Origin and age of the eclogites and amphibolites of the Ojén nappe (Betic Cordilleras, Spain): Insights about their protoliths and metamorphism

The results of whole rock geochemical and zircon dating analyses on amphibolite and eclogite samples of the Ojén nappe (Alpujárride Complex, Betic Cordilleras, Spain) are presented, with the aim of deciphering the geodynamic setting for their protoliths and the age of both the protoliths and their metamorphism. The Ojén nappe rests below the Sierra Alpujata peridotite, the second largest massif of the Ronda peridotites after Sierra Bermeja. The lithological sequence of the Ojén nappe consists of two main parts: 1) a lower metapelitic member, with gneisses, migmatites, schists and quartzites of Paleozoic (and older?) ages and 2) un upper marble member of Triassic ages. Amphibolite and eclogite layers appear scattered at different levels of the metapelitic and marble members.

Geochemically, the eclogites and amphibolites are mainly classified as tholeiitic basalt and display weak crustal contamination, whereas normalized REE patterns and tectonic discrimination diagrams point to magmatic protoliths of basaltic compositions produced by partial melting of a transitional or enriched mantle source (T- or E-MORBs). The high Th/U ratios (0.19-0.74) of the zircon cores from the amphibolites and the eclogites support their magmatic origin. Ages of 192 and 185 Ma have been determined by means of LA-Q-ICP-MS dating, for eclogite and amphibolites respectively, and interpreted as the intrusion age of their magmatic protoliths.  Zircon grains from eclogites also show metamorphic rims that yield concordant ages of 19.9 ± 1.7 Ma (Sánchez-Rodríguez and Gebauer 2000). The integration of regional, geochemical and age data supports the interpretation of the protoliths as gabbroic sills that were emplaced in the thinned continental margin of a Jurassic rift. We propose that this Mesozoic rift would represent the southern connection of the Atlantic Sea with the Neo-Tethys Ocean, which spread from Early Permian to Cretaceous times between Laurasia and Gondwana. The age of 19.9 ± 1.7 Ma is attributed to the thermal peak of a Miocene stage of subduction-zone metamorphism, which we link to the hot-emplacement and thrusting of the Ronda peridotites.

 

Sánchez-Rodríguez, L. & Gebauer, D. (2000). Mesozoic formation of pyroxenites and gabbros in the Ronda area (southern Spain), followed by Early Miocene subduction metamorphism and emplacement into the middle crust: U-Pb sensitive high resolution ion microprobe dating of zircon. Tectonophysics, 316, 19-44.