MACRORYTHMIC GABBRO TO GRANITE CYCLES AND COMPOSITE DIKES, CLAM COVE, VINALHAVEN ISLAND, MAINE

The Vinalhaven intrusive complex, comprising the bulk of the bedrock of Vinalhaven Island, Maine and surrounding islands and tidal ledges, is an intrusive complex formed by successive magma injections. Due to excellent coastal bedrock exposures, the Vinalhaven intrusive complex is an excellent place to study silicic magmatic processes. A large, coarse-grained granite unit in the center and west of Vinalhaven is intruded by a smaller, fine-grained granite unit which is exposed in the east; a smaller gabbro-diorite unit comprises the bedrock in the southeast. Layering in the gabbro-diorite unit dips approximately 20° to the NNW, suggesting that the gabbro-diorite is at the base of the complex.

The rocks of the gabbro-diorite unit, on which this project focused, are rhythmically layered and show wide variation in mineralogy over relatively short distances (10s to 100s of meters). The ~1.5 km of coast examined in this study corresponded to ~200 m of pseudostratigraphy and contained one complete macrorhythmic unit, as well as the transitions to both the underlying and overlying units. Olivine-bearing, clinopyroxene and plagioclase-rich gabbros (as mafic as 48 wt% SiO2 and 7 wt% MgO) grade into diorites with rapakivi feldspar, clinopyroxene, and amphibole-rimmed quartz, formed by a reaction between the quartz and a mafic liquid, which, in turn, give way to granites ( >73% wt% SiO2) characterized by quartz, biotite, plagioclase and alkali feldspars.

The gabbro-diorite unit is cut by two different types of dikes: composite dikes and fine-grained granitic dikes. Composite dykes crystallized from commingling mafic and felsic magmas and often contain angular blocks of gabbro. The grain size of granite dikes vary from ~0.05 - 0.5 mm with the grain size of the coarsest dikes similar to that of the fine-grained granite unit. At one locality, a fine-grained granite dike appears to have cut a composite dike while the composite dike was a crystal mush. Large plagioclase crystals in the porphyritic matrix of the composite dyke appear as xenocrysts in the fine-grained granite dyke, although none of the mafic pillows are quenched against a fine-grained granite matrix. Petrography and mineral chemistry from this gabbro-diorite macrorhythmic cycle will be interpreted in the context of models for granite formation and evolution.