Exhumed continental lithospheric mantle in the Ronda peridotite massif, S. Spain

One of the most spectacular continental mantle peridotite massifs is exposed south of Ronda, southern Spain. It covers roughly 300 km2 and contains zones of garnet-, spinel-, and plagioclase-facies peridotite with layers of pyroxenite throughout. These facies are inverted and juxtaposed across a few km structural thickness, and have a steep thermal gradient. Previous work suggests the garnet facies mylonites at the top of the presently exposed body were deformed at 850 °C, while peridotite near the base was recrystallized in the plagioclase facies at 1200 °C. The steep thermal gradient alone indicates a substantial thinning event within the lithospheric mantle. This story is corroborated in the crustal rocks, where the overlying 50 km thick section has been thinned to just 5 km. During extension, a large thermal pulse heated the massif from the bottom, producing partial melts that percolated up to refertilize the lower units and cause annealing recrystallization. This process produced the inverted pressure-facies gradient currently observed; however, the stability of such a steep gradient over time is doubtful. Simple thermal calculations show that the ~350°C temperature difference between the marginal garnet mylonites and the plagioclase peridotites could not have been sustained over the present 3 km distance between the two zones for more than a quarter of a million years. Thus the section was probably greatly thinned after the heating and melt percolation event. Shear zones within the massif may be responsible for removing material during the exhumation of the peridotite body. We conduct thermobarometry on the mylonitic garnet peridotites, garnet pyroxenite layers, and spinel peridotite units combined with thermal models in order to better constrain the duration of the thermal gradient and refine the currently proposed P-T paths for the body. In addition, we present microstructural and map-scale evidence suggesting the existence of these section-removing shear zones.