Polymetamorphic Amphibole Veins in Metabasalts from the Betic Ophiolitic Association at Cóbdar, Southeastern Spain: Relics of Ocean-floor Metamorphism Preserved through the Alpine Orogeny

Our aim in this work is (1) to characterize mineralogically and geochemically the brown amphibole and other minerals filling millimetric veins in the Cóbdar metabasalts of the Betic ophiolites in southern Spain, and (2) to deduce their genetic conditions, mainly with the aid of the optical microscopy and analytical electron-microprobe analysis, complemented by transmission and analytical electron microscopy. The vein-filling minerals can be grouped into two assemblages according to their textural relationships. The first assemblage comprises brown amphibole (titanian pargasite and kaersutite) and sodic–calcic plagioclase, and the second one comprises green amphibole (mainly katophorite, barroisite, taramite and tremolite), sodic plagioclase, rutile, clinozoisite and calcite. Brown and green amphiboles present similar, unusually high chlorine contents, up to 0.8 wt.%. Halite micro-inclusions are present in the brown amphiboles, whereas chlorine is incorporated in the structure of the green sodic–calcic amphiboles. Differences in Ti, IVAl, VIAl and BNa values between the brown and green amphiboles clearly indicate different genetic conditions, which are, in turn, similar to those corresponding to amphiboles developed in ocean-floor and orogenic metamorphic conditions, respectively. P–T estimates yield temperatures ranging from 660° to 880°C at very low pressure for the first assemblage, and conditions lower than 550°C and 8 kbar for the second assemblage. These data suggest that the minerals filled the Cóbdar veins during the pre-Alpine ocean-floor metamorphic stage (dated by 40Ar/39Ar laser probe on brown amphibole as Upper Jurassic), and were re-equilibrated under conditions of the albite–epidote amphibolite facies during the Alpine orogeny.