The CGS Field School: Field School 11 Itinerary

Before we begin our geological mapping exercise around the Tjakastad region we will spend a day familiarising ourselves with the general geology and deformation characteristics of the Barberton Greenstone Belt. Remember to check out the geological overview given here.

Reminder - NO GEOLOGICAL SAMPLING is permitted during this trip.

Simplified geological map and general overview localities

Stop 1

We will start the day with a walk into the eastern part of the Komati River. Here we will investigate the upper portions of the Onverwacht Group, mainly the Hooggenoeg, Kromberg and Mendon Formations. These are predominantly composed of pillow- basalts and komatiitic basalts that are often interlayered with chert. We will also investigate the H6 volcano-clastic unit toward the top of the Hooggenoeg Formation. We will also focus on the contact relationship (stratiform vs tectonic) between these different units and discuss their implications.

Stop 2

The next stop will be the c. 3215 Ma potassic, post-tectonic Dalmein Pluton. This unfoliation, slightly porphyritic granite cross-cuts highly deformed ultramafic units of the Hooggenoeg Formation.

Stop 3

We will now look at the Komati Formation, of the Onverwacht Group at the type-locality of Komatiites. Here we will see classical pillowed komatiites and komattitic basalt with characteristic plagioclase-varioles and olivine spinifex texture. We will also discuss methods of determining flow structures and relative age of volcanic flows.

Stop 4

The next stop will be to look at the very lower part of the Onverwacht Group, namely the Theespruit and Sandspruit Formations and the intrusive relationship with the second oldest group of TTG's, the c. 3445 Ma Theespruit Pluton. Here we will investigate features related to the main c. 3.2 Ga deformational event. Furthermore, since our group would have done Thermocalc courses, we will discuss the metamorphism exhibited in these lower Onverwacht rocks.

Stop 5

The next stop is travelling a bit further to investigate the contact between the highly-foliated c. 3445 Ma Stolzburg Pluton and the Tjakastad Schist Belt. We will also discuss the implications of this high strain zone (could this be a tectonic boundary?). Note, the geological mapping region is located south of this location.

Stop 6

Here we will stop to appreciate the not-too visible Inyoka Fault. This fault separates the Northern and Southern Domains of the Barberton Greenstone Belts. We will also discuss the implications of this feature toward the geodynamics of the early Earth.

Stop 7

Here we will look at the largest of the TTG plutons in the Barberton region, the c. 3229 Ma Kaap Valley Tonalite. Besides being the largest, this is also the only Tonalite sensu stricto, with other plutons being Trondhjemitic, plagioclase vs oligioclase, respectively.

Stop 8

Finally, around the Sheba Hills in the north, we will do a detailed road-section on a limb of the Eureka Fold structure. Here we will see the contact region between the upper Fig Tree and lower Moodies Group. We will also discuss the occurrence and mineralisation of gold within the Barberton Greenstone Belt.

Tools you will need:

Of course, the most important is your wits and geological compass, but also: