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Photographed by Michael P. Klimetz

Phyllitic Metabasalt/Greenstone

Catoctin Metabasalt Formation
Late Proterozoic-Early Cambrian

I-70 at Arch Bridge

Frederick County

Greenstones consist primarily of volcanic rocks, dominated by basalt, with minor sedimentary rocks inter-leaving the volcanic formations. Greenstones have been interpreted as having formed at ancient oceanic spreading centers and island arc terranes and occur in laterally extensive greenstone belts. There is a change in the structure and relationship of greenstone belts to their basements between the Archaean where there is little clear relationship, if any, between basalt-peridotite sheets of a greenstone belt and the granites they abut, and the Proterozoic where greenstone belts sit upon granite-gneiss basements and/or other greenstone belts, and the Phanerozoic where clear examples of island arc volcanism, arc sedimentation and ophiolite sequences become more dominant. This change in nature is interpreted as a response to the maturity of the plate tectonic processes through time. Archaean plate tectonics did not take place on mature crust and as such the presence of autochthonous greenstone belts is observed. By the Proterozoic, magmatism was occurring marginal to cratons and with established sedimentary sources, with little recycling of the crust, allowing their collective preservation. By the Phanerozoic, an extensive continental sedimentary veneer and lower heat flux from the mantle has resulted in further preservation of sediments and a dominant signature of continental crust. Greenstones, aside from containing basalts, also give rise to several types of metamorphic rocks which are used synonymously with metabasalt.