Matthew Engleman

A heterogeneous Sebago batholith: A petrographical and geochemical study of the granitis lithologies of the southern Sebago

ENGELMAN, Matthew, Geology Dept., Bates College., 138 Bates College, Lewiston, Maine 04240,

Recent detailed mapping of the bedrock geology within the Sebago batholith in southwestern Maine have been successful in delineating several distinct granitic lithologies that combine to comprise the batholith. These lithologies exist as mappable bodies characterized by differing mineralogies. This thesis project analyzes these different granites from a mineralogical and geochemical perspective in order to better understand their genetic relationships with each other and the magmatic processes that created them. The mineralogical portion of this project was carried out using optical thin sections and standard geological optical microscopy. Samples were divided into three groups based on these observations. Group 1 includes samples with both biotite and muscovite in roughly equal proportions, group 2 consists of samples with muscovite and garnet, and group 3 includes samples with only biotite. Geochemical data on 15 selected samples covering all three groups were obtained using inductively coupled plasma emission spectrometry and mass spectrometry (ICP and ICP/MS). Trace element abundances and chondrite-normalized rare earth element trends indicate a similar source in the generation of the two-mica and biotite granites with incomplete partial melting or crystal fractionation playing an important role in their developments. The highly depleted nature of the muscovite-garnet granite indicates that it was formed very early in a partial melt environment. These interpretations are consistent with a model of the three granite lithologies as comprising different phases of a single intrusive event during which there was a change in crystal/melt equilibria conditions.