K-Ar ages in biotites and K-feldspars from the Catalan Coastal Batholith: Evidence of a post-Hercynian overprinting

A K-Ar isotopic study was carried out on biotite, K-feldspar and amphibole separates from several of the main acid and intermediate plutonic units of the epizonal late-Hercynian Catalan Coastal Ranges batholith (NE Spain). According to the study, apparent ages of biotite separates from granites, granodiorites and tonalites, containing more than 7% potassium, ranged from 282 +/- 6 to 296 +/- 6 Ma with an isochrone age of 284 +/- 4 Ma. This value is considered to be the best estimate of a common closure biotite age for all the corresponding intrusions. In contrast, most of the potassic feldspars showing argon loss to different degrees, gave apparent ages reaching down to 187 +/- 4 Ma. This indicates that a slight post-Hercynian disturbing event partially opened the K-Ar feldspar system, but not the biolite system. Nevertheless, K-feldspars from a few localities (even those affected by some deuteric alteration) gave ages close to that of the biotite. Thus, it is suggested that K-feldspar can preserve its radiogenic Ar for a long period of time, as long as it remains unaffected by subsequent processes of recrystallization or overheating. The disturbing event must have happened after the exhumation of the plutons, since the Lower Triassic rocks lie unconformably over the eroded granitoids thence they would have been nearly at surface temperature about 250 Ma ago). The age and characteristics of this event are loosely constrained, but the cluster of apparent ages around 200 Ma, together with the fact that K-feldspars younger than 187 Ma have not been found, suggest that it occurred during the Mesozoic. In addition to argon loss in K-feldspars, a significant increase in apparent ages of chlorite-bearing separates, ranging from 282 +/- 6 to 311 +/- 6 Ma, was observed in correlation with their chlorite content. We suggest that, in the studied granitoids, chlorite contains extraneous Ar-40* whereas pure biotite does not seem to trap significant amounts of radiogenic argon. Consequently, since interlayered chlorite cannot be easily separated from biotite, some K-Ar ages obtained from biotites of regions that have undergone slight thermal or hydrothermal overprints may be considerably increased.