Zircon-bearing chlorite schists in the Sierra Bermeja massif (Ronda peridotites, Betic Cordilleras)

Category Tectonic & Seismotectonic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Universidad del Paیs Vasco, Spain
Holding Date 20 September 2008

The Ronda peridotites, one of the largest masses of orogenic lherzolites in the world, outcrop in the western part of the Betic Cordilleras (Southern Spain), sandwiched into the continental crust as allochthonous slabs. In the southwestern part of the Sierra Bermeja massif, the largest massif of the Ronda peridotites, zircon-bearing chlorite schists form layers of several meters long and variable thickness (< 50 cm). They cut the peridotite foliation, as also do the abundant granite dykes that are approximately perpendicular to the peridotite foliation in the same area. Some chlorite schists appear at the contact between peridotites and granite dykes. The chlorite schists are crumbly and soft rocks, apparently lack deformation and are mainly composed of chlorite (> 90 %), minor proportions of ilmenite, sphene, epidote, zircon and monazite and some nodules composed of prehnite and thomsonite intergrowths. Modal abundance of zircon and monazite may exceed 2 %. The analysed samples display parallel chondrite-normalised REE patterns with a marked negative anomaly in Eu and enrichment of light REE.
While the high content of elements as MgO, Ni, Co, Cr, V is consistent with a mantle signature of the chlorite schists, their REE patterns indicate that these rocks have been enriched by fluids with high LREE contents. The field relationships of the chlorite schists, the unusual presence of large amounts of zircon and monazite and the finding of rodingites within the Ronda massifs indicates that such fluids probably come from the granite dykes. The temperature conditions of chlorite formation range from 289 to 322 °C. The use of fission-track analyses to determine the timing of the chlorite formation provides a mean age of 19.2 ± 1.1 Ma, in agreement with the low temperature evolution of the Ronda peridotites after their emplacement into the continental crust. From geochemical, structural and textural data, the chlorite schists probably represent blackwalls formed by focussed Mg-metasomatism acting upon the granite dykes. This assumption is consistent with the parallelism between the chlorite schists and the granite dykes and the unusual abundance of zircon, because zirconium probably was concentrated from late Zr-saturated melts related to the dykes. An evolutionary model for the chlorite schists formation in three main stages could be: 1) Development of fracture-systems that cut the main structure of the peridotite massif, 2) Opening of previously developed fracture-systems favouring the intrusion of the granite dykes originated from the partial melting of the continental crust 21.8 Ma ago and at temperature intrusion of some 720 °C. 3) Cooling below 500 °C and beginning of serpentinization of the peridotites, at a minimum age of 19 Ma, coeval with the formation of the zircon-bearing chlorite schists.