Retrograde crystallization of clay minerals in metamorphic rocks linked to fluid circulation related to fault activity

Abstract. The study of two outcrops of schists in the Padul Fault area has allowed us to corroborate the enrichment in clay minerals in the fault zone. In the case of the dark schist outcrop, the fault rocks show a reduction in mineral diversity with aluminosilicates, characteristics of the regional metamorphism losing and showing an enrichment in quartz and kaolin-group minerals. In the calc-schists, there is a clear presence of chlorite in the fault rocks, which is practically absent in the fresh rocks, as well as R1 illite/smectite and smectite. The geochemical characterization also showed differences between fresh and fault rocks, more evident in the dark schists than in the calc-schists, with an increase in SiO2 but a significant decrease in Al2O3, Fe2O3, and MgO in the fault rocks with respect to the protolith. The presence of authigenic chlorite in the fault rocks from the calc-schists allowed for the application of semi-empirical thermometric methods for the fault rocks giving predominant temperatures < 225 °C. All these data are consistent with the circulation of low-temperature fluids along the fault zone that interact with the rocks, promoting clay mineral formation. Such was also the findings in a previous study focused on the predominant rocks of the Padul Fault area, which are dolostones, affected by the dynamics of this fault. Interestingly, the circulation of fluids promoted by the existence of the Padul Fault has not been restricted to the fault rocks, given that some of these clay minerals (kaolinite and smectite) are also present in the fluid-accessible zones of the metamorphic protolith. These data highlight the importance of faults in the retrograde mineralization processes of metamorphic rocks.