A petrographic and geochemical study of the Somesville granite in the Cadillac Mountain intrusive complex : the role of basaltic infusions in silicic magma chamber processes

Schuh, Kathleen

'The Silurian intrusive complex located on Mount Desert Island, Maine has been referred to as the Cadillac Mountain intrusive complex (Wieve and Chapman, 1993). It consists of two felsic plutons, the Cadillac Mountain and Somesville granites, and a layered gabbro-diorite unit. This composite pluton represents a mafic-silicic layered intrusion. Plutons of this type are thought to be long-lasting, floored silicic magma chambers that were recurrently injected by basaltic magma. Recent studies suggest that many large accumulations of acidic volcanic rocks have erupted from such systems. This study is concerned with studying the role of the Somesville granite in the composite pluton on Mount Desert Island in an attempt to further the understanding of magma chamber processes and the connections that can be made between the plutonic record and volcanic activity.' 'The Somesville granite, a two feldspar biotite granite, forms an oval-shaped body between the gabbro-diorite in the north and southwestern areas of the complex and the eastern portion of the Cadillac Mountain granite. Textural features of outcrop scale observed at contact areas in the field, as well as petrography, indicate that the Somesville granite was affected by hybridization with mafics, although this may have had little effect on the granite's compositional evolution. Geochemical data was obtained through ICP, INAA, and XRF major and minor element analysis. The geochemical characteristics of the Somesville granite indicate that the unit is calc-alkaline, peraluminous and demonstrates an upper contenental crust signature.' 'Several discrimination diagrams have been applied to the geochemical data in an attempt to resolve the tectonic setting and source of the unit. These diagrams suggest that the Somesville granite is likely an I-type granite having subduction related origins. Two hypotheses have been established explaining the relationship between the Somesville granite and the Cadillac Mountain granite. Associated with these hypotheses are two models for the origins of the granites: a) the Somesville granite may have differentiated from the Cadillac Mountain granite through the fractionation of potassium feldspar. This would indicate that an A-type granitic source magma for the Cadillac Mountain granite was generated during the initiation of rifting in the earliest stages of melting of an anhydrous lower crust. The upper continental crust signature in the Somesville granite would reflect a rifting environment in which the upper crust has retained calc-alkaline signatures from prior subduction-related events. b) the Somesville granite may represent the most siliceous magma in the intrusive complex which mixed with the gabbro-diorite unit to form the Cadillac Mountain granite. If this is the case, the Somesville granite represents a pre-collisional calc-alkaline intrusion which is mostly derived from mantle modified by a subduction component, with the Cadillac Mountain granite representing a post-collision alkaline intrusion which may be derived from mantle lithosphere beneath the collision zone.' /STUDY LOCATIONS:/ /MOUNT DESERT ISLAND/