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.