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Journal of the Geological Society

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Tectonic implications of the granite dyke swarm in the Ronda peridotites (Betic Cordilleras, Southern Spain)

Departamento de Geodinámica, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado 644, 48080 Bilbao, Spain (e-mail: julia.cuevas@ehu.es)

This paper deals with the origin and structural relationships of the leucocratic dykes that intruded the Ronda peridotites. Plagiogranites are frequent in ophiolitic sequences, but similar Si-rich melts are scarce in orogenic lherzolites. The Ronda peridotites could be an excellent target in which to look for Si-rich melts of mantle origin, as they form the largest mass of orogenic lherzolites in the world and contain many granite dykes. However, petrographic and geochemical data provide evidence that the dykes come from felsic magmas originated from partial melting of the continental crust. Structural studies of the deformed and undeformed dykes reveal that they are non-Andersonian dykes that exploited pre-existing fractures. Undeformed dykes display N15°E strike, whereas the deformed ones show N50°E or N100°E trends and sinistral and dextral shear senses, respectively. These structural data point to reactivation of pre-existing fractures either as tensional cracks (undeformed dykes) or transtensional shear zones (deformed dykes) during the ENE-directed thrusting of the Ronda peridotites. Consequently, the dominant N15°E trend of the undeformed dykes can be used to infer an ENE-thrusting direction for the buried basal shear zone of the Ronda peridotites.