Abstract
An amygdaloidal, ophitic, and slightly porphyritic basalt occurs beneath Cambrian or Precambrian sedimentary rocks in a deep well in Lawrence County, Indiana. The rock has been highly altered and contains more than 35 per cent chlorite. Many of the plagioclase $$(An_{56})$$ microlites and small orthopyroxene crystals have been chloritized, but clinopyroxene, which typically is found as phenocrysts, is unaltered. Plagioclase $$(An_{46-62})$$ phenocrysts have been altered to muscovite, and large orthopyroxene crystals have been replaced by biotite. The alteration of chlorite has produced pumpellyite. Quartz, calcite, dolomite, analcite, potash feldspar, chlorite, and pumpellyite fill veins and cavities. Sparse magnetite and secondary pyrite are present, and red iron oxides are abundant in the upper part of the basalt. The basalt from Lawrence County is very similar texturally, mineralogically, and chemically to the Precambrian Keweenawan flows of Michigan and to a Precambrian augite andesite microporphyry f...
Published Version
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