Presque Isle peridotite

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

Peridotite (thoroughly serpentinized). Pseudomorphs of serpentine after olivine (dark green) and anthophyllite after pyroxenes (linear patterns of brown, orange, and yellow) retain the shapes of original crystals. I've seen the Presque Isle peridotite described as "highly deformed". This doesn't look highly deformed to me. A rock body whose protolith is a billion-plus years old and still preserves the original crystal shapes seems to me to hardly be deformed at all.

Michael Lewan's (1972)* M.S. thesis reports on three samples ranging from 54.9 percent to 59.2 percent serpentine by volume. Olivine plus augite account for another 15.7 to 21.2 percent total, about evenly divided. Lewan also found 4.4 percent to 7.8 percent anorthite in his three samples, but I've looked at several thin sections and haven't seen any. Presque Isle Park, City of Marquette, Marquette County, northern Michigan. XPL. Imaged area 1.3 x 2 mm. Link to photo of outcrop.

*Lewan, Michael D., 1972, Metasomatism and Weathering of the Presque Isle Serpentinized Peridotite, Marquette, Michigan, unpublished M.S. thesis, Michigan Technological University.