Structural petrology of the Ronda peridotite, SW Spain: Deformation history

Abstract

Solid bodies of upper-mantle peridotite, emplaced in the Betic-Rif chains of SW Spain and North Morocco, show a variety of structures developed under different metamorphic conditions. These structures and related metamorphism reflect tectonic processes in the West Mediterranean mantle during orogeny in the Betic-Rif realm. The largest oj the peridotites, the Ronda massif, has preserved three structural domains which are spatially associated with metamorphic domains previously defined by Obata (Journal of Petrology, 21, 533-572, 1980). These structural domains include: (1) porphyroclastic spinel peridotites (spinel tectonites) and mylonitic garnet-spinel peridotites (garnet-spinel mylonites), developed during progressive strain localization at ambient conditions changing from the Ariégite subfacies to garnet peridotite facies; (2) seemingly undeformed granular peridotites, developed during a phase of extensive recrystallization affecting the spinel tectonites and garnet-spinel mylonites at Seiland subfacies conditions, and separated from the spinel tectonites by a well-preserved recrystallization front which forms a marked structural, metamorphic and possibly geochemical boundary probably unique to orogenic peridotites; (3) porphyroclastic plagioclase peridotites (plagioclase tectonites) developed at the expense of the granular peridotites during progressive shear localization allied to ductile emplacement of the Ronda massif into the crust. Our structural and microstructural data from the Ronda massif allow us to assess the relative ages of the different metamorphic facies seen in the West Mediterranean peridotites. In order of decreasing relative age, these are: Ariégite-subfacies, garnet peridotite facies, Seiland subfacies and plagioclase peridotite facies. In addition, the associated microstructures provide some insight into the microphysical conditions controlling the development of the different structures and, as a result, the structural and chemical heterogeneity of the West Mediterranean peridotites.