Petrochemistry of a Mafic Layer Within a Bimodal Granite Pluton, Vinalhaven Island, Coastal Maine
Richard Henterly
Department of Geology and Geography
Denison University, Granville, Ohio 43023
Introduction
The Vinalhaven pluton exposes the crystallized remains of a bimodal plutonic system once composed of a granitic magma chamber that was injected repeatedly with mafic magma. Each successive batch of mafic magma pooled on the aggrading floor of the magma chamber, creating a stratified sequence of mafic layers and accumulated granitic crystal mush (Wiebe, 1997; Wiebe and Collins, 1998). This stratigraphy not only reveals the evolution of the magma chamber, but also preserves evidence of interactions between the mafic layers and the residing granitic magma that produced hybridized magmas locally.
This study focuses on the field, petrographic and geochemical characteristics of a single mafic layer exposed along the shore of Coombs Neck on the east side of Vinalhaven Island. The 45 meter-thick layer is well exposed along the shoreline of Coombs Neck over 400 meters along strike. My investigation reveals variations within the layer of both lithology and whole rock geochemical composition that suggest hybridization resulted from a selective, rather than bulk mixing process. Moreover, preliminary results indicate that mafic layers on Coombs Neck are more highly evolved than other mafic layers on Vinalhaven.