, amphibolite, sulfidic garnet amphibolite and highly-deformed mafic gneiss, quartz veins, pegmatite, and undeformed Mesozoic alkali basalt dikes. Initial magmatism is probably represented by the felsic gneisses into which mafic magmas were later intruded. The earliest phase of metamorphism is preserved in the dominant foliation and layering. This metamorphism changed granitic rocks to gneisses, and basaltic rocks to amphibolites. Metamorphism of these rocks likely occurred during the Middle Proterozoic Grenville orogeny, with emplacement of quartz veins and partial melting to form pegmatites. High-grade metamorphism and deformation during the Grenville developed the obvious layering and foliation and probably also the somewhat younger spectacular open folds. The foliation is defined by parallel alignment of micas in all rock types. In thin section photographs there is a younger weak foliation that cuts the dominant foliation, with the parallel development of fine-grained mylonitic zones and growth of epidote and other minerals that indicate a second metamorphic episode, which probably occurred during the Ordovician Taconic Orogeny. A third phase of magmatism emplaced several alkali basalt dikes that have large augite phenocrysts and are likely related to regionally extensive Mesozoic dike swarms. The alkali basalt dikes intruded as magmas hot enough to melt adjacent gneisses, visible as thin zones of melt, some being mixed into the basalt magma. This road cut is an excellent one stop field trip where several geologic features can be readily studied.