Zircon triple dating (U–Pb, Raman, and U–Th–Sm/He) constraints on the thermotectonic evolution of the Araçuaí Orogen at the craton–orogen interface (Brazil)

Thermochronology has advanced through the development of new methods and applicable mineral systems, with multi-method approaches proving essential for bridging temperature–time gaps and improving the resolution of thermal history reconstructions. Here, we apply a zircon-based multi-method approach to investigate the thermotectonic evolution of the Araçuaí Orogen, along the São Francisco Craton in Brazil. While the Mesozoic–Cenozoic evolution of the orogen is relatively well constrained, its earlier thermal history remains poorly understood. To address this gap, we expand an existing apatite fission-track (AFT) dataset of 20 samples by adding new zircon (U–Th–Sm)/He (ZHe) ages, extending thermal constraints from the apatite partial fission-track annealing zone (APAZ; ~60–120 °C) to higher-temperature conditions (~140–220 °C). In addition, four representative samples were selected along a north–south transect across the craton–orogen interaction zone for zircon Raman multi-band thermochronology and zircon U–Pb analyses. Zircon Raman multi-band thermochronology, a recently developed approach, further extends thermal constraints to mid- and high-temperature conditions (~260–370 °C).

The ZHe dataset reveals a systematic relationship between effective uranium (eU) concentration and single-grain ages. Zircons with low eU contents (<500 ppm) yield predominantly Paleozoic ZHe ages, ranging from the Cambrian to Carboniferous (ca. 500–350 Ma), whereas grains with progressively higher eU concentrations record younger ages spanning the Late Paleozoic to Early Cretaceous (ca. 350–100 Ma). This inverse age–eU relationship is consistent with radiation-damage–controlled helium diffusion in zircon, as predicted by established diffusion models.

Raman ages derived from multiple zircon bands (ν1, ν2, ν3, and external bands) indicate distinct thermal responses across the transect. Two samples record Raman ages overlapping with or being older than the Araçuaí orogeny, suggesting preservation of pre- to syn-orogenic thermal signatures. In contrast, Raman ages from the other two samples correspond to the late stages of the orogeny or post-date it.

Comparison of Raman-derived ages with ZHe and AFT data provides constraints on cooling rates through successive temperature windows. Samples showing convergence of Raman, ZHe, and AFT ages indicate relatively rapid cooling through mid- and low-temperature regimes, whereas increasing separation between these chronometric results reflects more prolonged cooling histories. Variations in the thermal sensitivity of individual Raman bands, reflected in their accumulated radiation damage, constrain the rate of cooling across mid-temperature ranges: synchronous band resetting indicates faster cooling whereas differential band behavior suggest slower, stepwise cooling.

These results reinforce evidence that the craton–orogen interaction zone of the São Francisco Craton experienced significant thermal overprinting associated with the development of the Araçuaí orogenic front (ca. 500 Ma), even though the underlying crust is of Rhyacian (~2.1 Ga) and Archean (~3.2 Ga), as indicated by zircon U–Pb data. Thermal history modelling indicates that following orogenesis, the region underwent substantial cooling, allowing samples to pass through progressively lower-temperatures and reach shallow crustal levels by the end of the Paleozoic. Subsequent opening of the South Atlantic Ocean preferentially affected structurally weakened domains, particularly areas associated with deep-seated faults and shear zones.