Petrology and metamorphic evolution of ultramafic rocks and dolerite dykesof the Betic Ophiolitic Association (Mulhacen Complex, SE Spain): evidenceof eo-Alpine subduction following an ocean-floor metasomatic process

The Betic Ophiolitic Association, cropping out within the Mulhacen Complex (Betic Cordilleras), is made up of numerous metre- to kilometre-sized lense s of mafic and/or ultramafic and meta-sedimentary rocks. Pre-Alpine oceanic metasomatism and metamorphism caused the first stage of serpentinization i n the ultramafic sequence of this association, which is characterized by lo cal clinopyroxene (Cpx) breakdown and Ca-depletion, and complementary rodin gitization of the basic dykes intruded in them. Subsequent eo-Alpine orogen ic metamorphism developed eclogite facies assemblages in ultramafic and bas ic lithotypes, which were partly retrograded in Ab-Ep-amphibolite facies co nditions during a meso-Alpine event. The heterogeneous development of the o ceanic metasomatism in the ultramafic rock-types led to the patchy developm ent of highly serpentinized Ca-depleted domains, without gradual transition to the host, and less serpentinized, Cpx-bearing ultramafites, mainly Iher zolitic in composition. The high-pressure eo-Alpine recrystallization of th ese ultramafites in subduction conditions originated secondary harzburgites in the Ca-depleted domains, consisting of a spinifex-like textured olivine + orthopyroxene paragenesis, and a diopside + Ti-clinohumite paragenesis i n the enclosing Iherzolitic rocks. During the meso-Alpine event, secondary harzburgites were partly transformed into talc + antigorite serpentinites, whereas the diopside and clinohumite-bearing residual meta-lherzolites were mainly transformed into Cpx-bearing serpentinites. Relies of mantle-derive d colourless olivine may be present in the more or less serpentinized secon dary harzburgites. These relies are overgrown by the eo-Alpine brown pseudo -spinifex olivine, which contains submicroscopic inclusions of chromite, il menite and occasional halite and sylvite, inherited from its parental ocean ic serpentine. The same type of mantle-derived olivine relies is also prese rved within the Cpx-bearing serpentinites, although it has been partly repl aced by the eo-Alpine Ti-clinohumite. The dolerite dykes included in the ul tramafites wen partly rodingitized in an oceanic environment. They were the n transformed during the eo-Alpine event into meta-rodingites in their bord er zones and into eclogites towards the innermost, less-rodingitized portio ns. Estimated P-T conditions for the high-pressure assemblages in ultramafi c and basic lithotypes range from 650 to 750 degrees C and 16-25 kb. (C) 19 99 Elsevier Science B.V. All rights reserved.