June 12, 2017

Dispatches from field camp: The Big Sky rocks

Posted by llipuma

By Brian Balta

In my last post, I showed off some of the metamorphic rocks we can see that formed about 1.8 billion years ago during a mountain building event known as the Big Sky Orogeny.

The textures in these rocks are fascinating. They contained a variety of protolith lithologies, making them immediately complicated. They are highly deformed; so one lithology bends into another very rapidly. In fact, we had one area within about 10 meters where we counted a metamorphosed limestone, shale, and sandstone amongst the protoliths – an indicator of how tightly folded they are. They reached the upper amphibolite facies, where they started partially melting. They were then strained while they were partially molten, stretching the melt pockets out into veins. They also contain layers with huge porphyroblasts of garnet, which grew more quickly than the surrounding minerals. These are some of the neatest textures I’m getting to walk through.

This rock shows one of the lighter melt veins (we call them leukosomes) and darker sections that haven’t melted yet (melanosomes). The name for this type of metamorphic rock that has partially melted but that still has melt in it is a “migmatite.” You can clearly see that the melt veins are being folded during the strain the rock is undergoing.

Here’s an even cooler example of the rock being foliated while it was melting – boudinage, one of my favorite metamorphic textures. The vein of melt has literally been strung out and stretched out like silly putty as the soft, solid, metamorphosing rocks flowed around it. This sample is coming back to my office with me for a teaching collection.

Here’s a larger scale fold in that same unit – bush for scale. When these rocks are foliated, the layers in them are twisted and folded over themselves so tightly that the minerals give up and reorient themselves in a new direction – that’s the basic mechanism that forms a foliation.

This is an awesome rock. That big, reddish thing is a single, 5 centimeter sized garnet crystal. Garnet tends to grow very rapidly in the amphibolite facies – get to the right pressure and temperature conditions with the right rock and it grows almost out of control. I wasn’t willing to try to break this one out of the rock, but did pull some 3 cm garnets out of the surrounding soil.

That’s it for this sequence, see you again soon from Field Camp in Montana.

— Brian Balta is a a visiting professor of petrology at Texas A&M University. Follow his twitter feed at https://twitter.com/theearthstory for more content.