Tourmaline KAr ages compared to other radiometric dating systems in Alpine anatectic leucosomes and metamorphic rocks (Cyclades and southern Spain)

Abstract

KAr ages of tourmalines from Alpine anatectic leucocratic rocks in the migmatite dome on Naxos, Greek Cyclades, and along ultramafic intrusions in southern Spain are older than the corresponding mineral ages. They are interpreted as cooling ages closely approaching the age of magmatism, suggesting a closure temperature exceeding that of hornblende. RbSr whole rock analyses from metagranites and gneisses of the Palaeozoic suite of the Mulhacen Complex of the Betic Cordilleras, southern Spain, yield errorchrons of between 275 and 191 Ma. Several KAr tourmaline ages of these rock units are higher than the corresponding whole-rock ages, pointing to the incorporation of excess radiogenic Ar. Of the young tourmalines, three of them give ages of 80–85 Ma, indicating possible examples of incomplete degassing and/or possible incorporation of excess radiogenic Ar in the mineral during the Alpine metamorphism. The tourmaline ages are compared with the ages of the coexisting micas. These have a range of KAr ages consistently higher than the corresponding young Alpine RbSr ages of 15.6–12.5 Ma, pointing to a complex behavior for radiogenic Ar during the Alpine metamorphism.