The recrystallization front of the Ronda peridotite: Evidence for melting and thermal erosion of subcontinental lithospheric mantle beneath the Alboran basin

Evidence for a major heating event accompanied by decompression was recently reported from crustal rocks drilled in the Alboran basin. The metamorphic evolution recorded by these rocks implies complete removal of lithosphere mantle during the Cenozoic, a process that is confirmed by geophysical modelling indicating thin lithosphere beneath the Alboran domain. In this region, the Ronda lherzolite massif (Betic Cordillera, southern Spain) provides a unique opportunity for the observation of mantle processes associated with lithospheric thinning. A striking-feature of the Ronda peridotite is a narrow recrystallization front, which has been ascribed to kilometrescale porous-melt flow through the massif. The front separates the spinel tectonite domain, interpreted as old, veined lithospheric mantle, from the granular domain where the lithospheric microstructures, mineralogical assemblages and geochemical signatures were obliterated by grain growth coeval with pervasive infiltration of basaltic melts. On the basis of trace-element abundances In peridotites collected over a distance of 12 km along the recrystallization front, our study confirms that the front is a relatively sharp (less than or equal to 400 m) geochemical discontinuity at the scale of the Ronda massif. Compared with the spinel tectonites, the coarse-granular peridotites are more homogeneous, more refractory in terms of major elements and more depleted in incompatible trace elements. These features are consistent with a process involving partial melting, kilometre-scale migration of melts by diffuse porous flow and limited melt extraction (2.5-6.5%). Hence, the Ronda recrystallization front is interpreted as the narrow boundary of a partial-melting domain (the coarse-granular peridotites) formed at the expense of subcontinental lithospheric mantle (the spinel tectonites). The existence of melt-consuming reactions in the transitional peridotites, a few hundred metres ahead of the melting front, demonstrates that the front was thermally controlled. This implies that a smooth thermal gradient existed across the Ronda massif during he development of the recrystallization front. Differences in pyroxene compositions on either side of the front may be explained by a transient heating event at greater than or equal to 1200 degreesC (similar to 1.5 GPa) coeval with partial melting. Consistent with the geodynamic scenario proposed for the Alboran domain during the Cenozoic, the evolution of the Ronda recrystallization front is considered as an example of thermal erosion and partial melting of lithospheric mantle above upwelling asthenosphere.