Preservation of sub-microscopic structural relicts in micas from the Gran Paradiso Massif (Western Alps): Implications for 40Ar–39Ar geochronology

Abstract

Permian igneous biotite and white mica that were re-heated at 500–550°C in the Middle-Late Eocene were investigated by laser step-heating and in situ 40Ar–39Ar techniques, in conjunction with scanning electron microscopy (SEM), electron microprobe (EMP) and transmission electron microscopy (TEM) to assess the influence of short-lived metamorphism on K–Ar systematics. Large intra- and inter-grain age variations, ranging from the Triassic to the Late Eocene, are primarily related to the extent of metamorphic re-equilibration. Brown biotite porphyroclasts from weakly re-equilibrated samples are characterized by core-to-rim zoning in major element composition, mineral structure and 40Ar/39Ar ratios. Titanium concentration and Fe/(Mg+Fe) ratios decrease from crystal cores towards the rims, which are compositionally indistinguishable from green biotite aggregates in metamorphic coronas. Compositional changes are coupled with structural modifications towards the crystal edges, where the disordered igneous 1M–2M1 stacks are replaced topotactically by highly ordered metamorphic 1M polytypes. Apparent ages up to ∼66Ma were determined in crystal cores by in situ laser-probe analyses, while ages as young as ∼45Ma are typical of crystal rims. Step-heating experiments of these zoned biotites yielded a discordant saddle-shaped age spectrum, with a concordant central segment yielding an error-weighted mean age of 44.9±0.3Ma. Biotite porphyroclasts from a more re-equilibrated specimen gave markedly different results, with uniform major elements compositional profiles, homogeneous argon distribution throughout the crystals and a flat age spectrum at 36.5±0.3Ma. TEM investigations of these optically homogeneous porphyroclasts revealed that the original igneous structure had been replaced by highly ordered 1M polytypes during Alpine metamorphism. White mica yielded comparable results, with spot ages up to ∼218Ma in domains characterized by high Na/(Na+K) ratios and low Si and Ti content, representing the best preserved igneous relicts. Significantly younger apparent ages, down to 44–39Ma, are typical of domains affected by partial metamorphic re-equilibration at high-pressure conditions, as indicated by increasing Si and Ti contents, or by exhumation-related exsolution of amoeboidal quartz. These results indicate that different intra-grain and inter-grain Ar concentration patterns arise primarily from the relative amount and distribution of relict igneous and metamorphic crystal-chemical domains. Core-to-rim age gradients in biotite result from inward-directed recrystallization from the crystal edges, eventually leading to flat age profiles as metamorphic replacement of pre-existing porphyroclasts runs to completion. This study calls for caution when estimating thermal histories from apparent age gradients or from age vs. grain size correlations, unless the presence of pre-metamorphic mineral relicts can be ruled out.