THE ORIGIN OF COMPOSITIONALLY GRADED LAYERS IN THE VINALHAVEN INTRUSION, COASTAL MAINE

The Silurian Vinalhaven intrusion is located along the Maine coast and consists mainly of cg granite with inward-dipping sheets of interlayered gabbro and hybrid intermediate rocks. Superb exposures on Narrows Island, close to the SE shore of Vinalhaven Island, provide sections through two mafic sheets that grade upward through hybrid cumulates to granite. Each compositionally graded layer is about 100 meters thick and dips about 35 NW beneath the main Vinalhaven granite body.

The chilled gabbroic base of each layer rests with load casts on cg granite. Granitic pipes and flames extend upward into the base of the gabbro. Chilled gabbro grades upward to mg and cg olivine gabbro. Subtle chilled margins at higher levels suggest the entry of new pulses of basaltic magmas. The upper intermediate and granitic parts of the layers range from only a few to about 20 meters thick. Hornblende increases upward in the gabbro, becoming dominant in intermediate, texturally and compositionally heterogeneous dioritic rocks. Further upward in the layer, rocks become more felsic with subhedral plagioclase, hornblende, and biotite with interstitial K-feldspar and quartz. Reaction textures suggest variable mixing between mafic and felsic melts and crystals. The top of the layer consists of cg biotite granite comparable to normal Vinalhaven granite.

These layers are sharply cut by basaltic, granitic and composite dikes of granite with chilled basaltic pillows. Granitic dikes are fine-grained and homogenous with CI < 5. The chemical compositions of basaltic dikes and pillows in dikes are identical to the basal chilled margins on the mafic sheets. The granitic dikes have compositions consistent with low P minimum melts and are close in composition to the granitic tops of the layers, though enriched in Rb and depleted in Ba. Dike compositions suggest they were feeders to the bimodal Vinalhaven intrusion. When basaltic magma entered the granitic magma chamber, it ponded on a floor of granite crystal mush. Mixing between it and overlying granitic magma produced an intermediate layer of hybrid magma. Compositional variation in the graded layers suggests that the dominant process was crystal accumulation: from gabbroic magma at the base, granitic magma at the top, and from heterogeneous hybrid magma in the middle.