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Mixed-up with Mongolian Migmatite

When I was a novice geology student, the world appeared kind of simple. Faced with a rock, I could ask myself the most fundamental question – is this rock sedimentary, metamorphic or igneous? And there was some sort of expectation that the question could be answered clearly. It seemed the same in First-year Botany. We were taught about xylem, tracheids and parenchyma cells and shown specimens where they could be easily distinguished under a microscope. It seemed kind of simple. Then came the day when our teacher, Dr Brenda Shore (a very capable scientist, who I much admired), suddenly thundered” “You Lot! You think you know it all – I’ll show you something and you won’t have a clue what it is!!” Or something to that effect. I’m not sure what prompted it, I think the background chat in the class had gone above the acceptable level. But the comment scored a point, because it stuck in my mind ever since. The world does not break down into the categories we want it to.

Back to geology – it’s when you find yourself alone in some place, without a detailed map (you’re the guy making that), without a library of academic papers to get clues from, and you find yourself faced with some rock that makes you think – What the Hell is this? Where’s my Prof to tell me? That is really comes home. The universe doesn’t follow our rules. And just to be perverse, said Universe might even make the rock is deeply weathered, and place you under a tropical rainforest canopy in a storm where its so dark you can barely see anyway. Fortunately Mongolia is not a place where a geologist has to deal with those extra problems. In Mongolia, many rocks seem to look as fresh as the day they were minted, and in the land of the Eternal Blue Sky, there’s generally no problem seeing a rock clearly (absent snow). So it’s perturbing to still come across stuff that leaves you puzzled.

One kind of rock that Mongolia has good examples of is ‘migmatite’. This is a rock that’s part-way between a metamorphic rock (recrystallised as a result of pressure and heat) and igneous (formed from molten rock). Note that it’s not a term for when you are not quite sure whether its a metamorphic or igneous rock and just want some word to cover your ignorance and sound smart. A migmatite is a metamorphic rock that has actually started to melt. So it’s right between a metamorphic and an igneous rock. And there’s the rub – as a geologist facing an outcrop in the field – are you sure that quartz has formed from a melt, or from solution, or as some sort of recrystalisation? You can find your self-confidence waning rapidly. I have seen migmatites where my colleagues were sure of what it was, but I’ve also come across rocks where I’m still stumped (where’s a Prof when you need one?).

A typically highly-folded migmatite in the Gobi of Mongolia. There has been a segregation into light-colored fraction (leucosome) and dark-colored (melanosome).

A typically highly-folded migmatite in the Gobi of Mongolia. There has been a segregation into light-colored fraction (leucosome) and dark-colored (melanosome).

It’s some consolation to learn that the whole field of migmatites is pretty esoteric stuff, and perhaps way beyond a non-specialist in the field with no access to microscopes, thin-sections and so-on. Migmatites are tied up in the debate over the origin of granites – a huge, complex issue by itself. So nice to come across a whole book devoted to beautiful photographs of migmatites – an ‘Atlas of Migmatites‘ (Sawyer 2008). I find there’s nothing like a nice, visual guide. But as well as that, Sawyer’s Atlas concisely explains what to look for, the different types of migmatite, and what they mean. Migmatites are not just ‘mixed-up’ rocks. For the geology of Mongolia, migmatites are a critical part of understanding how Mongolia was put together. They are clues to where and when ancient terranes collided to form the geologically complex land that is Mongolia. The Atlas of Migmatites is a large, pricey tome, but boy, I wish I had this sitting on the bookshelf in the ger…..


Sawyer , E.W. 2008. Atlas of Migmatites: v. 9: The Canadian Mineralogist Special Publication.


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