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Exsolution microstructures in amphiboles from metabasalts of the Betic ophiolitic association (SE Spain)

¹ Departamento de Química Inorgánica, Cristalografía y Mineralogía, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain
² Instituto Andaluz de Ciencias de la Tierra (CSIC-UGRA), Facultad de Ciencias, Avda. Fuentenueva s.n., 18002 Granada, Spain

^* Corresponding author, e-mail: mdruiz{at}uma.es

Exsolution microstructures in a complex amphibole assemblage from metabasalts of the Betic Ophiolitic Association (SE Spain) have been studied using transmission and analytical electron microscopy (TEM, AEM). Three main types of amphibole were identified, filling veins and vesicles: brown amphibole, with mean composition of pargasite, originated during an early ocean-floor metamorphism, and two types of green amphiboles with compositions of edenite and actinolite-magnesiohornblende, respectively formed during a later orogenic metamorphic event. The exsolution in the brown amphibole include minute rutile and amphibole rods. The brown amphibole also contains numerous halite inclusions. Amphibole exsolutions, with compositions of magnesiohornblende and actinolite appear to have formed paired rutile + amphibole intergrowths. All the exsolution products appear oriented with respect to the lattice of the host amphibole. Green amphiboles do not contain either submicroscopic rutile or halite inclusions, but they show oriented lamellae of magnesiokatophorite, exsolved in the edenite. These thin lamellae (300 to 500 Å in thickness) are coherently intergrown with the host. The studied amphibole pairs evidence two types of miscibility gaps: the one gap between amphiboles of the calcic group (brown amphibole) and the other gap between amphiboles of the calcic and sodiccalcic groups (green amphibole).

Key-words: ocean-floor metamorphism, orogenic metamorphism, Betic Cordillera, amphiboles, miscibility gap, exsolution, TEM/AEM.