At the end of yesterday's post about ultramafic rocks included in the Piedmont meta-accretionary wedge complex, I showcased a few boulders and cobbles found in our local streams. The last one I showed was a migmatite: a rock which is a complex swirl of high-grade metamorphic rock and granite magma. Here, gneiss has "sweated out" a liquid melt of its most easily-melted minerals (the felsic ones: quartz, potassium feldspar, muscovite mica). Minerals which have higher melting temperatures didn't melt, and are left behind as a dark-colored, well-foliated residual gneiss. The magma it spawned has joined together with little rivulets of felsic magma emerging from neighboring areas of hot gneiss, and then congealed & moved along as a blob. That blob eventually cooled and solidified into the (light-colored) granite rock you see on the front of the boulder. Lens cap is 5 cm in diameter.

The idea here is called partial melting: as the original graywacke sediments of the Iapetus Ocean floor got heated up during mountain building, some of the minerals therein melted, but others didn't. The melted portion escapes as a buoyant, mobile liquid, but the unmelted portion stays where it is as a solid, dark-colored (mafic) residue. A migmatite therefore is a really interesting rock: it has one foot in the metamorphic camp, and another foot in the igneous camp. A migmatite is the rock cycle in action; the Earth's dynamic processes caught red-handed!

Sometimes chunks of the mafic residue get broken off and go spinning wildly through the pockets of magma. When the magma cooled and solidified into solid granite, these mafic chunks were trapped as xenoliths. The xenoliths in the following three photos were all photographed in outcrops along Four Mile Run, in Arlington, Virginia near Columbia Pike. Note how the xenoliths have their own internal foliation, which is not necessarily aligned with the regional foliation:

Here's the contact between the migmatitic gneiss and the granite magma it has sweated out:

I'm not totally sure what's going on in this image, but it looked cool, so I photographed it:

More complex relationships between intermediate-composition source rock and derivative granite, with a new player added in as well: hydrothermal quartz veins.

These quartz veins were likely the last of these three components to be emplaced. In most places, they are straight, and if they are deformed, it's brittle deformation (as in the left-lateral fault seen below) and not ductile (flowing) deformation. This indicates their emplacement along fractures after the bulk of orogenic heat & differential pressure has left the rock.

The gneiss/migmatite was intensely squeezed during the process of partial melting, as this folded foliation shows. You can also see the contact with a more massive body of granite at the top of this outcrop, and "fingers" of granite intruding along the "plane" of foliation. I wonder how much of a role differential pressure (squeezing) plays in generating a granite. Yeah, you have to heat the rock up enough to melt out the quartz, potassium feldspar, etc. But if you squeeze it too, perhaps that helps separate the melted component from the solid component, much as a cheesemaker uses cheesecloth and some judicious squeezing to separate solid curds (future cheese) from liquid whey.

Lastly, the Four Mile Run outcrops show a nice waterfall, which is pockmarked with lots of lovely smooth potholes. I'm less into geomorphology than I am into orogeny (can you tell?) but they're neat features, and well worth a photo or two:

Here's a nice "flume" (sort of a sideways-oriented pothole) channeling a small amount of water over the top of the waterfall ledge. You can see it starts off as two lateral chutes, which then converge in the middle, merging into a single channel. It was beautifully smooth, like a fine sculpture (which I guess it is!).