Twenty million years of post-orogenic fluid production and hydrothermal mineralization across the external Araçuaí orogen and adjacent São Francisco craton, SE Brazil

Abstract

The Araçuaí orogen (AO) is a hot orogenic system that is known for its widespread and long-lived magmatism (630–480 Ma). Within the last c. 20 million years of its evolution, the AO saw the widespread development of hydrothermal systems in its lower-grade external fold-and-thrust belt and its Foreland Domain (eastern margin of the adjoining São Francisco craton). UPb crystallization ages for hydrothermal monazite, rutile, and xenotime indicate that fluid circulation along the entire AO occurred between 515 to ca. 495 Ma, largely overlapping with UPb ages of hydrothermal systems and mineralized zones in the adjacent São Francisco craton. Titanium-in-quartz thermometry suggests that most hydrothermal systems closed at temperatures of ca. 360 °C. The SmNd isotope composition of the vein monazite samples ranges from εNd500Ma −16.8 to −17.8, and one less-evolved sample with εNd500Ma of −5.9. The SmNd isotope compositions overlap with the bulk rock SmNd data for the Espinhaço Supergroup, suggesting that the hydrothermal veins in this study are mainly related to metamorphic fluids derived from the prograde dehydration of low-grade metasedimentary rocks. The UPb ages, SmNd isotope composition, and vein mineralogy are all consistent with a 20 My period of channelized fluid flow, characterized by an oxidizing, low pH, REE-, P-, and Ti-bearing aqueous metamorphic fluid. The expression of supercritical fluid flow in the core of the AO is given by the emplacement of large volumes of bare/mineralized pegmatite bodies and regional resetting of monazite ages in host rocks. On a regional context, this widespread fluid flow is associated with a high-T, low-P metamorphism that marks the beginning of the post-collisional decompression of the orogeny, which is associated with lithospheric mantle delamination. The reorganization of mass and heat, due to extensional collapse, resulted in multi-sourced fluid production and mineralization from deep to shallow crustal levels that affected an area of over 400.000 km2 along the eastern border of the São Francisco Craton.