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| BASALT PORPHYRY
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Typical Minerals Minerals too small to identify (except Ca plagioclase phenocrysts). Composition will be from the top of Bowen's Reaction series (i.e. mafic) and we would expect, in addition to Ca plagioclase, pyroxene, and perhaps some olivine. Note that these minerals are present in this specimen, just too small to see.
| Description
A mafic igneous rock from the top of Bowen's Reaction Series. Typically dark colored, as is the aphanitic portion of this rock, although the large plagioclase phenocrysts give it a lighter appearance. The phenocrysts indicate this rock had a two stage cooling.
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Tectonic Association
See original specimen for general tectonic associations. The large phenocrysts in this specimen tell us it had a two stage cooling, having spent some time below the surface cooling slowly enough for the plagioclase crystals to form, and then being brought toward the surface for rapid cooling. But even the fine grained portion of this rock is more granular than the previous specimen, indicating it probably did not cool at the surface, but perhaps in a dike, sill or laccolith.  Tectonic Cross Section - pdf version
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| Last Update: 9/29/00 | e-mail: (Fichtels@jmu.edu) |