Evidence of extensional metamorphism associated to Cretaceous rifting of the North-Maghrebian passive margin: The Tanger-Ketama Unit (External Rif, northern Morocco)

Authors

  • M. VÁZQUEZ Andean Geothermal Center of Excellence (CEGA), Universidad de Chile. Santiago, Chile. Department of Geology, Universidad de Chile. Santiago, Chile.
  • L. ASEBRIY Institut Scientifique, B.P. 703, Université Mohammed V Agdal-Rabat (Morocco).
  • A. AZIMOUSA Laboratoire LGA, Faculté des Sciences, Université Mohammed IOujda (Morocco).
  • A. JABALOY Departamento de Geodinámica, Facultad de Ciencias, Universidad de Granada. C/Fuentenueva s/n. 18071, Granada Spain.
  • G. BOOTH-REA Departamento de Geodinámica, Facultad de Ciencias, Universidad de Granada. C/Fuentenueva s/n. 18071, Granada Spain. Departamento de Geodinámica, Instituto Andaluz de Ciencias de la Tierra (IACT), CSIC-Universidad de Granada. Facultad de Ciencias, 18002, Granada (Spain).
  • L. BARBERO Departamento de Ciencias de la Tierra, Facultad de Ciencias del Mar y Ambientales, Universidad de Cádiz. 11510 Puerto Real, Cádiz (Spain).
  • M. MELLINI Dipartimento di Scienze della Terra, Università di Siena. Via del Laterino 8, 53100 Siena (Italy).
  • F. GONZÁLEZ-LODEIRO Departamento de Geodinámica, Facultad de Ciencias, Universidad de Granada. C/Fuentenueva s/n. 18071, Granada Spain.

Keywords:

Low-grade extensional metamorphism, Illite crystallinity, Kübler index, Tanger-Ketama unit, External Rif, Northern Morocco

Abstract

The distribution pattern of diagenetic conditions to very low-grade metamorphism in the eastern Rif has been determined based on a study of clay-mineral assemblages and illite crystallinity of Mesozoic metapelites. Low-grade conditions were reached in marbles and also in the Beni-Malek serpentinites, as suggested by the mineral assemblages present in the marbles and antigorite growth in serpentinites. Previous thermochronological data are based on i) 40Ar/39Ar in amphiboles from greenschists, ii) K/Ar in white micas from metasandstones, and iii) fission tracks in apatites and zircons from metasandstones. These data indicate a Late Cretaceous age (80 Ma) for the very low- to low-grade metamorphism. We propose an evolutionary model for the Tanger-Ketama Unit consisting of a Lower Cretaceous sequence deposited in half-graben basins over an exhumed serpentinized mantle in a setting similar to the West Galician non-volcanic margin. The sediments underwent diagenesis to very low-grade metamorphism under relatively high heat flow in this extensional setting. Miocene contractional deformation of the Tanger-Ketama Unit resulted in a penetrative crenulation cleavage associated to asymmetric inclined folds. This crenulation developed, mostly by solution-transfer processes, without significant mineral growth. Miocene metamorphism reset the apatite fission-tracks, but metamorphic conditions were not high enough to reset either the K/Ar ages or the zircon fission tracks.

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