This proposed research on the Vinalhaven intrusion and related volcanic rocks, exposed on the coast of Maine, attempts to understand (1) how this bimodal, plutonic-volcanic complex was constructed, and (2) how input, magma chamber processes and loss by eruption affected the compositions and textures of the Vinalhaven granite. This complex, which contains both plutonic rocks with evidence for a wide range of magma chamber processes and coeval volcanic rocks, is an ideal magmatic system in which to test the model of incremental pluton growth. High-precision U-Pb zircon geochronology provides a time frame for pluton construction and the eruption of coeval volcanic rocks. Prior studies established highly contrasted Sr and Nd isotopic compositions of mafic (87Sr/86Sri = 0.70234; epsilon Nd = +6) and felsic (87Sr/86 Sri = 0.70713; epsilon Nd = -2) input. The research will document the magmatic evolution of the Vinalhaven complex, including the history of mafic and silicic replenishment, rejuvenation, crystallization, and the evolution of coeval volcanic rocks. Intensive fieldwork will be combined with petrography, whole-rock and mineral chemistry. Mapping and study of depositional features in the granite will establish the spatial and temporal consolidation history of the granite. The crystallization history of granite, silicic feeders and rhyolite will be constrained by high-resolution imaging of compositional zoning in zircon, feldspar and quartz. Finally, whole rock Sr, Nd and Hf isotopic compositions and the Hf isotopic composition of previously dated zircon crystals/fragments, combined with petrochemical data, should provide insights into the relative influences of the source region, magma chamber processes, silicic and mafic input, and the eruption of evolved rhyolite on the chemical and isotopic composition of the granite.
The results of this study will help identify firm petrologic links between plutonic and volcanic processes and products. It will also provide a plutonic perspective on a number of magma chamber processes inferred by volcanologists, including replenishment, magma mingling and mixing, rejuvenation of granitic crystal mush, convective stirring, and fractionation. Geochronologic data will provide fundamental constraints on the rates at which these processes operate and permit direct comparison to the timescales of analogous volcanic centers. Finally, this research will assess the relative contributions of the source region and magma chamber processes to the geochemical and isotopic composition of granite and should provide fresh insights into the interpretation of whole-rock isotopic data from both plutonic and volcanic systems.
