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parsing jargon
jms — (archived)

I drew you a picture because I can't wave my arms here and I didn't take a picture of the rocks when I was up there, just of the view.

http://members.aol.com/podrock/xenolith.jpg


The thin, almost parallel lines are the amphibolite. That's a metamorphosed basalt or mafic sediment composed mostly of the mineral amphibole and feldspars. Dark rocks, typically, sometimes with a salt and pepper look. Most likely marine basalts. When the rock was metamorphosed (around 1.75 billion years ago), it was folded into those tight folds (called isoclinal folds) which were then folded about a larger, more open fold. That's the F2 regional structure. (the "2" should be a subscript, unfortunately, I'm not typing on an IBM selectric and too lazy to do the html) We are only seeing a limb of that fold here, a few miles away the rocks dip in the other direction. The calc-silicates are the green areas. They are a "dirty marble", full of quartz and weird green minerals. The whole package is cooked up as well by high temps and folded in other ways as well.

The cross-hatched areas are the quartz monzonite igneous intrusion. We could just call it granite if we want. It showed up about 65 million years ago. (I'll have to confirm the date of this particular intrusion) Thing is, the granite melt temperature is nowhere near the melting point of the amphibolite, so the amphibolite tends to break off and go floating into the melt, forming xenoliths. Foreign rocks - exactly right. If you are close to the edge of the intrusion, they may not rotate from their original orientation. See how the magma exploits the fabric of the amphibolite? It will also cut across the structure where there is a fault or a joint in the amphibolite. The xenoliths I was marvelling about were the size of cars and smaller. I'm sure there are bigger ones, but you could only tell that by detailed mapping.

Xenoliths are crucial to our understand of the lower crust and upper mantle. Diamonds are essentially xenoliths from the mantle.

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