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 recentl y 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 mode lling indicating thin lithosphere beneath the Alboran domain. In this regio n, the Ronda lherzolite massif (Betic Cordillera, southern Spain) provides a unique opportunity for the observation of mantle processes associated wit h lithospheric thinning. A striking-feature of the Ronda peridotite is a na rrow recrystallization front, which has been ascribed to kilometrescale por ous-melt flow through the massif. The front separates the spinel tectonite domain, interpreted as old, veined lithospheric mantle, from the granular d omain where the lithospheric microstructures, mineralogical assemblages and geochemical signatures were obliterated by grain growth coeval with pervas ive infiltration of basaltic melts. On the basis of trace-element abundance s In peridotites collected over a distance of 12 km along the recrystalliza tion front, our study confirms that the front is a relatively sharp (less t han or equal to 400 m) geochemical discontinuity at the scale of the Ronda massif. Compared with the spinel tectonites, the coarse-granular peridotite s are more homogeneous, more refractory in terms of major elements and more depleted in incompatible trace elements. These features are consistent wit h a process involving partial melting, kilometre-scale migration of melts b y diffuse porous flow and limited melt extraction (2.5-6.5%). Hence, the Ro nda recrystallization front is interpreted as the narrow boundary of a part ial-melting domain (the coarse-granular peridotites) formed at the expense of subcontinental lithospheric mantle (the spinel tectonites). The existenc e of melt-consuming reactions in the transitional peridotites, a few hundre d metres ahead of the melting front, demonstrates that the front was therma lly controlled. This implies that a smooth thermal gradient existed across the Ronda massif during he development of the recrystallization front. Diff erences in pyroxene compositions on either side of the front may be explain ed 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 geodyn amic scenario proposed for the Alboran domain during the Cenozoic, the evol ution of the Ronda recrystallization front is considered as an example of t hermal erosion and partial melting of lithospheric mantle above upwelling a sthenosphere.