Silicic Magma Chambers as Traps for Basaltic Magmas: The Cadillac Mountain Intrusive Complex, Mount Desert Island, Maine

Abstract

The Cadillac Mountain intrusive complex, located on Mount Desert Island, Maine, provides a superb record of episodic invasion of a floored silicic magma chamber by many pulses of basaltic magma. The complex consists of three units: the Cadillac Mountain granite (CMG), the Somesville granite (SG), and gabbro-diorite (G-D). Basaltic magmas were emplaced before, during, and after the emplacement and crystallization of the granitic plutons, and the chambers of silicic magma acted as traps for basaltic magma. Basaltic magma emplaced into the surrounding country rocks crystallized as homogeneous diabase and gabbro, while basaltic magma emplaced within the perimeter of the granite displays abundant evidence for commingling with silicic magma. The G-D unit (about 1.5 km thick) consists of interlayered gabbroic, dioritic, and granitic rocks. Although thin (<1 m thick) gabbroic layers in the G-D unit are typically chilled on all margins, thicker layers of gabbro commonly form the lower parts of macrorhythmic units that may grade upward from basally chilled gabbro through hybrid dioritic rocks to granite. These macrorhythmic units provide a cumulate record of the temporary stratification of mafic and silicic magmas in the CMG chamber. Widespread, small mafic enclaves occur at all levels of the CMG and were probably generated along turbulently stirred double-diffusive boundaries between mafic and silicic magma. Heat from basaltic infusions promoted convection in the overlying silicic magma, and this convection distributed enclaves thoughout the CMG chamber. Scarce, finegrained enclaves of hybrid, intermediate rock in the CMG may represent disrupted portions of a hybrid magma that evolved near the base of the chamber due to repeated infusions of basaltic magma. Some zones of transgressive granophyre in the CMG probably represent highly silicic liquids that evolved within the CMG magma chamber. The Somesville granite may represent a mixture of magma that evolved within the CMG chamber and injections of new silicic magma from below.