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 tectonometamorphic 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 S 1 foliation preserved within the main S 2 foliation or enclosed in rotated albite porphyroblasts; (2) celadonite-poor white micas aligned along the main S 2 foliation. The S 1 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 S 2 foliation or is of later crystallization. In situ 40 Ar---39 Ar 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-D 1 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.