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Petrology and metamorphic evolution of ultramafic rocks and dolerite dykes of the Betic Ophiolitic Association (Mulhacén Complex, SE Spain): evidence of eo-Alpine subduction following an ocean-floor metasomatic process |
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| Authors: | E Puga J M Nieto A Dí az de Federico J L Bodinier and L Morten |
| Institution: | a Instituto Andaluz de Ciencias de la Tierra, CSIC–UGRA, Facultad de Ciencias, Fuentenueva s/n, E-18002 Granada, Spain b Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany c Departamento de Mineralogía–Petrología, Facultad de Ciencias, Univ. Granada, Fuentenueva s/n, E-18002 Granada, Spain d Géofluides-Bassins-Eau, URA 1767 CNRS, ISTEEM, Univ. Montpellier 2, F-34095 Montpellier Cedex 05, France e Dipartimento di Scienze della Terra e Geologico-Ambientali, Universitá di Bologna, Piazza di Porta S. Donato 1, I-40127 Bologna, Italy |
| Abstract: | The Betic Ophiolitic Association, cropping out within the Mulhacén Complex (Betic Cordilleras), is made up of numerous metre- to kilometre-sized lenses of mafic and/or ultramafic and meta-sedimentary rocks. Pre-Alpine oceanic metasomatism and metamorphism caused the first stage of serpentinization in the ultramafic sequence of this association, which is characterized by local clinopyroxene (Cpx) breakdown and Ca-depletion, and complementary rodingitization of the basic dykes intruded in them. Subsequent eo-Alpine orogenic metamorphism developed eclogite facies assemblages in ultramafic and basic lithotypes, which were partly retrograded in Ab-Ep-amphibolite facies conditions during a meso-Alpine event. The heterogeneous development of the oceanic metasomatism in the ultramafic rock-types led to the patchy development of highly serpentinized Ca-depleted domains, without gradual transition to the host, and less serpentinized, Cpx-bearing ultramafites, mainly lherzolitic in composition. The high-pressure eo-Alpine recrystallization of these 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 in the enclosing lherzolitic 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. Relics of mantle-derived colourless olivine may be present in the more or less serpentinized secondary harzburgites. These relics are overgrown by the eo-Alpine brown pseudo-spinifex olivine, which contains submicroscopic inclusions of chromite, ilmenite and occasional halite and sylvite, inherited from its parental oceanic serpentine. The same type of mantle-derived olivine relics is also preserved within the Cpx-bearing serpentinites, although it has been partly replaced by the eo-Alpine Ti-clinohumite. The dolerite dykes included in the ultramafites were partly rodingitized in an oceanic environment. They were then transformed during the eo-Alpine event into meta-rodingites in their border zones and into eclogites towards the innermost, less-rodingitized portions. Estimated P–T conditions for the high-pressure assemblages in ultramafic and basic lithotypes range from 650 to 750°C and 16–25 kb. |
| Keywords: | Betic Cordilleras Mulhacén Complex Meta-ophiolites Oceanic metasomatism High-pressure metamorphism Spinifex-like textures |
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