Journal of Petrology, Volume 40, Issue 1: January 1999.

Petrological and Structural Evolution of High-Grade Metamorphic Rocks from the Floor of the Alboran Sea Basin, Western Mediterranean

J. I. SOTO¹ AND J. P. PLATT²

¹INSTITUTO ANDALUZ DE CIENCIAS DE LA TIERRA & DEPARTAMENTO DE GEODINÁMICA, C.S.I.C.-UNIVERSIDAD DE GRANADA, FACULTAD DE CIENCIAS, CAMPUS FUENTENUEVA S/N, 18071 GRANADA, SPAIN
²DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY COLLEGE LONDON, GOWER STREET, LONDON WC1E 6BT, UK

The metamorphic basement beneath the Alboran Sea, a Neogene extensional basin drilled during Ocean Drilling Program (ODP) Leg 161 (Site 976), consists of high-grade pelitic schist overlying migmatitic pelitic gneiss. Inferred assemblages in the high-grade schist evolved from garnet + staurolite + biotite + muscovite + plagioclase (Assemblage 1) to biotite + sillimanite + K-feldspar + plagioclase ± garnet (Assemblage 2) to andalusite + biotite + K-feldspar + plagioclase (Assemblage 3). In the gneiss, which shows abundant migmatitic textures, the present assemblage is biotite + andalusite + sillimanite + muscovite + cordierite + K-feldspar + plagioclase ± garnet. Phase relations and thermobarometric calculations suggest that the high-grade schist experienced decompression accompanied by heating from ~500°C at 10·5 kbar through 600 ± 30°C at 6-7 kbar (Assemblage 1), and thence to 650-700°C at 3-4 kbar (Assemblage 2), followed by cooling through 500-600°C at 2 kbar or less (Assemblage 3). Peak temperature and melting occurred under low-pressure conditions. The widespread disequilibrium and overstepping of metamorphic reactions suggests that decompression was rapid. The P-T evolution is consistent with metamorphism in a late orogenic extensional basin, and suggests the existence of an external source of heat, probably indicating the complete removal of lithospheric mantle beneath the extending region.

Keywords: decompression;extension;melting;P-T path;phase relations

Pages 21-60

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