Tavşanlı Zone field trip, part 1

Before the Tectonic Crossroads conference two weeks ago, I had the good fortune to participate in a Istanbul-to-Ankara geology field examining the Tavşanlı Zone, a tectonic suture zone where a portion of the Tethys Ocean basin closed. This paleo-convergent boundary is marked by a suite of interesting rocks, including blueschists, ophiolites, and eclogites. I’d like to share with you some of the things I saw along the trip.

This is one of the trip leaders, Aral Okay (pronounced “Oh-kai,” okay?), discussing the general geology of the area at our first stop. (The other trip leader was Donna Whitney.)

tav_1_01

I think in general, you can make out the east-west trend of the rock units on Aral’s map (where they aren’t obscured by alluvium). This reflects the approximate north-south convergence of the Tethys closure in Turkey. To visualize this, I’d like to call your attention to a paleogeographic interpretation of the Tethys Ocean from Ron Blakey, the talented mapmaker from Northern Arizona University:

tethys

See all those colliding east-west-oriented crustal fragments in the northwestern Tethys? Those are the pieces that will comprise future Turkey. As you can imagine, rocks caught up in these tectonic collisions got both deformed and metamorphosed. Some of them were even subducted to ~80 km depth, and then brought back up to the surface! At our first stop, we saw some blueschist-grade rocks that had a phyllitic texture. Here’s two of them:

tav_1_02

As usual, my eye was drawn towards the structures visible in these rocks. Here are a couple of nice little folds:

tav_1_03

tav_1_05

(The Turkish 1-lira coin is the same size as a U.S. quarter.)

I found this to be an interesting portion of the outcrop:

tav_1_04

That’s green phyllite on the left, and blue phyllite on the right. Allow me to annotate it for you:

tav_1_04anno

“Blueschist” and “greenschist” refer to two assemblages of minerals which supposedly represent different combinations of temperature and pressure. They are examples of metamorphic “facies,” as illustrated in this image:

facies

Image redrawn and modified by me from Figure 3 of Bousquet, et al. (2008), which is itself modified from Oberhänsli, et al. (2004), and also from University of British Columbia (1997), which is modified from Yardley (1988).

Theoretically, blueschists and greenschists should be forming at different combinations of pressure and temperature. Blueschist forms at high pressures, but relatively low temperatures. But here we have an outcrop of blueschist that is right adjacent to a greenschist (medium temperature and pressure), with no faulting in between. It was suggested to me by a blueschist expert that this was likely a reflection in differences in the initial composition of the protoliths. I found this explanation less than completely satisfying, but there was no time to discuss, for we were being called back to the bus, already gunning its engine and ready to roll down the road.

At our second stop, we found some metamorphic rocks that showed clear textural evidence of having had pyroclastic protoliths:

tav_1_06

There were lots of chunky bits in there.

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So it wasn’t just pelitic (muddy) rocks that were getting metamorphosed in this Tethyan suture zone, but volcanic rocks too!

More later… when we move on to stop #3

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9 Responses

  1. The “compositional differences” argument is also invoked for the juxtaposed blueschists and eclogites at Jenner, California (e.g., Blueschist/Eclogite Deskcrop). Doesn’t seem unreasonable to me, though I also haven’t explored the literature on it.

  2. If I don’t come back and try to explain this in a couple hours, remind me (via Twitter or e-mail). Short answer: bulk composition differences are probably the explanation, but without knowing exactly which green minerals are in your “greenschist,” it’s hard to give you a precise answer. (Shorter answer: blueschist-facies rocks aren’t necessarily blue.) And I might need to draw diagrams, which would require dusting off my blog.

  3. I’ve seen this in Anglesey, Wales too. I seem to remember part of the argument was compositional and what the ‘green’ and ‘blue’ minerals actually are.

  4. Ok, class is done and I’ve got a moment to try to explain this.

    First big concept: the minerals found in metamorphic rocks depend on both the original composition of the rock and the metamorphic conditions (T&P) that the rock experienced. (There’s also a third possibility – changing the composition of fluids that run through the rock – but that complicates things even more.) For rocks like quartzite or pure calcite marble, the role of composition is obvious, but it’s more subtle in rocks like metamorphosed shales or basalts, which form all the cool minerals that make metamorphic petrologists happy.

    That metamorphic facies diagram is based on typical mineral assemblages in metamorphosed basalts. So the amphibolite facies, for instance, is the range of temperatures and pressures at which hornblende and intermediate plagioclase coexist in a metamorphosed basalt.

    Basalts are a great system for the amphibolite facies, in particular, because hornblende has a huge range in composition, so you can get the same minerals over quite a range of rock compositions and pressure and temperature conditions. But if the minerals that form have a smaller range of compositions, or if they are more sensitive to temperature and pressure differences, it’s possible to get a much wider variety of minerals. In metamorphosed shales in the amphibolite facies, for instance, you can have kyanite or sillimanite or andalusite or staurolite or garnet or biotite coexisting with quartz and muscovite (and that’s just varying K, Al, Fe, and Mg – think about Na or Ca or Mn and you’ve got even more freedom). In fact, in metamorphosed pelites, it’s possible to get quite different mineral assemblages in different layers of the same sedimentary rock, just based on the amount of Al (or Fe vs Mg) in the rock. (I think some of your pictures from New Hampshire show this effect.)

    So what about blueschists? Well, at low temperatures, there are a lot of different minerals that can form in basaltic composition rocks. The key minerals that define the blueschist facies are coexisting glaucophane (the blue amphibole) and lawsonite (a Ca Al silicate). But unlike in the amphibolite facies, a lot of other minerals are also potentially stable, depending on the composition: chlorite and albite and quartz, and maybe epidote or a white mica or a slightly greener amphibole. A lot of those minerals are green, so it doesn’t take much compositional difference to make part of a blueschist-facies outcrop look green.

    (And at the high pressure side of things, the reaction that forms the green pyroxene, omphacite, happens at different conditions depending on the amount of Ca and probably Fe and Mg in the rock. So it’s also possible to get stuff that looks like eclogite and stuff that looks like blueschist interlayered together.)

  5. (Also, can I add that working in retrograded blueschists is my personal definition of hell, simply because of how difficult it is to tell whether mineral differences are the result of composition or P/T conditions? When I got out of grad school, I never wanted to see another high-P/low T metamorphic rock again.)

  6. Kim’s answer does a fairly good job, but I just want to give a shout out to the fluid issue. Depending on protolith, you might be dealing with fluid-rich vs. fluid-poor rocks and rocks with higher amounts of fluid are much more likely to actually react to form stable mineral assemblages. Rocks that are fluid-poor tend to have slower reaction progress and are much more likely to preserve metastable assemblages. This also would impact the system if they were the same protolith originally (say fluid-poor) and the fluid-flow through the system was localized instead of pervasive.
    Basic question to ask, though, would be what was the currently mineral assemblage in each rock and then we could backtrack to the protolith.

  7. [...] Yesterday, I shared a few thoughts about the first couple of stops on the field trip I took earlier this month from Istanbul to Ankara, prior to the Tectonic Crossroads conference. Today, we’ll pick up with some images and descriptions from the next few stops. [...]

  8. [...] Geologists on the field trip ponder the meaning of greenschist in with the blueschist, just like with did back at our first stop: [...]

  9. [...] Today’s geology word (according to Evelyn) is “fabric.” I spent a little time this afternoon playing with my ultra-cool Nikon microcamera, and decided to photograph the S-C fabric in one of the rock samples I collected in Turkey. [...]

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