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