Evidence for protracted fluid mobility from 40Ar/39Ar stepwise crushing of quartz-rich veins and high-grade rocks

Several decades of research into the tectonics, metamorphic petrology, and geochronology of the Agly Massif and adjacent areas, eastern Pyrenees (France) have revealed a complex tectono–metamorphic history including Variscan magmatism, Cretaceous tectono–metamorphic overprinting, and subsequent Alpine uplift and exhumation during the Cenozoic. However, the timescale and episodicity of fluid mobility in the area remain poorly constrained. In this study, we present stepwise crushing 40Ar/39Ar data from quartz-rich veins and high-grade rock samples, and fluid inclusion analyses that reveal the presence of brines (ca. 25–26 wt% NaCl equivalent (NaCleq)). The stepwise crushing analysis was performed to liberate fluid from inclusions and evaluate its association with tectono–metamorphic processes. This approach allows to resolve distinct reservoirs of fluids, which can be identified chemically and isotopically using three-isotope correlation diagrams. The inverse isochron diagrams further facilitate the distinction between primary and secondary isotopic alignments. The results indicate the occurrence of several fluid pulses that produced new quartz veins at ∼140 Ma, ∼120–90 Ma, and ∼80–60 Ma, while also affecting both earlier formed quartz-rich veins and high-grade rocks. These specific age groups relate to separate phases of fluid mobility induced by Cretaceous crustal extension and later exhumation during Paleogene convergence. Our findings put published maximum apparent ages into the context of episodic deformation and fluid activity and testify to the pervasive nature of fluid–rock interaction in the area.

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