Into the Bushveld #2: ‘Look at the size of that thing!’

We’ve established that the differentiated mineral unit that I showed you yesterday is part of a larger, slow cooling intrusive igneous body. More specifically, it it found within the imaginatively named ‘Upper Zone’ of the Rustenburg Layered Suite, which is also exactly what it says on the tin: a fairly substantial sequence of compositionally layered igneous rocks. To see just how substantial, we’re going to have to step back a little:

outcrop of Rustenburg Suite of Bushveld complex

The Rustenburg Suite, which reaches thicknesses of almost 9 kilometres in places, is but one part of the larger Bushveld Igneous Complex (pdf): prior to the intrusion of the layered unit about 2 billion years ago, up to 3km of basalts were erupted at the surface, and a granitic body was intruded a little bit later. The total weight of all of this dense volcanic rock appears to have bowed down the crust in the centre of the intrusion, causing everything to dip inwards – including the very hard quartzites beneath the lower contact of the intrusion. These baked sediments form the prominent ridges you can see running around the edges of the Rustenburg outcrop (particularly along the southeastern and western boundaries).
The scale of the layering that we were looking at yesterday seems rather insignificant in a formation that you can quite easily see from space. However, the fact that our magnetite layer is sitting on top of several kilometres’ worth of olivine and pyroxene-rich igneous rocks at least partially explains the weird magmatic chemistry that Kim was wondering about – all of the other minerals that should be forming along with magnetite and plagioclase have been removed from the melt already. Oh, and the chromite, too:

Dwars River Chromite

Categories: fieldwork, geology, volcanoes

Comments (1)

  1. Not without reason, but this brings back fond memories of the very confusing time I was privilaged to spend several years back looking at the Stillwater Complex in Montana. I’m no igneous petrologist, but it was damned impressive and a rather bizarre thing to have to wrap my head around the idea of a “layered igneous intrusion”! Plus, I got to see (and hear about) all kinds of weird rocks that I’ll probably never see again, like troctolite and an unusual striped rock consisting (IIRC) of thick bands of quartz divided by paired bands of mica separated by a thin interval of a couple of centimeters. Great post!