- © 2016 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
The age of peak-metamorphic conditions of the ultrahigh-pressure (UHP) metamorphic rock sequence constituting the Dora-Maira Massif is well established at 35 Ma. In order to understand the behaviour of excess argon during sequential pressure–temperature stages of exhumation from >3.5 to less than 0.5 GPa, distinct generations of chemically and petrographically well-characterized white mica were studied with the multigrain, stepwise-heating 40Ar/39Ar dating technique. Mica separates were obtained from boudins of coesite-bearing UHP pyrope quartzite and phengite schist enclosed within them, from retrograde assemblages evolving from the pyrope quartzite, and from biotite–phengite orthogneiss surrounding the boudins. Assuming the 35-Ma age as a benchmark, it is possible to reconstruct directly the quantity of excess argon during these various stages of the exhumation history. The amount of excess Ar roughly correlates with the Si content of phengite, which is an indicator for the pressure at which phengite incorporated excess Ar. All mica samples crystallized at pressures >1.0 GPa exhibit significant excess argon and aberrant ages significantly higher than 35 Ma. Between 1.0 and 0.5 GPa excess Ar diminishes and realistic ages are approached. Phengite (Si3.55) separates from two localities of fresh coesite-bearing pyrope quartzite equilibrated at ca. 3.8 GPa show similar U-shaped spectra with high apparent ages in the first or second degassing steps, dropping to a plateau-like value of 71.0 ± 1.1 Ma in one case and yielding an apparently trustworthy plateau age of 103.4 ± 1.0 Ma in the second. Retrograde phengite generations from pyrope quartzite crystallized between 3.2 and 1.5 GPa also display a U-shaped spectrum with a broad depression corresponding to a minimum age of 96.9 ± 1.6 Ma. Retrograde phlogopite, representing part of the breakdown assemblage of pyrope and formed between 1.9 and 1.2 GPa, documents several sharp steps in age values from 548 Ma towards 850 Ma, whereas further degassing steps are characterized by a nearly linear increase of age values from 852 to 973 Ma. A plateau age of 56.3 ± 0.9 Ma was calculated for phengite from a phengite-schist inclusion within pyrope quartzite. The spectra of phengite from the country-rock biotite–phengite gneiss surrounding the pyrope quartzite boudins are hump-shaped. The total degassing ages decrease with decreasing grain size (and decreasing metamorphic pressure from 1.6 to 0.3 GPa, as signalled by a concomitant decrease of Si per formula unit) from 43.25 ± 0.60 Ma (1–2 mm) to relatively realistic values of 33.50 ± 0.50 Ma (90–180 μm) and 31.55 ± 0.50 Ma (<100 μm). The latter age correlates with very low calculated excess argon values. The data demonstrate that even retrograde micas are affected by excess argon, and that “ages” derived from inverse isochron diagrams (36Ar/40Ar vs. 39Ar/40Ar) as well as “perfect” plateau ages need to be treated with caution and constrained by other geochronological systems.