Repeated thermal resetting of phengites in the Mulhacen Complex (Betic Zone, southeastern Spain) shown byAr/Ar step heating and single grain laser probe dating

This study reports the results of the first⁴⁰Ar/³⁹Ar combined induction furnace and laser probe dating of phengites from the Mulhacen HP/LT metamorphic complex in the Betic Cordilleras, southern Spain. Laser step heating and spot fusion analyses on different halves of a split single grain were made with a continuous laser probe. Spot fusion analysis resulted in ages of about 30–31 Ma in the core and ages as low as 25–26 Ma in the rim. Laser step heating on the other half of the grain gave a spectrum with apparent ages increasing from about 25 Ma to 29.5 Ma. The age spectrum and the decreasing ages towards the rim of the grain may imply that resetting essentially occurred by volume diffusion of radiogenic⁴⁰Ar due to late stage reheating resulting from extensional tectonics. Ages around 30 Ma in the core of the grain are interpreted as minimum estimates of the cooling age of the main tectono-metamorphic phaseD₂.

Induction furnace step heating on phengite separates from mica schists and one gneiss resulted in two types of age spectra. Type I spectra show monotonously rising apparent ages from14.5 ± 1.9 Ma to20.7 ± 0.2 Ma, and in a second sample from16.9 ± 0.7 to29.7 ± 0.2 Ma. Type II spectra are characterized by plateaus of14.4 ± 0.1 Ma (the gneiss sample),17.3 ± 0.1 Ma and17.6 ± 0.1 Ma. Type II spectra show low temperature apparent ages significantly below the plateau age, implying resetting subsequent to initial cooling. Modelling of the age spectra demonstrated that the plateau ages are possibly the result of strong resetting (75–85% of argon loss) of an older isotope system. Total fusion of a number of phengite single grains from marbles taken close to type II mica schists yielded ages of15.4 ± 1.2 Ma and17.0 ± 0.7 Ma. The observed repeated resetting is coeval with major volcanic activity in basins adjacent to the metamorphic ranges, pointing to a resetting by advective fluid transport related to volcanism.