Magnetite-silicate inclusions in olivine of ophiolitic metagabbros from the Mulhacen Complex, Betic Cordillera, southeastern Spain

Submicroscopic magnetite-silicate inclusions in igneous olivine in metagabb roic rocks from the Betic Ophiolitic Association (BOA) of the Mulhacen Comp lex, Betic Cordillera, southeastern Spain, have been studied by TEM-AEM. Th e metamorphic history of these rocks includes a stage of ocean-floor metamo rphism followed by polyphase Alpine metamorphism, its metamorphic climax be ing developed under conditions of subduction. The grains of olivine have a brown color, interpreted as due to blebs of magnetite that formed during th e metasomatic and metamorphic processes superimposed on the igneous crystal s. The magnet ire precipitates display a fixed orientation relative to the olivine host, and form intergrowths with silicate phases. These include mon oclinic amphibole, orthorhombic amphibole and monoclinic pyroxene, the asso ciation magnetite - monoclinic amphibole being the most common. Chemically, the exsolved amphiboles are calcic and may be subdivided into two types: a ctinolite, without Na, and edenite and pargasite, containing Na. The presen ce in the exsolution-induced blebs of a hydrated silicate phase, together w ith the existence of saline inclusions in the olivine, point to the ocean-f loor stage for the influx of seawater, which contributed to the exsolution process. Petrographic observations and comparison of the chemical compositi on of the exsolved amphiboles with that of the amphiboles developed during the several metamorphic stages registered in the host gabbros allow us to i nfer the P-T conditions during the exsolution process. Exsolution probably began during ocean-floor metamorphism, and continued during the prograde st age of the eo-alpine event, at which time the kelyphitic amphibole bearing coronas formed in the olivine.