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Journal of Petrology | Volume 45 | Number 5 | Pages 1013-1043 | 2004
Journal of Petrology 45(5) © Oxford University Press 2004; all rights reserved.
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
1 ISTITUTO DI GEOSCIENZE E GEORISORSE–CNR, VIA MORUZZI 1, I-56124 PISA, ITALY
2 DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DI PISA, VIA S. MARIA 53, I-56126 PISA, ITALY
3 MUSEO NAZIONALE DELL'ANTARTIDE, UNIVERSITÀ DI SIENA, VIA LATERINA 8, I-53100 SIENA, ITALY
* Corresponding author. Telephone: +39 050 3152270. Fax: +39 050 3152360. E-mail: g.divincenzo{at}igg.cnr.it
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.
KEY WORDS: K–Ar systematics; 40Ar–39Ar laserprobe dating; white mica; Barrovian metamorphism; Variscan belt
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