Rounded olivine grains in peridotite of the Los Pinos pluton.

Click on image to enlarge.   Sample: Michael Davis   Image: Dan Snyder

   Rounded olivine grains surrounded by amphibole. The Los Pinos pluton is one of several gabbroic plutons intruding the Peninsular Ranges batholith in the southernmost part of California. "Olivine and amphibole...are the dominant phases and form up to 88% of the rock." (Walawender, 1976)*. Walawender describes the petrogenesis of the peridotite as an example of Bowen's classic reaction series. Olivine was first mineral to solidify from melt, along with minor plagioclase (not shown in this image). Outer parts of olivine grains were resorbed by melt as pyroxene (not shown) solidified. Amphibole solidfied last and occupied interstices between other minerals. Red material in large grain at center is iddingsite, occupying part of an irregular fracture.  Los Pinos Mountain, Peninsular ranges, San Diego County, California. XPL. Imaged area 2.7mm by 4mm.

See post of December 17, 2012 for a macrophotograph of the entire thin section.

Many thanks to Professor Michael Davis, of the University of California at Riverside, for the specimen.

Click on image to enlarge.   Sample: Michael Davis   Image: Dan Snyder

   PPL image of same area as above. Dark smudges in amphibole are made up of "aligned, rod-like, opaque minerals" (Walawender, 1976)*.


* Walawender, M. J, 1976. Petrology and Emplacement of the Los Pinos Pluton, southern California. Canadian Journal of Earth Sciences, v. 13, pp. 1288-1300.