As mentioned previously, I went on a post-conference field trip that involved a transect from the Blue Ridge in VA to the Valley & Fold belt in West Virginia. Callan originally live-blogged both days (end of day 2) and is now working his way through several synthesis posts, so I’m going to leave the majority of the actual trip description to him.
Several vans were involved in the trip, but mine had several discussions about what exactly a charnockite was and how it formed. Callan has pictures in hand sample, but I found a photomicrograph from approximately equivalent Blue Ridge rocks to the ones we saw:
In the labeled image: q = quartz, f = feldspar (mainly plagioclase), and p = orthopyroxene. The rocks are anhydrous, which explains the lack of biotite, hornblende or other OH-bearing mafic minerals.
The majority of pyroxenes contain iron and/or magnesium, silica and oxygen. Clinopyroxenes (augite, diopside, hedenbergite) include calcium as well and have a lower amount of symmetry than the orthopyroxenes (enstatite, ferrosilite, hedenbergite). In thin section, both clinopyroxenes (cpx) and orthopyroxenes (opx) have cleavage at about 90 degrees and tend to be more equant than elongate in shape. Cpx, however, has inclined extinction (as you rotate the stage in crossed-polars, the mineral goes black when the cleavage does not line up N-S or E-W with the cross-hairs) and tends to have higher orders of interference colors (2nd-3rd order on a Michel-Levy diagram). Opx has parallel extinction (black when cleavage N-S or E-W) and only mid to upper 1st order interference colors. Of the two, normally cpx is present in more mafic (iron / magneisum / calcium rich samples) and opx in rocks that are less mafic. Most modern magmas contain at least minor amounts of water, which would lead to the formation of biotite or hornblende as the magma crystallizes more intermediate and then felsic rocks (silica, sodium, and potassium rich rocks). This normally produces a more “standard” granite composition of quartz + plagioclase + k-feldspar +/- biotite +/- hornblende. In this case, the lack of water removes the potential for biotite or hornblende forms a charnockite or “orthopyroxene granite.”
