The Vinalhaven intrusion comprises 3 main units: homogeneous cg granite, a layered gabbro-diorite-granite (GDG) unit, and late intrusions of fg granite. The GDG unit forms a curved, sheet-like body, up to about 1 km thick, which dips 10-30? N and W beneath the granitic core of the complex and rests on heterogeneous granitic rocks exposed mainly in outer islands. Contact metamorphic assemblages in country rock blocks within the GDG unit indicate cooling at depths of 3 to 6 km. Volcanic rocks with overlapping ages and comparable compositions occur along the northern (upper) contact of the intrusion.

Field and petrographic relations indicate that (1) a long-lasting silicic magma chamber was periodically replenished, (2) the chamber underwent active convection, and (3) much of the intrusion formed by crystal accumulation on the floor of the chamber. Aphyric basaltic and granitic dikes in the outer parts of the intrusion have compositions that strongly suggest they are the major feeders for this essentially bimodal intrusion. Mafic dikes that breached the chamber spread out on a floor of granite mush to form the mafic sheets of the GDG unit. The resulting mafic layers grade upwards through hybrid rocks to cg granite, probably reflecting convective mixing of the upper part of the mafic sheet with overlying crystal-poor silicic magma. Layering in the GDG unit is punctuated by discrete horizons of magmatic breccia containing angular blocks of country rock that record episodes of roof collapse, perhaps related to explosive eruptions. Granitic layers within the GDG unit, as well as the overlying homogeneous cg granite, are interpreted as crystal accumulations because they exhibit variable packing of large feldspar and quartz crystals, they contain a variety of current structures defined by mafic schlieren, and they exhibit large variations in Rb/Ba at constant wt. % SiO2.

These relations suggest the pluton grew from a crystal-poor, dominantly silicic magma chamber that waxed and waned as the rates of replenishment and eruption varied. The pluton grew largely by crystal accumulation on the chamber floor. The present basinal form of the intrusion formed gradually as the basal cumulate material sagged downward deforming the underlying country rocks. The magma chamber was always significantly smaller than the final size of the pluton.