Magma mixing in the Cadillac Mountain intrusive complex, Sommes Cove, Mount Desert Island, Maine

Bergh, Frederick

From Abstract: "Mount Desert Island, located off the southern coast of Maine, is dominated by the Silurian Cadillac Mountain Intrusive Complex (CMIC), one of the many plutons within the Coastal Maine Magmatic Province (CMMP) (Hogan and Sinah, 1989). The major phase of the CMIC is the Cadillac Mountain Granite (CMG). It is bounded on the west by a complexly layered Gabbro-Diorite unit (G-D) (Wiebe, 1993) which has excellent exposures along the coastal inlets and bays of the island. The G-D unit is suspected of being the result of magma mixing (Wiebe and Chapman, 1993). In order to examine this hypothesis, a study conducted during June of 1993 in Sommes Cove, Mount Desert Island (MDI in Figure 1) involved mapping detailed field relations and gathering samples for petrographic and geochemical analysis. These data allowed the determination of crystallization histories and petrogenetic relationships between the various rock types within the unit, and in particular within Sommes Cove. Field relations in the layered section of Sommes Cove strongly suggest the interaction between magmas. Chilled gabbroic layers, in close association with more felsic, heterogeneous units, grade to coarse gabbro. This implies a non-isothermal relationship, thus eliminating liquid immiscibility and undermining simple magmatic differentiation. Tabular biotite within the felsic unit near the contact is parallel to the boundary, appearing flow laminated; biotite within the chilled margins of the mafic unit is also flow laminated. Lamination in both units controlled by the contact between them strongly suggests that both were liquid when they came in contact. Macroscopic and petrographic mineral assemblages and textures provide abundant evidence for the interaction of magmas in the G-D unit. Intermediate layers consist of mafic and felsic glomerophyrocrysts several millimeters across, and display textures that imply disequillibrium conditions, such as partially resorbed and zoned plagioclase and rapakivi quartz and kspar xenocrysts. Geochemical data support the petrography and field relations. Several major oxides plotted against silica conform well to linear mixing trends, and trace element modeling indicates magmatic differentiation is an inadequate explanation for the observed chemistries. Tectonic discrimination diagrams classify the granitic unit of Sommes cove as a peraluminous, subalkaline within-plate granitoid, consistent with the A-type classification of the Cadillac Mountain Granite. The basaltic dikes may be tholeiitic basalts, and both the granite and the basalt plot near rift tectonic fields. Although the exact timing of the pluton's emplacement in relation to regional tectonics is not fully resolved, Hogan and Sinah (1989) have suggested extension of the crust as a possible model for magmatism in the area. The model proposed by Wiebe (1993) for the layered sequence of the CMIC states that it formed from periodic infusions of basic magma into a granitic magma chamber. This mechanism explains well the field, petrographic, and geochemical relations of Sommes Cove."