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

1962; University of Chicago Press; Volume: 70; Issue: 5 Linguagem: Inglês

10.1086/626851

1537-5269

Carleton A. Chapman,

Geology and Paleoclimatology Research

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 "pseudochill" 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 enclosed partly by granite and partly by diabase. Gabbro xenoliths and wall rock maintain sharp, straight contacts and have not been encroached upon by the granite. It is concluded that originally simple diabase dikes with angular gabbroic xenoliths cut gabbro wall rock on Mount Desert Island. Subsequently material was introduced metasomatically, principally along fractures in the diabase, to form granite net veins and produce a pillow-like structure. As granite encroached upon diabase, the veins were enlarged and diabase masses disintegrated and reduced. A more local effect was the selective replacement and recrystallization of certain diabase masses and their transformation to skialiths.