The Relationship between Tectono-metamorphic Evolution and Argon Isotope Records in White Mica: Constraints from in situ 40Ar-39Ar Laser Analysis of the Variscan Basement of Sardinia

Abstract

The basement of Sardinia represents a nearly complete section of a segment of the Variscan belt that experienced a polyphase tectono-metamorphic evolution and Barrovian metamorphism. This basement is well suited to investigate the relationship between tectono-metamorphic evolution and argon isotope records in white mica. Micaschists from the garnet zone (maximum T of up to 520–560°C) contain two texturally and chemically resolvable generations of white mica: (1) deformed celadonite-rich flakes, defining a relict S1 foliation preserved within the main S2 foliation or enclosed in rotated albite porphyroblasts; (2) celadonite-poor white micas aligned along the main S2 foliation. The S1 foliation developed earlier and at a deeper crustal level with respect to that at which the thermal peak was reached. From the staurolite zone (T of up to 590–625°C) to the sillimanite + K-feldspar zone, white mica is nearly uniform in composition (muscovite) and is predominantly aligned along the S2 foliation or is of later crystallization. In situ40Ar–39Ar laser analyses of white mica yielded ages of ∼340–315 Ma in the garnet zone, and ∼320–300 Ma in the staurolite and sillimanite + K-feldspar zones. Results highlight a close link between argon isotope records in white mica and both textures and structure-forming major elements. The oldest ages were detected in samples where the earlier syn-D1 white mica generation did not texturally and chemically re-equilibrate at upper-crustal levels. This study suggests that the white micas that escaped recrystallization retain argon isotope records of an earlier metamorphic stage that survived a later event characterized by temperatures higher than 500°C.