In situ Pb-Pb garnet geochronology as a tool for investigating polymetamorphism: a case for Paleoarchean lateral tectonic thickening

Abstract

The Barberton Granite-Greenstone Belt remains a key location in the debate concerning the nature of Archean tectonic processes. Much work has focused on deciphering the tectonic significance of the c. 3.23 Ga metamorphism, as this has been correlated with lower geothermal gradient conditions potentially indicating Archean subduction. However, several studies also found evidence of an earlier, 3.45 Ga metamorphic episode, overprinted by the 3.23 Ga event. Here we apply in situ Pb-Pb dating and P-T modelling to a large (3 cm diam- eter) garnet crystal, allowing for the direct dating of the metamorphic conditions obtained from the garnet. The garnet core produced an isochron age of 3435 + 45 Ma, corresponding to an increase in P and T evolution reaching peak conditions of at least 7 kbar and 700°C. Analyses obtained from the garnet rim give an isochron age of 3245 + 41 Ma, corresponding to P-T conditions reaching 8-9 kbar and 700°C. The preservation of two moderate- to high-pressure events occurring 200 million years apart is consistent with lateral tectonic processes producing crustal thickening at 3.2 Ga and may also be a viable process for the earlier event.