Saturday, March 22, 2008

High-pressure metamorphism before 1.7 billion years ago?

The further back in time we look, the more difficult it is to figure out how plates have moved, or whether plate tectonics was even active. The ocean basins give us less than 200 million years of history, and in long-lived continental crust, younger events can wipe out the record of older events.

And some types of evidence are quite fragile. For instance, when oceanic crust is subducted into the mantle, it moves more quickly than heat can flow into it, and is metamorphosed into distinctive rocks that form only at unusually low temperatures for their depths. These rocks, blueschists and eclogites, are convincing evidence for an old subduction zone... but they rarely survive their return trip to the surface. It doesn't make much heating to replace the unusual minerals of a blueschist with more common lower-pressure minerals.

In the 1980's, when I first learned about blueschists, there weren't any known that formed in Precambrian time. Did that mean that plate tectonics was a new phenomenon, or that the subduction-related rocks just hadn't survived more than 500 million years? In the years since then, older high-pressure rocks have been found, but they're still very, very rare. Precambrian geology is interpreted in terms of plate tectonics, for the most part, but the blueschists and eclogites aren't part of the evidence in most mountain belts.

That's been true of North America. My corner of North America is thought to have formed from collisions between lots of volcanic arcs, from around 1.8 to 1.6 billion years ago. The evidence comes mostly from metamorphosed volcanic rocks, and from Precambrian deformation. The arcs are here, but their subduction complexes are gone.

Or, at least, their subduction complexes are mostly gone.

At the Rocky Mountain/Cordilleran section Geological Society of America meeting that I was at last week, Nina Fitzgerald and Mark Colberg of Southern Utah University showed evidence of retrograded eclogites from southwestern Utah. The rocks weren't pristine, by any means, but the authors had good evidence that the rocks had been at much higher pressures. (Not extremely high pressures - the pyroxenes didn't contain as much sodium as they do in, say, the Franciscan eclogites from north of San Francisco. And they didn't find coesite or diamonds, like in the ultra-high-pressure rocks of Norway or China or the Alps.) But they were the best candidates for subduction-zone rocks I've seen described in the Precambrian of the American Southwest. And they're the oldest high-pressure rocks I've heard of (though I've been out of the HP loop for a while now).

And they were found by people at a school with no grad students, doing good field mapping and looking at thin sections.

Ref: Fitzgerald, N.E., and Colberg, M.R., 2008, Evidence for Paleoproterozoic high-pressure metamorphism and decompression melting in the Mojave-Yavapai suture zone, Beaver Dam Mountains, Utah: Geological Society of America Abstracts with Programs, v. 40, n. 1, p. 64.

3 comments:

BrianR said...

Do you think the lack of old subduction-related rocks is essentially all about preservation (as you allude to)?

Or, does anyone in this area of study think there was something fundamentally different about plate tectonics >1 Ga or so?

This is way out of my field ... just curious.

Kim said...

My impression as an undergrad in the 80's was that Precambrian geologists questioned how much modern plate tectonics could explain of their observations. There's still talk about that, but my impression is that it's the Archean, not the Proterozoic, that's the most suspect.

Warren Hamilton had a talk about the problems with applying plate tectonics to the Archean, and he got people talking in the hallway afterward. I didn't see it, though. (I was listening to a geoscience education talk, instead.)

Chris Rowan's doing paleomag on Archean rocks, though, so he's deep into Archean tectonics. And he's worked in young tectonics in New Zealand. We should bug him to blog about the Big Questions about the differences between Archean and Phanerozoic plate tectonics.

As far as high-pressure metamorphism goes, though, I think that preservation is a huge problem. (There aren't good blueschists along either of the old orogenic boundaries in New England, although there are rocks in Vermont that are thought to be a subduction complex, and I think there's about one eclogite locality in the entire Appalachians. The Norwegian ultra-high-pressure eclogites make up for the rest of the mountain belt, though!) The Franciscan is really an exception, rather than the rule, for the preservation of high-pressure metamorphic rocks.

BrianR said...

"The Franciscan is really an exception, rather than the rule, for the preservation of high-pressure metamorphic rocks"

Yeah...good thing the San Andreas transform margin came along to help preserve it!