40Ar/39Ar dating of white micas from an Alpine high-pressure metamorphic belt on Naxos (Greece): the resetting of the argon isotopic system

Abstract

Overprinting of white micas from high pressure, low to medium temperature (M 1) metamorphic assemblages in pelitic schists on Naxos during subsequent thermal dome (M 2) metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome. The original (M 1) minerals are phengites (Si4+=6.7–7.0) and the overprinting minerals are muscovites (Si4+=6.0–6.45). 40Ar/39Ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra, characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between. These upward-convex age spectra are shown to be caused by mixing of two generations of micas, each of which has a different age spectrum and argon release pattern. Seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages. Further, the upward-convex age spectra have been used to trace the isotopic signature of phengites toward increasing M 2 metamorphic grade, and suggest that as long as phengites can be observed in the rocks upward-convex age spectra occur. On Naxos, crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas. However, there is also good evidence for argon loss by volume diffusion from phengites. Simple diffusion calculations suggest that the M 2 metamorphism was caused by a shortlived heat source.