Field relations, petrology, and geochemistry of layered gabbro-diorite units on the west coast of Mount Desert Island, Maine

Seckler, Michael

'A repeating sequence of layered mafic to intermediate plutonic rocks makes up the western edge of the Silurian Cadillac Mountain Intrusive Complex that dominates Mount Desert Island, Maine. In an attempt to understand the petrogenesis of these rocks, a detailed study was taken of three distinct macrorhythmic units, as defined by Wiebe (1992), along 130 meters of wavy coast at Stewart Head. Each sequence of layers in the gabbro-diorite constitutes a macrorhythmic unit with a chilled gabbroic base that grades upward into more silicic compositions and ends with the chilled base of another unit. The chilled zones record influxes of basaltic magma into a cooler evolving silicic magma chamber.' 'The three macrorhythmic units' hybrid nature is the result of a succession of processes. When the two magmas came into contact at the bottom of the magma chamber, the base of the hotter basalt chilled against the cooler, more silicic magma. This contact with the cooler magma also induced rapid crystallization and fracturing along the bottom of the injected magma. The underlying more silicic magma, kept partially crystalline by the heat of the basaltic magma, was filter pressed up through the fractures in the overlying basalt. This contact of the basalt and the underlying cumulates produced most of the mingling observed in the field and minor amounts of mixing as well.' 'Other processes developed at the basalt's contact with the overlying silicic liquid. Here, the literature suggests, a double diffusive interface developed and the silicic melt began convecting. The amount of mixing produced at theis interface is ambiguous but seems minor in comparison with other studies of magma mixing. The major processes involved at this interface appear to have been the transfer of heat and volatiles. These interactions altered the fractionation sequences in each macrorhythmic unit.' 'The nature of these interactions at both the top and bottom of the basalt influx, however, appears to have varied between units. Unit 2 evolved much differently than the other two units. This can be explained by an extended period of calcium-rich cumulate accumulation or an extended period of basalt influx. Complete explanation of these variations is impossible due to the spacing of samples and the lack of vertical exposure for the three units.' 'Further work must evaluate why pipes form in some layers and not in others; what accounts for the parallel relationship of Ba and Zr; and, most generally, how do the observed trends relate to the variable sizes of replenishments and the nature of the basaltic magma's interaction with the resident magma?' /STUDY LOCATIONS:/ /MOUNT DESERT ISLAND