DETERMINING METAMORPHIC AND TECTONIC EVENTS FROM DETAILED BEDROCK GEOLOGIC MAPPING OF MATINICUS QUADRANGLE, MAINE

Field and petrographic studies suggest that the geologic history of rocks on Matinicus Island, Maine is complex and unresolved; this research attempts to establish a timeline of metamorphic and tectonic events. Mapping of the Matinicus Island quadrangle during the summer of 2011 revealed quartzites, biotite-garnet schists, and amphibolites exposed along the western coast. Mineral assemblages suggest that these rocks were regionally metamorphosed to amphibolite-facies conditions. Massive undeformed coarse-grained granite exposed along the eastern coast contains cobble- to boulder-sized angular inclusions of foliated garnet-bearing rocks in a 1-10 m wide contact zone. Randomly oriented andalusite up to 3 cm long occurs and overprints the foliation in particularly aluminous metasedimentary layers. Up to three overprinting subvertical fabrics commonly accompanied by subhorizontal lineations occur locally in metasedimentary. Preserved, unrecrystallized C-S fabrics, mica fish, and quartz ribbons observed in thin section suggest a late shearing event, possibly related to late faulting along the Norumbega fault zone, may have locally affected these rocks after intrusion of the granite. En-echelon porphyritic basaltic dikes cut all other lithologies. Emerging petrographic evidence will help establish the grades and relative timing of metamorphic and tectonic events that affected rocks on Matinicus Island and will correlate bedrock units on Matinicus to other mapped units in coastal Maine.