This image clearly shows the distinct compositional domains of the garnet as it was formed – magnesium (Mg) and iron (Fe) and manganese (Mn) and calcium (Ca). Image: Kathryn Cutts
Andalusite-bearing rock from Barberton.
Image: Kathryn Cutts
(uranium-lead) dating system and generally gives ages associated with metamorphism. The monazite from this sample gave an age of 3.45 billion years, indicating that this sample experienced a metamorphic event before the 3.23 billion year metamorphism found in the other sample. When the garnet compositions are used to determine P-T conditions, the garnet core occurs within the andalusite stability field while the rim occurs within the kyanite stability zone. This suggests that the cores are 3.45 billion years old and record high T, low P metamorphism while the rims are 3.23 billion years old and record burial metamorphism similar to the other sample. The early low P, high
T metamorphism could be contact metamorphism. Contact metamorphism occurs when hot molten rock from the lower crust moves to the upper crust and heats up the surrounding rocks. This is a plausible scenario because there are plutonic rocks (formerly lower crustal melts) close to where this sample comes from that are the right age (also 3.45 billion years old). Importantly, this sample indicates that the BGGB has experienced at least two metamorphic events, one 3.45 billion years ago perhaps related to pluton intrusion and one 3.23 billion years ago as a result of collision. The P-T path for the 3.23 billion year old metamorphic event suggests that mountains were being created in the
These images show how garnet is made up of different elements calcium (Ca), irons (Fe) and magnesium (Mg)) as it is formed. The way in which these elements were laid down shows us the P-T path involved. Image: Kathryn Cutts
Archean, potentially as a result of plate tectonic processes. These results are important because debate is ongoing as to whether plate tectonics was operating during the Archean or if crust was being created/recycled by some other mechanism. The results here, which suggest a collisional tectonic setting for the 3.23 billion year metamorphic event, are consistent with previous work by researchers from Stellenbosch. â?‘ Kathryn Cutts completed her PhD on the metamorphic evolution of rocks from Scotland and Australia at the University of Adelaide, Australia. She is presently conducting research on Barberton as a SARChi Postdoctoral Researcher at the Centre for Crustal Petrology at Stellenbosch University.
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