Hydrous minerals replacing plagioclase and pyroxene in metabasalt

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Click on image to enlarge.          Photo © Daniel R. Snyder

Above: Ophitic-texture pyroxene and plagioclase in a metabasalt are being replaced by hydrous minerals of the greenschist facies. The large orange and brown cluster was a pyroxene grain enclosing the now-corroded plagioclase laths. The pyroxene has been replaced by uralite, a massive amphibole usually composed mainly of actinolite (less commonly of hornblende). The scattered plagioclase laths show varying degrees of replacement by epidote, and probably by other minerals too small to identify. Extensive replament has occurred in the two adjacent, parallel plagioclase laths at right of center (See second image below for greater detail). Greenstone cobble in glacial gravel, southeastern Michigan, Specimen No. 49. Provenance likely northern Ontario. XPL. Imaged area 1.3mm by 2mm.

Below: PPL image of above. Green color of uralite is characteristic of actinolite (as well as hornblende).

Click on image to enlarge.          Photo © Daniel R. Snyder

Below: Higher-magnification image of area right of center in top photo. Note the cluster of small epidote grains that have replaced much of the two adjacent plagioclase laths. The replacement of plagioclase by epidote in this location was likely speeded by a channel between the two grains (visible as a narrow strip of uralite) that provided access by percolating fluids. XPL. Imaged area 0.5mm by 0.8mm.

Click on image to enlarge.          Photo © Daniel R. Snyder

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Sub-greenschist facies metamorphism in Keweenawan (Pre-cambrian Y age) Basalt

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Click on image to enlarge.          Photo © Daniel R Snyder

Above: Above: Fine-grained metamorphosed basalt from Pre-cambrian Mid-Continent Rift. Note abundant corroded plagioclase. No relict pyroxene is evident. Most of the black masses are magnetite (sample is strongly magnetic). Round black mass at lower left is a chlorite vesicle filling. The light rim is calcite. Mamainse Point, Ontario (eastern end of Lake Superior). XPL. Imaged area 1.3mm by 2mm.  Photo made with 4x objective.

Below: Higher-magnification image of a portion of image above. Note predominance of small grains with upper first-order and second-order interference colors. Black masses are magnetite (compare with PPL image below). XPL. Imaged area 0.5mm by 0.8mm. Photo made with 10x objective.

Click on image to enlarge.          Photo © Daniel R. Snyder

Below: As above, PPL.

Click on image to enlarge.         Photo © Daniel R. Snyder

Below: Anheadral to subhedral grains of crystalline replacement mineral(s) surrounded by partially-altered plagioclase. Dark areas are magnetite. Left image XPL, taken with 10x objective, digitally enlarged to match right image. Right image PPL, taken with 20x oblective. Imaged area of each panel 0.24mm(W) by 0.32mm(H).

Click on image to enlarge.          Photo © Daniel R. Snyder

Below: Higher-magnification image of a field of mineral grains characteristic of this specimen. PPL. Imaged area 0.13mm by 0.2mm. Thus, the long dimension of the imaged area is 200 microns. Photo made with 40x objective.

Click on image to enlarge.          Photo © Daniel R. Snyder

I will write more about the difficulty of making positive identification of the green mineral. It could be any of several hydrous calcium-aluminum silicates. If YOU know what it is, please comment!

More Text coming