Growth and Impact of a Mafic–Silicic Layered Intrusion in the Vinalhaven Intrusive Complex, Maine

The Silurian Vinalhaven intrusive complex (VIC) is dominated by granite with subordinate gabbrodiorite. It was emplaced at a shallow crustal level (1–2 kbar) into pre-Silurian metamorphic rocks and apparently cogenetic basalts and high-silica rhyolite. The main granite body is about 12 km in diameter and includes an ∼1 km thick section of inward-dipping, originally subhorizontal, gabbrodiorite sheets that extends more than 10 km along strike, creating a mafic–silicic layered intrusion (MASLI). Fine-grained granite dikes are probably feeders for the silicic magma chamber, and basaltic dikes are probably feeders for the gabbroic sheets. Gabbroic rocks form the bases of macrorhythmic layers from 10 to 150 m thick that grade upward from chilled gabbro through gabbroic cumulates to hybrid rocks and granite; many layers can be traced for several kilometers. Abundant country rock blocks (up to 100 m) occur at many levels within the MASLI. The granite has widespread arcuate schlieren and settled enclaves, which define aggrading magma chamber floors and suggest a depositional origin for most granite below, within and above the MASLI. Mafic magmas are equivalent to low-K tholeiites, and silicic magmas are comparable with high-silica rhyolites (72–78% SiO2). Compositional variation in the MASLI reflects both hybridization with granite and crystal accumulation at the base of a compositionally zoned magma chamber. Mafic input heated and enhanced convection in the felsic magma chamber and locally rejuvenated older granite, producing a porphyry that crystallized at higher T and lower H2O contents. Thermal arguments suggest that the thickness of the silicic magma chamber was at least hundreds of meters, and perhaps as much as 1 km. Because the extent of the basaltic sheets approached 75 km², at times the volume of eruptible silicic magma ranged from 10s of km³to 100 km³. The VIC provides a possible smaller-scale template for the magmatic plumbing system beneath large bimodal volcanic systems such as Yellowstone.