The record of plume-arc interaction in the Southern São Francisco Craton – Insights from the Pitangui greenstone belt
Greenstone belts are important windows to the geodynamic evolution of the early Earth. Comprehensive and continuous stratigraphic columns of greenstone belt sections are rarely preserved in South America due to polyphasic deformation, metamorphism and recycling at the cratonic margins and thick regolith layers mainly yielded by heavy weathering. Some recent studies in rocks intersected by drill cores have been pivotal to unveil the stratigraphy of Archean and Paleoproterozoic belts in covered greenstone belts in the São Francisco and Amazonian cratons. This work investigates drill cores from the Pitangui Greenstone belt (PGB), in the Southern São Francisco Craton (SSFC). Rapid localized tectonic transformations are revealed by lithogeochemistry. Two distinct lithochemical associations occur spatially intercalated. The first one is the komatiite-tholeiite association that presents plume signature (low LREE/HREE and no negative HFSE anomalies), with associated low Al2O3/TiO2 and elevated (Gd/Yb)N indicative of a garnet-bearing source, which was also slightly depleted, as suggested by the subchondritic Nb/Ta ratios. In contrast the arc-basalts, andesites and graywackes association displays high LREE/HREE and negative Nb and Ti signatures, which may indicate a subduction-like environment or of crustal contamination. Important part of the subduction-like package are the Nb-enriched basaltic andesites and related volcaniclastic rocks. The presence of this lithotype indicates that metasomatism of the mantle by slab-derived melts was probably an important mechanism in the PGB. It is argued that the assemblages mark a transient stratigraphy from a thick mafic-ultramafic domain generated in an oceanic-plateau to an overlying volcaniclastic-sedimentary dominated package likely generated in an arc-like environment at 2.77 Ga. The intercalation of plume and arc products in the PGB is interpreted as an example of plume-arc interaction, which seems to have been more common in the Archean, as evidenced by numerous occurrences interpreted as such worldwide.