A revised thermal history of the Ronda peridotite, S. Spain: New evidence for excision during exhumation

The Ronda peridotite massif of southern Spain exposes subcontinental lithospheric mantle that records pressure–temperature data and microstructures formed during exhumation beneath the rapidly extending Alboran domain. The peridotite is zoned from garnet- and spinel-bearing mylonites at the structural top, to spinel-bearing tectonites, to melt-percolated spinel-bearing granular peridotites, to plagioclase-bearing tectonites at the structural base. We find microstructural evidence of melt present in the spinel zones prior to the deformation event which exhumed the peridotites, and we therefore reinterpret the spinel tectonites as being a result of deformational overprinting of part of the granular domain. We also reinterpret garnet intergrown with spinel in the mylonite zone as part of the pre-mylonitic porphyroclast assemblage, rather than as a syn-mylonite assemblage. This places mylonite formation within the spinel field, rather than right on the garnet-spinel transition (18 kb). Two-dimensional thermal modeling indicates that these conditions require removal of lithospheric mantle below 100 km followed by exhumation along a low angle shear zone. Excision of material during exhumation is required to explain the steep thermal gradients observed. These results shed light on the mechanisms of back-arc extension, as well as the emplacement of orogenic lherzolites.