<p>White mica commonly yields younger ⁴⁰Ar/³⁹Ar dates than ⁸⁷Rb–⁸⁷Sr isochron dates in greenschist to amphibolite facies rocks, which are often interpreted as ⁴⁰Ar/³⁹Ar cooling dates. However, recent petrochronological studies showed that younger ⁴⁰Ar/³⁹Ar dates can result from chemical re-equilibration. We investigate the frequency and conditions under which white mica chemically (re)equilibrates during an orogenic cycle in common upper crustal rocks and its implications for ⁴⁰Ar-loss and ⁸⁷Sr/⁸⁶Sr-(re)equilibration. We have applied double-dating mapping by in-situ ⁸⁷Rb–⁸⁷Sr and ⁴⁰Ar/³⁹Ar geochronology, quantitative chemical mapping, and δ¹⁸O analysis to Mesoproterozoic metasedimentary and igneous rocks from the Black Hills, South Dakota. Intragrain chemical disequilibrium is pervasive in white mica, with primary phenocrystic and porphyroblastic compositions overprinted by reaction-grown phases along cleavage planes and grain boundaries. Re-equilibration involved metasomatic exchange of incompatible elements (Ar, Na, Rb, Sr, Cs, Ba, B, La and Li) during fluid-assisted alteration in a meta-tuff and the Harney Peak Granite. ⁸⁷Rb–⁸⁷Sr white mica isochron dates of 1763.24 ± 17.05 Ma and 1677.95 ± 9.99 Ma for the meta-tuff and the Harney Peak Granite, respectively, are a consequence of high-temperature chemical equilibrium. In contrast, ⁴⁰Ar/³⁹Ar dates of reaction-grown white mica of 1300.36 ± 15.84 Ma and 1284.89 ± 4.87 Ma, record fluid-driven re-equilibration at 364 ± 50&#xa0;°C (chlorite-thermometry). These results indicate that fluid-assisted processes can reset ⁴⁰Ar/³⁹Ar dates in white mica, while having a minor effect on the ⁸⁷Rb–⁸⁷Sr system, rendering cooling-age interpretations ambiguous when they lack petrological control.</p>