I’ve spent the last two weeks towing students around the Barberton Greenstone Belt.
The belt consists of a sequence of volcanic (Onverwacht Group) and sedimentary (Fig Tree and Moodies Groups) rocks which have been heated up to around 400 degrees Celsius at some point after they formed, prompting the growth of new metamorphic minerals such as chlorite (which provides the ‘green’ in ‘greenstone’), and which are folded in and around a number of large granite intrusions. The Barberton region is a place of great importance to Archean geology, because it is one of the very oldest greenstone belts we know of (the lowermost volcanic rocks are almost 3.5 billion years old), and is therefore one of the oldest pieces of contiguous crust on the planet. More importantly, the processes which formed and deformed greenstone belts seem to be intimately associated with the formation of the first cratons, or stable continental interiors.
The volcanic rocks are thought to have principally formed as submarine basalt flows, due to the widespread presence of pillow lavas:
The overlying sediments demonstrate that by about 3.2 billion years ago, fairly significant areas of crust were being uplifted, possibly by the intrusion of the granites, and being eroded. The granites themselves cover a much larger area than the greenstones and form the core of the Kaapvaal craton, the oldest bit of continent proper on the planet, and the platform on which the rocks I’m presently studying (or would be, if I didn’t keep getting diverted into other fieldwork) were deposited.
The exact processes which led to the formation of the greenstones and granites are still poorly understood, and are the subject of some rather vigorous debates. On the one side are those who try to relate everything to plate tectonic processes – the basalts formed at a mid-ocean ridge, and were later thrust together and deformed at a subduction zone, which also generated the granites. However, it is unclear how justified such a comparison is: a higher concentration of radioactive elements, and more leftover heat from accretion, mean that the mantle of the early Earth was probably a couple of hundred degrees hotter than it is today. Thus there is no guarantee that plate tectonics as we know it was the main mechanism of heat loss as it is today (there may have been more mantle plumes, for example). It is certainly true that field evidence for large scale subduction thrusting within greenstone belts seems particularly hard to come by. Fortunately, even if we don’t know precisely what was going on, the outcrops at Barberton give us a fair amount of information about what the Earth was like at a mere 1 billion – some of which I hope to share with you all in the next couple of days.