The relationship between tectonometamorphic evolution and argon isotope records in white mica: Constraints from in situ 40 Ar-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 situ 40Ar-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. © Oxford University Press 2004; all rights reserved.