Journal of Petrology | Volume 40 | Number 1 | Pages 21-60 | 1999
© Oxford University Press 1999
Petrological and Structural Evolution of High-Grade Metamorphic Rocks from the Floor of the Alboran Sea Basin, Western Mediterranean
1 Instituto 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
2 Department of Geological Sciences, University College London Gower Street, London Wc1E 6Bt, UK
Received September 30, 1997; Revised typescript accepted May 14, 1998
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Abstract |
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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.
KEY WORDS: decompression; extension; melting; P–T path; phase relations
* Corresponding author. Telephone: +34-958-243341. Fax: +34-958-243384 and +34-958-248527. e-mail: jsoto{at}goliat.ugr.es
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