EFFECTS OF WATER CONCENTRATION VARIATION AND MAGMA INTERACTION IN THE TUNK LAKE PLUTON, MAINE

The Tunk Lake pluton is a compositionally zoned, alkaline granitic intrusion located ~30 km northeast of Mount Desert Island, Maine. The 366 +/- 1 Ma pluton probably postdates the accretion of its host rocks to North America in the early Devonian (West et al., 1992; 1995). The Tunk Lake pluton preserves evidence for thermal and mechanical interaction between the granitic magmas of the pluton and gabbroic magma that partially surrounded and possibly underlay the granitic pluton. The outermost zone of the pluton is a hypersolvus, single feldspar, aegirine-augite and hornblende granite. This zone grades into a central rapakivi feldspar, biotite, and hornblende granite. The rapakivi feldspar zone grades inward to biotite two-feldspar subsolvus granite. Initial eNd of granite from the three concentric zones of the pluton are nearly identical (+2.17 in the inner granite to +2.46 in the outermost granite) but increases outward from the center of the pluton. Gabbro that surrounds the pluton on its northeastern border is more primitive (initial eNd=+5.32). The increase in initial eNd from the interior of the pluton outwards toward the gabbro, along with many magma mingling textures, suggests that increasing degrees of mixing took place between the granitic and gabbroic magmas from the interior (upper) part of the pluton toward the exterior (possibly lower) part of the pluton. Variation in water concentration in the pluton may have also been controlled by the gabbroic magma. The relatively dry, aegirine-augite-bearing hypersolvus granite is in contact with the gabbro, and the relatively wet, biotite-bearing subsolvus granite is most distant from the contact with the gabbro. In between these two zones is the rapakivi granite zone. FTIR spectroscopy indicates that OH fluctuates considerably between higher concentrations in potassium feldspar and lower concentrations in plagioclase feldspar rims, suggesting that fluctuations in water concentration, as well as magma interaction, both related to the proximity of the gabbroic magma, influenced the growth of rapakivi feldspar. These relationships illustrate the control on variations in mineral assemblages in granitic plutons exerted both by the thermal influence of mafic magma bodies on water migration in granitic magmas, and by mechanical interaction of contrasting magmas.