FIELD GUIDE TO BUCHAN-TYPE METAMORPHISM OF THE WATERVILLE FORMATION, MAINE

Field-based learning is a concept that is particularly well-suited for an undergraduate education in geology. The close proximity of Colby College to classic examples of various metamorphic and igneous rocks provides a means to incorporate field observations within a standard three-hour lab period. A second lab period can be allocated to sample examination via petrographic microscopes or other quantitative techniques, such as the Scanning Electron Microscope. The field and lab exercise can be further supplemented by incorporating primary literature relevant to the field site. Blending field observations, laboratory data, and literature experience into a cohesive exercise facilitates students’ understanding of lecture material presented in the classroom.

Located within the Central Maine Synclinorium, Colby College is surrounded by Silurian-aged intercalated meta-volcanic and meta-sedimentary rocks and associated igneous intrusions. Specific to this study is the Waterville Formation, which is comprised primarily of slates, phyllites and schists, with infrequent calc-silicate layers. It is a classic example of low pressure Buchan-type metamorphism that typifies the region’s response to collisional events experienced during the formation of the Appalachian Mountains. Specific mineral assemblages record variations in peak metamorphic temperature or grade, which increases toward the south and west. From lowest to highest grade, metamorphic zones are characterized by the index minerals chlorite, biotite, garnet, staurolite, and sillimanite.

The goal of this project is to compile a field guide for use in Metamorphic Petrology, with outcrops selected in each of the latter three zones. An outcrop of the Hallowell Granite was also selected as an example of one of the many igneous intrusions that contributed to the metamorphic event. Hand samples were collected and thin sections were prepared for mineralogical and textural analysis. Field locations were mapped and a road log compiled. Students will learn from this field experience the effects of increasing temperature on the same protolith composition as well as examining local variations in protolith type having experienced the same metamorphic temperature.