⁴⁰Ar/³⁹Ar and (U-Th)/He constraints on emplacement, exhumation, and weathering of alkaline-carbonatite complexes in the Alto Paranaíba Igneous Province (APIP), Brazil

Abstract

The timing and rate of emplacement, exhumation, and weathering of alkaline-carbonatite complexes associated with failed rift systems in the South Atlantic provide constraints on the thermotectonic conditions of West Gondwana before, during, and after break-up. ⁴⁰Ar/³⁹Ar geochronology of biotite crystals show that two alkaline-carbonatite complexes – Araxá and Catalão I – from the Alto Paranaíba Igneous Province were shallowly emplaced simultaneously at ∼85 Ma. After rapid cooling, the igneous complexes were rapidly exhumed between 85 and 66 Ma at Araxá (∼52 m Ma⁻¹) and 85 and 45 Ma at Catalão (∼25 m Ma⁻¹). Fast exhumation of the complexes occurred under relatively arid to semi-arid climates, conditions that favored physical weathering and erosion. The eroded material was transported in a NE-SW direction and deposited in the adjacent Paraná and São Sanfranciscana basins. After exhumation, wetter climates and more subdued tectonic conditions favored active widespread vegetation cover that favored reduced erosion and deepening of weathering profiles. After 66 Ma, the Araxá complex denuded at rates of 3.4 m Ma⁻¹; at Catalão I, denudation rates reduced to 4.7 m Ma⁻¹ after ∼45 Ma. Favorable geological and climatic conditions promoted the formation of deep and chemically stratified lateritic weathering profiles. The exhumed and weathered carbonatites underwent hypogene and supergene enrichment in Nb, P, Ti, and REEs, critical metals needed for the electrification of our energy base. The preservation of the lateritic profiles and their mineral wealth in the continental interior contrasts with the younger and shallower weathering profiles formed on similar lithologies and at equivalent elevations near the Brazilain South Atlantic margin, confirming that climatic and tectonic conditions along the margin were less conducive to the preservation of valuable mineralized systems because of enhaced erosion.

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