Linking kimberlite magmatism in the Brazilian Platform with Pangea break-up events using in situ Rb-Sr in phlogopite

Abstract

Kimberlite magmatism provides insights into periodic tectonothermal processes linked to the evolution of supercontinents. Paleozoic and Mesozoic kimberlites overlapping with the Pangea cycle are exposed in the Amazonian Craton (Pimenta Bueno field) and the Neoproterozoic Brasília Belt. Phlogopite in mantle xenoliths in kimberlites from the Amazonian Craton are characterized by low Cr2O3 (< 1 wt%) and an enrichment in TiO2 (> 2 wt%), akin to secondary phlogopite formed by kimberlite magma infiltration in the mantle. Low Ti-Cr (< 1 wt%) and high #Mg (91–92) grains are linked to extensive metasomatism of garnet peridotites resulting in clinopyroxene-phlogopite rocks. In situ phlogopite Rb-Sr isochron ages of 250–220 Ma constrain the emplacement of pipes in the Amazonian Craton with no age difference between distinct phlogopite compositions (higher and lower TiO2 phlogopite) and microstructures (porphyroclasts, neoblasts, reaction rims). Mantle xenoliths in kimberlite pipes from the Brasília Belt present high TiO2 (> 2 wt%) akin to secondary phlogopite formed by magma infiltration, and low Ti-Cr (1 wt%), and high #Mg (> 92) phlogopite akin to primary grains in equilibrium with garnet. In situ phlogopite Rb-Sr isochron ages from all Brasília Belt phlogopite types span 90–80 Ma, constraining the emplacement of diamondiferous and barren pipes, with no relic of older metasomatic events. The kimberlites from the Pimenta Bueno field (Amazonian Craton) are coeval with the beginning of the extension within Pangea that culminated with the Central Atlantic rifting. In fact, the Permian-Triassic kimberlites occur close to intracratonic basins that are crosscut by ca. 200 Ma dykes and sills of the Central Atlantic Magmatic Province. In situ phlogopite Rb-Sr isochron ages spanning 90–80 Ma in the pipes from central and southeastern Brazil (Brasília Belt) may be associated with propagation of far-field stresses linked to opening of the South Atlantic Ocean during the Pangea break-up.