Monday, March 16, 2009

Migmatites, dikes, pegmatites, and whatnot

Picking up where I left off last week with cool new pictures of rocks from the Billy Goat Trail, today we examine igneous beasties...

As you may have picked up from previous posts on this blog [e.g. here], the rocks of the Piedmont province are essentially the mangled remains of an ancient ocean basin: deep sea sediments, oceanic crust, volcanic islands, even microcontinents -- and all were crushed between North America and Africa during the mountain-building that closed the Iapetus Ocean and formed the supercontinent Pangea. Along the Billy Goat Trail, Piedmont rocks are exposed that started off as deposits of mud and dirty sand, but then were metamorphosed during mountain-building. From the bottom of the ocean to the center of a mountain belt: that forces rocks to change. In some places, they heated up so much that they began to melt.

When rock partially melts, but then the melt crystallizes in places (i.e., it doesn't completely drain out of the source rock), we call it a migmatite. The Billy Goat Trail has some spectacular exposures of migmatite. Here's three shots from the downstream end of the trail:




If migmatitic rock rips open while it is in this partially-molten state, that generates cavitites that the fluid magma flows into and fills. Here, for instance, you can see a rip in the foliated migmatitic metagraywacke that is filled with granite.

Further away from the source rock, mobilized magma can fill in planar fractures that cut across older rocks of many varieties. These cracks are filled in with magma that cools into igneous rocks, and we call them dikes. Here is a new dike I discovered on my hike last week: a vertical dike of granite about one foot thick, cutting across non-migmatitic metagraywacke:

Here's a granite dike cutting amphibolite; weathered out in high relief:

Same dike, from a slightly different angle (I leaned over to the left), to show how it pokes up above the amphibolite like a little wall:

Metamorphosed (some epidote present) granite dike cutting amphibolite:

These fractures didn't open up wide enough to admit large volumes of fluid (either magma or hydrothermal solutions), but there was some fluid flow along them. How do we know? The rock immediately adjacent to each crack weathers out in high relief, suggesting a higher proportion of stable, tough minerals (like quartz). [We've seen this before.] The base rock here is fine grained amphibolite.

Contact between a small granite pluton (or a large dike?) and neighboring amphibolite:

Tension gash in amphibolite, filled in with a mix of potassium feldspar and quartz:

Xenoliths of foliated biotite-rich rocks which I interpret to be metagraywacke that has had all its felsic melt expressed from it, then ripped off by the growing granitic magma chamber (stoping) and dropped into the magma (relatively low temperature, so the biotite doesn't melt), and rotating around to new orientations which do not match the regional foliation orientation. I'm seeing these as shreds of the 'depleted' migmatitic source rock...

Closer-up of these xenoliths #1:

Closer-up of these xenoliths #2:

Another cool thing I saw on last weekend's hikes was pegmatite. Pegmatites are present where there is a particularly watery magma. Water, the universal solvent, helps act as a courier, ferrying atoms around to where growing crystals can access them and add to their bulk. As a result, pegmatites are characterized by really large crystals. These potassium feldspars are highlighted by lichens which grow at the interface between the feldspars and the surrounding milky quartz:

Those same black-colored lichens can also highlight the cleavage planes of the feldspars:

Another big-ass K-spar:

...and another:

I love this stuff. Hope you enjoy these igneous treats as I much as I enjoy sharing them.

Labels: , , ,


Anonymous Anonymous said...

Hope you enjoy these igneous treats as I much as I enjoy sharing them.

I enjoy them very much.[thumbs up]

I have been trying to re-aquaint myself with igneous/metamorphic petrology and structure, and your posts are very informative and helpful. Thanks

March 16, 2009 12:32 PM  
Blogger Lockwood said...

Deee-licious! Again, I'm surprised to see such high-grade metamorphism in the piedmont. Perhaps I shouldn't be (as he struggles to recall historical geo from nearly 30 years ago), but you've got stuff there I would have guessed was Grenvillian in provenance.

March 16, 2009 2:07 PM  
Blogger Callan Bentley said...

Hey Lockwood -- all our Grenvillian-aged rocks are in the Blue Ridge province, not in the Piedmont. These have metamorphic ages (and magmatic crystallization ages) of ~460 Ma, the Taconian episode of mountain building.

March 16, 2009 3:18 PM  
Blogger Donald said...

Thank You, I was looking for migmatite pictures to put into my lecture on metamorphic rock types(Loyola Marymounty University, Los Angeles) and ran across your blog. Great shots

September 27, 2009 5:14 PM  

Post a Comment

<< Home