Garnet lherzolite and garnet-spinel mylonite in the Ronda peridotite: Vestiges of Oligocene backarc mantle lithospheric extension in the western Mediterranean

Abstract

International audienceUplift and exhumation of vast exposures of diamond facies, subcontinental mantle peridotite in the Western Mediterranean arc are attributed to tectonic scenarios including pure extension, transpression or subduction followed by delamination-driven or rollback-driven stretching. In the Ronda peridotite (southern Spain) the strong overprint of low-pressure assemblages has precluded accurate determination of the pressure and temperature conditions for the onset of exhumation that formed the spinel tectonite and garnet-spinel mylonite domain in this massif. Here we report unequivocal petrographic evidence for the existence of prekinematic, coarse-grained garnet lherzolite assemblages from the garnet-spinel mylonite domain of the Ronda peridotite. Application of well-calibrated geothermobarometers yields prekinematic minimum equilibration conditions of 2.4-2.7 GPa and 1020-1100 degrees C, demonstrating that the Ronda peridotite equilibrated at similar to 85 km depth before shearing. We also show the existence of synkinematic garnet and spinel assemblages that overprinted garnet lherzolite assemblages at 800-900 degrees C and 1.95-2.00 GPa. The decompressional cooling path and high pressure recorded by garnet-spinel mylonites rule out their formation by near-isobaric cooling above a subduction-collision wedge or during or after the emplacement of the peridotite massif into the crust. Ronda garnet-spinel mylonites represent the vestiges of subcontinental mantle ductile shear zones formed at early stages of lithosphere extension during backarc extension in the western Mediterranean. Southward to westward retreat of the African slab during the Oligocene-Early Miocene accounts for intense backarc lithosphere extension and development of the Ronda extensional shear zone, coeval with extreme thinning of the Alboran domain overlying crust

Orogenic, Ophiolitic, and Abyssal Peridotites

“…However, it should be stressed that these rocks have been found only in strongly deformed orogenic peridotites that have undergone an HP to a UHP evolution in a mantle wedge above a subduction zone, before their exhumation to the surface (Becker, 1996;Davies et al, 1993;Garrido et al, 2011;Van der Wal and Vissers, 1993). Therefore, the 'HP/UHP' deformed pyroxenites may represent crustal material that was first entrained into the subduction and then exhumed to the surface with their host peridotites (see, e.g., Brueckner and Medaris, 2000, and references herein).…”

The composition of nanogranitoids in migmatites overlying the Ronda peridotites (Betic Cordillera, S Spain): the anatectic history of a polymetamorphic basement

“…The mylonitic gneisses are in contact with the underlying Ronda peridotites along a high temperature ductile shear zone; this contact is parallel to the mylonitic foliation developed in both crustal and mantle rocks, and to the penetrative foliation and lithological contacts in the sequence (Balanyá et al 1997;Platt et al 2003;Garrido et al 2011;Précigout et al 2013). …”

“…The metamorphic grade of the Alpujárride units increases from east to west in the orogen, such that in the western Betics granulitic migmatites occur at the base of the sequence and appear spatially associated with slices of subcontinental mantle peridotites (i.e. the Ronda peridotites ;Lundeen 1978;Obata 1980;Van der Wal and Vissers 1996;Garrido et al 2011;Précigout et al 2013). Thus, in the vicinity of the Ronda peridotites, crustal rocks show systematically the highest metamorphic grade and extensive melting (Loomis 1972;Torres-Roldán 1981Balanyá et al 1997;Tubía et al 1997Tubía et al , 2013Argles et al 1999;Acosta-Vigil et al 2001, 2014Platt et al 2003;Esteban et al 2008;Barich et al 2014).…”

Coesite and diamond inclusions, exsolution microstructures and chemical patterns in ultrahigh pressure garnet from Ceuta (Northern Rif, Spain)

“…It was also assumed that diamond, now graphitized (Davies et al, 1993;Pearson et al, 1989;Slodkevich, 1980), formed during this stage. Nevertheless, the mechanism and age of emplacement of the peridotite massif within the nappe pile of the Betic-Rif orogen are yet matter of debate (e.g., Garrido et al, 2011;Pearson and Nowell, 2004;Platt et al, 2003;Saddiqi et al, 1988;Zeck, 1996).…”