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Paper No. 30-7
Presentation Time: 8:00 AM-5:30 PM
DESCRIPTION OF THE INTRUSIVE CONTACT OF THE RED BEACH GRANITE, CALAIS, ME
The Devonian-aged Red Beach Granite, downeast Maine, consists of a composite intrusion of six phases of biotite, biotite and hornblende, and hornblende granite emplaced into the shallow crust. The shallow emplacement levels are indicated by the granophyric textures and abundant miarolitic cavities observed within the granite. Additionally, published Rb/Sr ratios suggest the granite has experienced some degree of open system behavior. The granite intrudes the Calais diorite of the large volume Moosehorn Plutonic Suite (421+/-5 Ma) and basalt member of the Eastport Formation (410 Ma) and is unconformably overlain by the Upper Devonian Perry Formation. The intrusive contact with the Calais diorite form a three-dimensional network of interfingering dikes and is gradational over approximately 500 meters. The granite is much finer grained near the contact. The margins of the contacts between the diorite and granite are rounded, embayed, and crenulate. Compositional maps of major elements collected using an XRF microscope indicate little chemical exchanges occur over the distance of 2 cm (i.e. there are sharp compositional contacts). Microscopic analysis of the contact indicates there is the potential for exchange of individual mineral grains, however, the exchange of material seems to be minimal. This suggests that, at least for the northern margin, assimilation of country rock is not likely to play a major role in the petrologic evolution of this intrusion. Therefore, the Rb/Sr ratio variations are likely inherited from the magma source terrane.