Thermo-chronology of the Barlet metamorphic basement unit: evidence for a Stephanian thermal event linked to Sb mineralization in the Haut Allier, France

A thermo-chronological analysis of the Barlet basement unit (French Massif Central) reveals a four-stage history. Peak metamorphism (650 °C and 7 kbar) was followed by retrograde growth of albite blasts and development of the main foliation at 600 °C, and chloritization of the ferromagnesian minerals. The third stage is a marked reversal of cooling, with recrossing of the biotite isograd and local reappearance of garnet at 450 °C. This thermal event, inferred to result from hot fluid infiltration, is also recognized in the adjacent basin of Langeac, where it gives rise to anomalous coal grades (recording 200 °C at 1 km). A Stephanian age for this event correlates with a regional thermal event recognized throughout the Variscan, where it has been linked to delamination of the continental crust. This work represents the first instance in the Massif Central that recognizes this event in the shallow basement itself. Final cooling is accompanied by extensive fluid-induced sericitization, starting immediately after the peak of the thermal event and continuing to temperatures inferred for Sb–As ore deposition. This continuum leads us to conclude that reheating-related silicate reactions and ore deposition are caused by the same fluid and related to the wider regional Variscan thermal event.