Composition and P-T conditions of the lithospheric mantle beneath the Azimuth 125º Lineament, Northern and Southeastern Brazil: constraints from peridotite xenoliths enclosed in diamond-bearing kimberlites

Abstract

We present new petrographic and chemical data together with calculated P-T equilibrium conditions of peridotite xenoliths enclosed in kimberlites from Rondônia, Northern Brazil (Cosmos-1 and Carolina-1) and Minas Gerais, Southeastern Brazil (Canastra-1) located in the Azimuth 125º Lineament. The composition of the mantle minerals is distinct in both areas, which can be related to the diversity of the lithospheric mantle beneath the southwestern portion of the Amazonian Craton and the Brasília Belt. New and compiled chemical data indicate that subcalcic G10 garnet occurs in samples from the Canastra-1 kimberlite and other occurrences of the Alto Paranaíba Igneous Province and can be related to the remnants of the Archean lithospheric mantle of the São Francisco Craton beneath the area. The garnets from Rondônia are mostly G5 (pyroxenitic) and G9 (lherzolitic) with a higher abundance of G3 (eclogitic) and G4 (pyroxenitic/eclogitic) relative to the Alto Paranaíba Igneous Province. Higher pressures and temperatures were calculated for the samples from Rondônia (40-60 kbar and 1030-1380 ºC) compared to samples from Minas Gerais (25-40 kbar and 730-1000 ºC). The peridotite xenoliths from Rondônia show P-T equilibrium conditions in the diamond stability field and can be the source of at least part of the diamond from the area. The P-T stability fields of the xenoliths from both locations are aligned close to the 40 mW/m2 geotherm. The data indicate that the cratonic 40 mW/m2 geothermal gradient in Rondônia may be related to a process of thermal relaxation of the lithospheric mantle after the Paleoproterozoic to Mesoproterozoic tectonothermal events of the southwestern Amazonian Craton until the sampling of the xenoliths by the magma in the Permian-Triassic.