Diabase-Granite Composite Dikes, with Pillow-Like Structure, Mount Desert Island, Maine

Abstract

Dikes of granite crowded with rounded masses of diabase are described in detail and their origin discussed. Where diabase masses are closely packed, outcrops appear as diabase dikes cut by a network of granite veins or may resemble pillow lava insinuated by thin veins of granite. Veins appear non-dilational and of replacement origin. Margins of diabase masses are very fine grained and resemble diabasic magma chilled against granite. These zones are explained as secondary features formed largely by recrystallization of solidified diabase. Skialithic bodies are locally numerous within the dikes and resemble the diabase masses in size, shape, and disposition. They are considered to represent metasomatically transformed diabase. Many contain abundant small mafic patches, petrographically identical with the pseudochill zones, which are interpreted as diabase relicts. Angular gabbro xenoliths, locally derived, are common and may be enclosed by granite, diabase, or skialithic rock. A few are enclos...