The evolution of the footwall to the Ronda subcontinental mantle peridotites: insights from the Nieves Unit (western Betic Cordillera)

Strongly heterogeneous deformation and extreme metamorphic gradients characterize the dominantly carbonate Nieves Unit in the footwall to the Ronda mantle extrusion wedge in the western Betic Cordillera. A well-developed foliation and mineral lineation, together with isoclinal intrafolial folds, occur in silicate-bearing, calcite/dolomite marbles within a c. 1.5 km-thick metamorphic aureole underlying the peridotites. For the inferred maximum pressure of 300 MPa, petrological investigations allow to define temperature ranges for the main zones of the metamorphic aureole: forsterite zone (> 510 ºC; probably c. 700 ºC), diopside zone (510–430 ºC), tremolite zone (430–360 ºC), and phlogopite zone (360–330 ºC). Field structural analysis integrated with petrological, microstructural and EBSD textural data document large finite strains consistent with general shear within the metamorphic aureole, associated with NW-ward thrusting of the peridotites. On the other hand, post-kinematic silicate growth suggests that heat diffusion from the high-temperature peridotites continued after the final emplacement of the Ronda mantle extrusion wedge, leading to final zoning of the metamorphic aureole and to local partial annealing of calcite marble textures, particularly in the highest-temperature zone of the thermally softened footwall carbonates. Following substantial cooling, renewed crustal shortening affected the whole Nieves Unit, resulting in widespread development of NE-trending meso-scale folds