⁴⁰ Ar/ ³⁹ Ar geochronology and petrophysics of Ediacaran granites, northern Borborema Province, Brazil: implications for cooling of the Brasiliano–Pan-African Orogeny

The Borborema Province, in northeastern Brazil, encompasses tectonic domains separated by high-temperature shear zones. Argon geochronology and thermal modelling of Ediacaran plutons, complemented by existing zircon U–Pb ages, provide insights into its magmatic cooling history and implications for its tectonic evolution during the Brasiliano–Pan-African Orogeny. Hornblende and biotite from eight plutons along a 700 km SE–NW transect (hosting zircons with U–Pb ages in the range 633–571 Ma) were analysed via laser incremental heating 40 Ar/ 39 Ar geochronology. Hornblende crystals yield plateau ages ranging from 570.8 ± 5.1 to 489.5 ± 3.2 Ma, whereas biotite crystals display plateau ages from 557 ± 4 to 491 ± 3 Ma. Thermobarometric calculations indicate pressures of 2.1–5.8 kbar and temperatures of 621–783°C for magma emplacement. Integration of all data reveals that the Borborema Province was assembled from blocks with different geothermal gradients (40–70°C km −1 ), featuring the highest temperature near the plutons and diverse cooling trajectories. Petrophysical data indicate an overall cooling rate (based on U–Pb zircon and 40 Ar/ 39 Ar hornblende ages) ranging from 0.3 to 4.0°C Ma −1 . This variation suggests (re)activation of low- and high-temperature shear zones by the end of the Ediacaran. This study also demonstrates the possibility of elucidating the cooling trajectory of plutons unaffected by shear zones.