Plagioclase in Yellow Dog peridotite (plagioclase-bearing lherzolite)

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

 Orthopyroxene at upper left (gray), enclosing plagioclase laths; clinopyroxene (magenta) at lower right; olivine at lower left, upper right, and top center. Yellow Dog Plains, Marquette County, northern Michigan. XPL. Imaged area 2.7 mm x 4 mm (2X objective).

This is an unusually rich concentration of plagioclase (center) for the Yellow Dog peridotite. Most of the plagioclase in my samples is scattered as single laths, perhaps two or three small grains in a typical frame this size.  Older geology texts rule out ANY plagioclase if a rock is to be called "peridotite", but the IUGS classification allows up to 10 percent, in which case they are called "plagioclase-bearing ____". However, the consultant's report states that the plagioclase can average 25 to 30 percent "over significant intervals". In order to accommodate this, the consultant uses a maximum of 30% plagioclase for peridotite. The report defines rocks with between 10% and 30% plagioclase as "feldspathic peridotite".  (This report is on the Web. You can get it in PDF format by Googling "Eagle Deposit Geology".) Under the IUGS classification, these rocks would either be olivine gabbro, olivine norite, or olivine gabbronorite.

Yellow Dog peridotite - clinopyroxene interstitial to olivine

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

The dark areas are olivine (partially serpentinized with stringers of magnetite), the magenta and violet shapes are clinopyroxene, and a few gray plagioclase laths show at the upper left. There appear to be several varieties of pyroxene in the Yellow Dog peridotite. One of the most noticeable is anhedral clinopyroxene occupying the interstices between fractured olivine crystals, as in this image. Apparently the pyroxene was the last major mineral to solidify from the melt (In his M.S. thesis, Morris (1977)* refers to these occurrences in the Yellow Dog peridotite as poikilitic texture). Also notice the dark rims around the olivine crystals, and the smooth interface between olivine and pyroxene, suggesting a reaction between the early-crystallizing olivine and the melt.  XPL. Imaged area 2.7 mm x 4 mm.

In the PPL enlargement (below) of the right center of the XPL image, the dark rims around the olivine appear to contain an iron-rich alteration product. The olivine fragments remaining in the core of the crystal are transparent in PPL. Imaged area of enlargement 1.3 mm x 2 mm.

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

Marquette County, northern Michigan.

*Morris, William J., (1977) Geochemistry and Origin of the Yellow Dog Plains Peridotite, Marquette County, Northern Michigan, unpublished master's thesis, Michigan state University.