Evolution of a kyanite-bearing belt within a HT-LP orogen: the case of NW Variscan Iberia

J. Reche, F. J. Martinez, M. L. Arboleya, C. Dietsch, W. D. Briggs

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Abstract

Kyanite replaces andalusite in a belt of Ordovician and Silurian pelitic rocks that form a narrow synform pinched between high-grade antiforms in NW Variscan Iberia. Kyanite occurs across the belt in Al-rich, black pelites in assemblages I: kyanite-chloritoid-chlorite-muscovite and II: kyanite-staurolite-chlorite-muscovite. In I, kyanite occurs in the matrix and in kyanite-muscovite aggregates that pseudomorph earlier andalusite porphyroblasts. The aggregates are found across the belt and can still be recognized in assemblage II and even in III: andalusite-staurolite-biotite-muscovite, this latter being a hornfelsic Silurian schist where kyanite is relic and staurolite occurs in the matrix, and is resorbed inside new massive pleochroic andalusite. KFMASH and MnKFMASH pseudosections have been constructed using Thermocale for Al-rich and Al-poorer compositions from the belt. Chloritoid zoning in Al-rich rocks containing assemblage I, plus chloritoid-chlorite thermometry complemented with garnet-chlorite thermometry in Al-poorer lithologies mean that the path is one of increasing pressure and temperature. Conditions prior to assemblage I, with earlier andalusite stable, are those of the andalusite-chloritoid-chlorite field as testified by chloritoid enclosed in andalusite porphyroblast rims. The passage from assemblage I to II implies a prograde path within the kyanite field. Assemblage III represents peak conditions, indicating a prograde staurolite-consuming reaction across a KFMASH field, leading eventually to a locally found andalusite-biotite-muscovite hornfels. The lowest pressure stages are recorded by cordierite-biotite in Al-poor pelites. Garnet-bearing MnKFMASH assemblages in Al-poorer pelites record conditions similar to assemblages II and III. The replacement of andalusite by kyanite in assemblage I is attributed to downdragging of andalusite-bearing rocks into a synform as testifed by the strained andalusite porphyroblasts affected by a subvertical crenulation cleavage. Prograde metamorphism in the eastern contact of the belt is due to heat transferred to the belt from the ascending high grade antiform across the Vivero fault.
Original languageEnglish
Pages (from-to)379-394
JournalJournal of Metamorphic Geology
Volume16
Issue number3
Publication statusPublished - 1 Jan 1998

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