6/13/2008

Charnockites.... what?

Hey geoblogosphere, help me out will ya?

I'm a brittle fault person by trade. Now I'm working on a project in the middle crust... slow, squishy and unfamiliar territory. Big mineral grains are nice though... I can identify them in hand sample! They fill a thin section! In most of my field areas, I was identifying minerals primarily through x-ray diffraction. So this is a nice change I suppose.

So here's the scene. The protoliths are a TTG suite - tonalites, granodiorites - metamorphosed at amphibolite facies. Luckily for us, the biotite foliation is pre-syn folding... which means it is sometimes axial planar but often folded. Leucosomes abound; they are solid albite+/-quartz. There are some gorgeous folded meta-dikes of nearly straight amphibole - most likely primary basaltic dykes into the TTG suite - which I am planning to use as strain markers. Here's one in a quarry in the middle of Nampula, Dr. Micheque for scale:

Outcrop here is limited to isolated, steep-sided mounds of rock (inselbergs), it's a common geomorphology in the subequatorial Gondwanaland. There are many inselbergs around Nampula, with completely flat land in between. I have questions about that as well... but I'll save those for another post. Anyway, one of the inselbergs has a unique zone crosscutting it, a networked zone of kspar-qtz-mt veins with a smoky, greenish patchy alteration zone around them (Lee-anne Rudd, Hons 07 and trooper extraordinaire, for scale). What you see looks like wet patches on a jointed rock, but I assure you, at 40° in blazing sun, there is no water here, you are seeing the greenish alteration zones:
Here's a closer look at the thing. The edges of the blocks in the network are sheared, the biotite foliation is curved into the boundaries - implying some rotation of the blocks. (excuse the white chalky patches, which are hammer blows):

Previous descriptions by previous workers called this a charnockite zone. This led me to a big literature search on charnockite (which I never, I swear it, heard of as an undergraduate). I found basically that charnockite is a sparcely defined facies? or rock type? resulting from (rock containing hydrous mafic minerals) + (CO2) -> (rock containing orthopyroxene) + (k-rich, volatile-rich melt). Basically, the idea is that if rocks with a little bit of water in them are flushed with CO2 at high enough temp, the hydrous minerals will dehydrate and the water will cause partial melting of the less refractory minerals, in this case biotite. (More recent work has pointed out that the fluxing fluid need not be CO2, but it needs to be thirsty for water). This fit well with our field observations and we were happy. Until we got home with the rocks. And found no opx, anywhere. Worse yet, our samples from the general background look exactly the same in thin section as the samples from the green patchy alteration zones. No textural difference, no change in mineralogy, even some (scanty, preliminary) microprobe work didn't turn up any difference between the green greasy-looking patchy rock, and the ordinary biotite gneiss. Hmmm.

From a geochemical standpoint, WTF? How can something so obvious in outcrop disappear in thin section? Luckily we have a new secret weapon on board, a metamorphic petrologist who knows THERMOCALC, knows migmatites, would prefer pelites but will help us out anyway.

From a structural standpoint (my standpoint), looky here. We have a network of veins of (I don't know what), at roughly 90° angles. This network occurs in a narrow linear zone about 20m wide. It clearly accommodated the flow of some reactive fluid. And take a look at the meta-basaltic dikes in the blocks in this zone:

Clearly, and this is a new level of wishywash for me, this is a "locus of strain" here in this zone.

Now we come to the question: Falling back on my old friend the Mohr Circle, I would say that 90° fluid conduits are a sign of hydrofracture. Can that be true? Slow, ever so slow hydrofracture in which the opening of joints is accommodated by volumetric strain in the blocks between joints? More importantly, can I use these folded dikes as passive strain markers? And people, what are charnockites? Truly? Do they require CO2, or low aH2O magma, or what? And how do I find out?

* just so you know, that is not my compass. I'm a brunton girl to the grave.
** theres been a lot of chatter lately about appropriate use of the geoblogosphere. Allow me to push the limits here by asking for your opinions on current research.

7 comments:

Kim said...

I think the definition of charnockite is an opx + kspar granitoid. I've seen it in the Adirondacks of upstate New York - it's metamorphosed there (so now it's a garnet meta-charnockite or something), though, so I've never seen it as a pristine igneous rock. It's associated with anorthosite in the Adirondacks, if that helps with the igneous association at all.

Chuck said...

AFAIK, a charnockite is an anhydrous granite- so the mafic phase can't be amph or mica, it has to be pyroxene. As far as I know, it doesn't have to be opx, but I think generally works out that way. CO2 fluid theories would suggest that the lack of water is metamorphic, not igneous, but I haven't studied them in detail.

As for your green alteration halos, check the LOI to see if they are more hydrated/carbonated.

JF said...

Hi Christie, you might want to cross the boerewors curtain and come here have a chat with Gary & me ... it would be easier than replying on your blog, what? There are also granulite guys (and gals) way more knowledgeable than me at Durbs, UJ and Pretoria that can probably help you.

There is lots of confusion around the names of granulite-facies rocks. You can brush up your French and have a look at what I've got here : http://jfmoyen.free.fr/spip.php?article196 , inspired by that : http://christian.nicollet.free.fr/page/Figures/charnockite/charnockite.html

It boils down to, charnockites are granitic magmas that crystallized in the conditions of the granulite-facies (i.e., above the biotite breakdown reaction). Therefore, they normally have opx and KSp, because Bt + Plag = Opx + KSpar + H2O.

However, to make our life miserable, there can be meta-granitoids, that crystallize as normal granites but get metamorphozed in granulite facies conditions -- they can therefore form "charnockitic gneisses". Not real charnockites (they did not crystallize in granulite facis) but rocks with similar mineralogy.

Furthermore, chranockites are commonly retrogressed, and opx reacts back to biotite (because, guess what, Opx + KSpar + H2O = Bt + Plag). So it's not uncommon to find rocks in the field that you would swear on our Brunton are charnockites and turn out to have no opx. Sometimes however, you can spot opx pseudomorphs and things like that (I do have nice examples of that in the Limpopo SMZ).

The greeny "charnockite" patches on granulitic gneisses (that are probably not charnockites s.s., as they are probably not igneous) are a relatively common occurence in granulite-facies terrains. One of the most spectacular locality (strikingly similar to your photos from Nampula, I would have to dig up my old 35mm slides to show you...) is the Kabbaldurga quarry SW of Bangalore, Ka., India. The locality came to some sort of fame in the 80's, was studied by Germans and Indians (Janardhan, Raith, Raase, Braun). Happy georef-ing... As far as I remember, the problem is the same -- obvious diffeencs in the field, nothing in thin sections or geochem.

Hmm, from a geochem perspective, I wouldn't mind having a look at what's happening, by the way...

Fault Rocks said...

Thanks everyone! I appreciate all the input. These "charnockites" are definitely NOT the igneous kind as they only occur along these pathways (fractures?) in an otherwise amphibolite-facies rock. Chuck -Good idea to check the LOI, I will look at that. JF - thanks I will come over! Emailing you now.

Silver Fox said...

I hope you will keep us posted on what you find out, and on your fascinating project. And great photos!

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I think the definition of charnockite is an opx + kspar granitoid. I've seen it in the Adirondacks of upstate New York - it's metamorphosed there (so now it's a garnet meta-charnockite or something), though, so I've never seen it as a pristine igneous rock. It's associated with anorthosite in the Adirondacks, if that helps with the igneous association at all.