EVIDENCE FOR MAGMA MIXING IN THE DEER ISLE PLUTON, COASTAL MAINE

The Deer Isle pluton, coastal Maine, shows evidence of interactions with mafic magma. This granitic pluton is texturally heterogeneous and three distinct facies are recognized. They range from the least evolved Oak Point (OP) facies in the NE, through the mixed Stonington (S) facies, to the most evolved Crotch Island (CI) facies in the SW. Enclaves in the granite are generally less evolved and occur in two forms: large, flattened enclaves associated with OP, and smaller ovoid enclaves, which sometimes occur in swarms, associated with S. The former commonly contain xenocrysts, while the latter occur in conjunction with cumulate domains and/or schlieren. Enclave abundance and their size decreases from NE to SW. Schlieren are found only in S facies and have various forms. Ring and planar forms are found in association with cumulate domains and enclaves, and are attributed to convective chaotic magma movement. Ladder dikes are thought to be the product of flow of magma. In addition to these forms, disrupted schlieren layers, broken pieces of the larger forms, are observed. Non-equilibrium mineral textures are also found throughout the pluton. Hornblende-mantled quartz, titanite-centered ocelli, complexly zoned plagioclase, and rapakivi alkali feldspar are examples. These decrease in abundance from NE to SW and are commonly associated with enclaves. The observed textures may be evidence for a number of processes, including: mixing, recharge, convection, and remobilization of melts and solids. Mafic and felsic dikes cut the granite. Highly evolved felsic dikes are thought to be late residual melt that was filter pressed out of the crystal mush. Textures within the dikes suggest multiple stages of development and intrusion into solid granite. Given these observations, a simple crystallization history for the pluton can be developed. Initial emplacement of the melt was followed by crystal accumulation. The cumulate pile was subsequently disturbed by input of more mafic magma. Enclaves formed from the input magma and some settle with the disrupted crystal mush forming OP. The heat input drove convective mixing in the crystal mush layer about the cumulates, forming the enclaves and schlieren of S. Further fractionation of the melt produced the silica-rich CI facies and felsic dikes.