THE RONDA PERIDOTITE, S SPAIN: EXHUMED LITHOSPHERIC MANTLE FROM BENEATH A COLLISIONAL OROGEN

The Ronda peridotite of southern Spain is an alpine-type peridotite massif exhumed from beneath thickened continental crust in the Africa-Iberia collision zone. It was emplaced during a late Tertiary extensional event, which formed the Alboran Sea and thinned the overlying crust by a factor of 10 (50 km thick sequence thinned to 5 km). Mantle units also show an apparent thinning, juxtaposing garnet-, spinel-, and plagioclase-peridotite facies across only a few km, but the timing and nature of this process is debated. Garnet in the marginal peridotite mylonites is variously interpreted as relict, or resulting from the breakup of garnet pyroxenite layers. Other complications include the apparent inversion of the pressure gradient and the steep thermal gradient within the body. Several authors have employed a zone of recrystallization or melt percolation to explain the apparent decrease of pressure down-structure. Simple thermal calculations show that the temperature difference between the marginal mylonites and the plagioclase melt percolation front (~350°C) could not have been sustained over the present structural distance between the two zones (2 km) for more than 130 kyr, indicating that the present geometry of the body was profoundly modified during exhumation. Convective removal or delamination of a lithospheric mantle root has been suggested to drive thinning and heating.

What was the initial depth of this thinning event? At what depth did melt percolation occur? Under what conditions did the peridotite mylonites form? We are conducting thermobarometry on the mylonitic garnet peridotites, garnet pyroxenite layers, and spinel peridotite units to refine the P-T path of the peridotite body before and during the thinning event. Comparison of the chemistry of garnets in peridotite with those in pyroxenite layers shows that they are significantly more Mg-rich and likely formed in equilibrium with the peridotite, suggesting that the mylonites were derived from true garnet peridotites. Field observations, however, suggest that the mylonitic deformation overprints the spinel peridotites, suggesting that it took place in the spinel field. This is consistent with mylonitization being related to emplacement into the crust, as suggested by the localization of the mylonites along the upper contact of the peridotite.