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Evidence of contrasting low-grade metamorphic conditions from clay mineral assemblages in Triassic Alpujárride-Maláguide transitional units in the Betic Cordilleras, Spain

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Abstract:

Triassic sequences from 'intermediate units' between the Alpujárride and the Maláguide complexes (Betic Cordilleras, Spain) of the westernmost part of the Cordilleras (Casares area) occur as four superimposed tectonic units; the uppermost unit shows lithological characteristics similar to those of the Maláguide complex, changing progressively at increasing depth, towards lithologies typical of the Alpujárride complex. The units studied, with a maximum thickness of ∼400 m, record important variations in metamorphic pressures, according to the b parameter of white micas: from low-pressure metamorphism (in the upper unit) to high-pressure facies series (in the deepest one). The mean b values range from 8.988 Å in the uppermost unit (Crestellina) to 9.042 Å in the lowermost one (Jubrique). The lowest metamorphic grade is represented by mineral assemblages consisting of phengite + intermediate Na-K white mica ± Fe-chlorite ± sudoite ± pyrophyllite, which record temperatures of ∼300°C and pressures of 1.5–3 kbar. At increasing tectonic depth, intermediate Na-K mica and pyrophyllite disappear and the metamorphic assemblages consist of phengite ± paragonite ± margarite + Mg-chlorite ± sudoite, which record minimum pressures of ∼7 kbar and temperatures in the order of 400–450°C. These mineral assemblages provide evidence of the passage from collisional to extensional geotectonic settings. The units showing different metamorphic patterns were juxtaposed tectonically, after the development of metamorphic mineral assemblages.

Keywords: BETIC CORDILLERAS; K WHITE MICA B GEOBAROMETRY; LOW-GRADE METAMORPHISM; MARGARITE; METAPELITE; PARAGONITE; PHENGITE; PYROPHYLLITE; SPAIN

Document Type: Research Article

DOI: http://dx.doi.org/10.1180/0009855064120209

Publication date: June 1, 2006

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