A NEW INTERPRETATION OF "PEGMATITE" LAYERS IN A BASALT FLOW FROM THE SOUTH RANGE QUARRY NEAR HOUGHTON, MICHIGAN

In an amygdaloidal basalt flow 4 miles southwest of Houghton, Michigan, H.R.Cornwall (1951, GSA Bul vol. 62 pgs. 159-202) identified evidence of internal differentiation forming pegmatite zones. This flow at the South Range Quarry is part of the Proterozoic Portage Lake Volcanics and is approximately 130 feet thick. The outcrop contains a series of progressively closer spaced zones of concentrated amygdules beginning approximately 50 feet from the bottom of the flow. The flow is a coarse-grained ophitic basalt containing heavily altered plagioclase and relatively unaltered clinopyroxene. Cornwall identified these amygdule string patterns as pegmatite layers. However, there was no sign of internal differentiation seen in pyroxene chemistry (determined by electron microprobe analyses) in that the composition remained the same in samples taken throughout the flow. The pegmatite zones show little grain size variation in comparison to the rest of the basalt and would have lacked the iron environment described in the internal differentiation model. The zones are in more likelihood concentrations of vesicles rather than pegmatites. The flow should then be described as a single flow crystallizing largely from the top down. Volatiles would have concentrated along the flow top from degassing, creating a series of vesicle concentrations. Prehnite, epidote and native copper, formed by secondary alteration, infill the vesicles formed by this process.