11
$\begingroup$

Blueschist has a mineral assemblage such that it must have formed at high pressures and low temperatures. It must therefore form in subducting slabs (and I believe this correlates with the locations in which it's found). However, that leaves the issue of how to get it to the surface rapidly enough that it doesn't heat up sufficient to reequilibrate and lose the characteristic mineral assemblage.

What are the currently favoured explanations/processes for such rapid exhumation from depth?

$\endgroup$
  • 1
    $\begingroup$ The rate of a thermally activated processes decreases exponentially with decreasing temperature. It doesn't require much cooling to slow the reaction down by a lot. High-grade metamorphic rocks contain meta-stable assemblages of high-temperature minerals, but virtually all also show some low-temperature alteration as well. This question would be improved by some explanation of why these assemblages are so surprising. Why do you think this case requires special explanation? $\endgroup$ – Mark Rovetta May 1 '14 at 21:16
  • $\begingroup$ Will attempt to expand question during daytime local. To address points briefly: blueschist is the one I know to ask about; in my second year of undergrad (2010ish) it was mentioned in passing that the exhumation process was poorly understood; and while I perfectly well understand melt migration from slab to arc, I've no idea at all as to how a solid piece of (cold, dense) subducting slab - which, by my understanding, is what blueschist is - should be able to migrate upwards through the wedge. $\endgroup$ – kaberett May 1 '14 at 22:52
6
$\begingroup$

Fast exhumation is not necessary to retain blueschist mineral assemblage. This is the classical difference between prograde and retrograde metamorphism, and it's not limited to blueschists. Why do we have eclogite on Earth's surface? What about granulites? Amphibolites? In fact, why do we see any metamorphic rocks on the surface instead of just various clays and quartz?

There are two main reasons for that.

  1. Prograde metamorphism results in dehydration and loss of water(1). Most of the original water content of the rock was in various hydrous minerals, and as you progress in your metamorphic path you lose more and more water to dehydration reactions. While it is true that in blueschist waster can be retained in minerals such as lawsonite, phengite, epidote and glaucophane, most of it is still lost. If you want to reequilibrate the rock to its previous mineral assemblage, you need to reintroduce the water. This can occur occasionally, for instance in this classic example from Norway:

    rehydration source: http://www.geol.ucsb.edu/faculty/hacker/geo102C/lectures/part11.html
    ...where a granulite was rehydrated along a joint to form eclogite, but it's usually very localised and uncommon.

  2. Most prograde metamorphic reactions are exothermic, i.e. they release heat. Retrograde reactions are usually endothermic. They require heat in order to proceed, but in the event of exhumation, temperatures are decreasing. There is simply not enough heat around to support the retrograde reactions. Furthermore, you have to remember that lower temperature also lowers the reaction kinetics.

This doesn't mean that fast exhumation cannot occur...

There are pieces of evidence that in some cases exhumation can be even faster than the erosion rate(2). This suggest that orogenic events may also result from simple uplift and not only compression and folding. What causes the uplift? This may actually be the result of crustal thinning and extensional stress regimes, as in the examples of Liaodong(3) and Brittany(4). This phenomenon is also known as a core complex, where extensional tectonics result in low angle normal detachment faults that results in rapid unroofing and exposure deep seated rocks. You may be familiar with oceanic core complexes, so this processes is quite similar.



(1) Forgive me for not including the zeolite facies in my definition of metamorphism.

(2) Rubatto, D., & Hermann, J. (2001). Exhumation as fast as subduction?. Geology, 29(1), 3-6. 10.1130/0091-7613(2001)029<0003:EAFAS>2.0.CO;2

(3) Yang, J. H., Wu, F. Y., Chung, S. L., Lo, C. H., Wilde, S. A., & Davis, G. A. (2007). Rapid exhumation and cooling of the Liaonan metamorphic core complex: Inferences from 40Ar/39Ar thermochronology and implications for Late Mesozoic extension in the eastern North China Craton. Geological Society of America Bulletin, 119(11-12), 1405-1414. 10.1130/B26085.1

(4) Brown, M., & Dallmeyer, R. D. (1996). Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France. Journal of Metamorphic Geology, 14(3), 361-379. 10.1111/j.1525-1314.1996.00361.x

$\endgroup$
4
$\begingroup$

I don't know whether its currently the favoured explanation, but in California people made reference to something informally referred to as the watermelon seed theory . The general idea being that the compression in a subduction zone managed to eject these rocks back up and out of depth by a process like squeezing (and shooting) a wet watermelon seed between your fingers.

$\endgroup$
  • $\begingroup$ Ah yes, Blueshist Knockers ;) $\endgroup$ – david valentine May 2 '14 at 20:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.