Evidence of periodic basaltic injections into the crystalline mush of the Cadillac Mountain granite magma chamber, Mount Desert Island, Maine

Symchych, Elizabeth

'The west coast of Mount Desert Island, Maine contains the interlayered macrorhythmic units of silicic, gabbroic, and heterogeneous intermediate rocks which resulted from the repeated injection of gabbroic magma into the Cadillac Mountain granite (CMG). While the CMG was crystallizeing within the magma chamber, gabbroic magma intruded the shallow-level, pre-existing florred silicic chamber, and ponded in topographic lows over the newly formed silicic crystalline mush. The purpose of this study is to use field, petrographic and geochemical data in order to determine the crystallization history of mafic and felsic magma in the CMG magma chamber.' 'Evidence that gabbroic magma was injected into the magma chamber before the CMG finished crystallizing is provided by a mafic feeder dike that cross-cuts lower mafic layers and connects to partly brecciated chilled mafic lobes in the overlying silicic mush. The introduction of new magma into the CMG produced two results: 1) the hybridization of the two magmas creating heterogeneous intermediate rocks and 2) the mobilization of underlying partially crystalline mush resulting in diapiric rise of fluid-rich, felsic magma. The mixing relationship of granitic, granodioritic and gabbroic rocks is apparent in geochemical plots. Thick gabbroic layers generally display prominent load-cast structures, or 'pillows', that project downward into underlying silicic layers. The weight of these 'pillows' has caused filter pressing to occur in the underlying partially crystalline granite. The growth of myrmekite in samples taken from the granite on the side of the 'pillow' demonstrates filter pressing and the mobilization of intercrystalline fluid. The mafic and granitic rock types found on the west coast of Mount Desert Island display a highly interactive and dynamic crystallization history.' /STUDY LOCATIONS:/ /MOUNT DESERT ISLAND/