The Petrology of the Cadillac Mountain Granite, Mount Desert Island, Maine

Holden, Jonathan; Franklin and Marshall College

'The Cadillac Mountain Intrusive Complex, located on Mount Desert Island, Maine consists of three major units: the Cadillac Mountain granite (CMG), the Somesville granite (SG), and the gabbro-diorite unit (G-D). This complex was emplaced along the Ellsworth schist - Bar Harbor unconformity. The CMG dominates the geology of Mount Desert Island and gravity data suggests that the CMG is underlain by a substantial mass of the G-D unit (Hodge et al., 1982). The G-D unit is a multilayered mafic intrusion that records multiple infusions of basaltic magma into the CMG magma chamber.' 'The CMG is bounded, except on the western edge by an intrusive breccia termed shatter zone by Gilman et al. (1988). It is a massive, hypersolvus granite with interstitial hornblende, comparable to A-type granite as defined by Whalen et al. (1987) and Clems et al. (1990). The eastern base of the CMG, above the shatter zone contact, is characterized by a feldspar cumulate that occurs as gradationally bounded lenses of varying thickness. Granitic dikes of varying size and mineralogy were injected into the country rock (shatter zone). Despite the variability of the dikes, most are characterized by early biotite and the presence of two feldspars.' 'The dikes crystallized from a relatively hydrous magma, while the CMG (basal cumulates and the interior) crystallized from a more anhydrous and/or higher temperature melt. This change of conditions within the magma chamber could have been triggered by an eruptive event which injected the original hydrous magma into the country rock. Subsequent degassing of the chamber could have sufficiently reduced the water content of the remaining liquid which crystallized the CMG. An increase in temperature could have been caused by basaltic infusions recorded by the G-D unit (Wiebe, 1994).' /STUDY LOCATIONS:/ /MOUNT DESERT ISLAND/