Bedrock Geology of Nampula Province (Rapale Gneiss)

Here's a bit of geology for those who care! We toured several quarries in and around Nampula, trying to get a grip on the variation within the Rapale Gneiss. At a quarry in the city (Lee-Anne and Jodie and I agreed to name it "Nampula Quarry" but I am now thinking we should have been more creative) I found them! The big Folds!!! In this exfoliated landscape, it's hard to get at a vertical fold with a sub-horizontal axis from atop a dome, especially since the quarries are the only desert varnish-free surfaces. I am truly fascinated with this landscape and I wish I could find a geomorphologist who would work on it.

The Rapale Gneiss is a tonolitic orthogneiss (that means it is a metamorphosed rock, which was formed from a tonalite, which is like a granite only it has basically no potassium feldspar and not so much quartz, either). The mineralogy is plag-bt-Hbl-quartz-sphene(?). Amphibolite-facies metamorphism (Pan African age, 600Ma) created a biotite foliation of variable intensity, which is roughly axial planar to the folds. The folds are hard to find, because the rock is almost never compositionally banded. In the first photo you see an anticline, plunging ever so gently east toward the charnockite quarry. The little cut face gives a cross sectional view of this assymetric fold.

Lest you get the idea that things are orderly in this gneiss, here's Dr. Micheque posing with a folded meta-mafic-dike? thing? which is west plunging. the dike is paper thin and has an incredible chocolate tablet structure, where a sheet of 3-5mm biotite books seems to have been brittley stretched by plane strain and the joints filled in with quartz/feldspar veins. Then the whole mess was recrystallized, was crosscut by felsic dikes, and then sat there quietly for a half a billion years until Dr. Micheque paid the quarry man $25US/ton to have somebody make it into brick-sized building stones for his new house in Nampula. Wooee!

Elsewhere in the Rapale gneiss, and never in the charnockite quarry, we see these Hbl poikioblastic things with sub spherical quartzes and also euhedral feldspars inside. They aren't overprinted by the biotite foliation; in fact, they form in low-biotite zones. My theory at the moment is that they have grown post deformation, during the oft-referenced isobaric cooling? Which may not have been isobaric in all directions, if you catch my drift. Anyway they will be Jodie's problem I think. Can't wait to see them in thin section. Could they be the zones which are readily charnockitized? They are about the size of the "opx" lumps in the charnockites.

Too bad I didn't take a whole lot of photos around the "ordinary" gneiss, at least not ones that are worth posting on a blog. Anyway here's Jodie, Micheque, and Lee-Anne at sunset on our first day in Nampula, after we hired a cab to take us to the Rapale gneiss "normal" quarry. I think this is a nice photo of Jodie explaining some wack-ass geochemistry. Ha just kidding Jodie!

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