The major elemental composition of chlorite is widely used for petrogenetic investigations on low-temperature geological processes. However, compositional variations of chlorite within a given tectonic environment are common and, when overlooked, can lead to erroneous petrological interpretations. We thoroughly investigate chlorites occurring within giant quartz veins (GQVs) in the basement rocks of the Pyrenees. These structures have widths of up to tens of meters and lengths of kilometers and form in both mid-crustal ductile and upper-crustal seismogenic domains. Texturally constrained chlorite analyses and spatially resolved whole-rock elemental analyses reveal a progressive chemical evolution of chlorite coupled to GQV formation. Six chlorite generations that were distinguished according to their texture show consistent chemical variations at the microscale. Host rock− and quartz vein−related chlorites are the textural and compositional endmembers. Between them, a progressive chemical refinement occurred in transitional chlorite compositions linked to host rock, vein quartz, and pressure-solution microstructures, in accordance with significant fluid-rock interactions leading to GQV formation. This rock alteration process is further confirmed at the macroscale by the progressive depletion of all major and trace elements but silica, with decreasing distance toward GQVs. We demonstrate that (1) inferring the temperature conditions of chlorite crystallization is not as straightforward as generally assumed and that (2) GQVs can be formed under rock-buffered conditions, at lower temperatures than previously thought. These results have implications for the practice commonly used in chlorite-based geothermometry, as well as for the modes of fluid, heat, and mass transport within Earth’s crust.

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