Provenance and Geodynamic implication from detrital zircon U-Pb LA-ICP- MS analysis of the Ojén Nappe (Betic Cordilleras, Spain)

The Ojén nappe underlies the largest worldwide exposure of subcontinental lithospheric mantle, the Ronda peridotites (Betic Cordilleras), and features the typical lithological Alpujarride sequence with Paleozoic or older metapelites at the bottom covered by quartzites and a Triassic marble formation to the top. With the objective of determining the radiometric age of the carbonate sequence and the provenance of the nappe, detrital zircons from two quartzite samples from layers interbedded within marbles and metapelites were processed by means of U-Pb LA-ICP-MS analysis. Both samples were collected far away from the peridotite contact, in order to avoid age resetting promoted by the high-temperature Alpine emplacement of the peridotites at Miocene times (Esteban et al., 2011).

The youngest zircon grains in both samples show ages of 219 and 240 Ma (Late-Middle Triassic) that support the regional correlation with paleontologically dated Alpujarrride marbles from the Central and Eastern areas of the Betic Cordilleras. Otherwise, the analyzed detrital zircon grains display age distributions with Cisuralian (280–290 Ma), Middle Ordovician (460–465 Ma), Ediacaran–Cryogenian (560–615 Ma), and Tonian–Stenian (950–975 Ma) peaks. These data contrast with the scarcity of Mesoproterozoic (1001–1561 Ma), Archean (2503–2976 Ma), and Mesozoic (219–248 Ma) zircon ages. The Permian zircons are well-arranged into three main populations of 292 ± 2, 278 ± 3, and 254 ± 3 Ma. Zircons in the aforementioned populations exhibit Th/U ratios higher than 0.1, with a mean value of 0.34, which points to felsic igneous rocks as the main protolith for the zircon-bearing sediments.

In summary, on the basis of the U-Pb LA-ICP-MS age determinations obtained for the analyzed detrital zircons the following interpretations are suggested: 1) the youngest detrital zircon population, 254 ± 3 Ma (Late-Permian), is considered as the more conservative and appropriate estimation for the maximum sedimentation age, 2) the three Permian zircon populations are in agreement with the main age clusters reported so far for rhyolites and shallow crustal basaltic–andesite subalkaline rocks emplaced in transtensional Permian basins of the Variscan Orogen during the break-up of Pangea, 3) the detrital zircon populations point to a sediment source from a Cadomian peri-Gondwanan terrane and, 4) the well-defined Middle Ordovician detrital zircon population (460–465 Ma) strengthens the hypothesis that the Alborán microplate (meso-Mediterranean domain) could be located along the southern passive margin of the European Hunic superterrane.

 

Esteban, J.J., Cuevas, J., Tubía, J.M., Sergeev, S., Larionov, A. (2011). A revised Aquitanian age for the emplacement of the Ronda peridotites (Betic Cordilleras, southern Spain). Geol. Mag., 148, 183-187.