Encarnación Puga

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

Abstract The Betic Ophiolitic Association, cropping out within the Mulhacen 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.

Tectonostratigraphic subdivision and petrological characterisation of the deepest complexes of the Betic zone: a review

Abstract The Veleta and Mulhacen Complexes, previously considered together as the Nevado-Filabride Complex, constitute the deepest units of the Betic Cordilleras. The Veleta Complex (VC) is mainly formed of several thousand metres of graphite-bearing micaschists and crops out as a series of tectonic windows below the Mulhacen Complex (MC). This latter complex comprises a pile of thrust nappes of crustal origin, composed of a Palaeozoic basement and Mesozoic cover series, between which a Jurassic–Cretaceous ophiolitic nappe is tectonically intercalated. The Alpine metamorphism developed in eclogite facies followed by Ab-Ep amphibolite facies in the MC, and in Ab-Ep amphibolite facies followed by greenschist facies in the VC. Pre-Alpine metamorphism in high gradient amphibolite facies is locally well preserved in some basement levels of the MC. In contrast, only scarce relics of pre-Alpine metamorphism, in greenchists facies, have been preserved in the VC. Within the MC nappes of crustal origin there appear metagranitic rocks, which were generated during two stages of late Hercynian magmatic activity: one of syncollisional acidic magmatism and the other of postcollisional intraplate acidic-to-intermediate volcanism. The ophiolitic nappe is composed of basic, ultramafic and sedimentary rocks affected by ocean-floor and orogenic metamorphism. The basic rocks partly retain their original E-MORB geochemical character. The ultramafic rocks are serpentinites and secondary harzburgites containing partly rodingitized basaltic dykes. The Soportujar Formation of the MC is made up of continental and evaporitic metasediments and meta-andesitic rocks that originated during an intraorogenic relaxation period between the eo-Alpine and meso-Alpine metamorphic events.

The Nevado-Filabride metaophiolitic association in the Cobdar region (Betic Cordillera, SE Spain): preservation of pillow structures and development of coronitic eclogites

AbstractThe Nevado-Filabride ophiolilic association is made up of the dismembered and metamorphosed remains of a Jurassic ocean floor. This associatiion is exceptionally well preserved, and locally unmetamorphosed, around the village of Cobdar (prov. of Almeria). It crops out in this area as lenticular metabasite and metasedimentary bodies, about 7 km in length and several hundred metres thick, tectonically intercalated between the Caldera and the Sabinas Units of the Mulhacen Group of Nappes. The ophiolitic association in this region is made up of three distinct sequences: a plutonic sequence consisting of cumulitic troctolites, olivine-pyroxene gabbros and dolerites, cut through by numerous dykes of porphyric basalts; a volcanic sequence composed of massive or vesicular olivine-pyroxene basalts, sporadically intersected by basaltic dykes and locally containing pillow-structures; and a frequently laminated sedimentary sequence made up mostly of ankerite-garnet calc-schists with interbedded lenses of mica...

Silicate and oxide exsolution in pseudo-spinifex olivine from metaultramafic rocks of the Betic Ophiolitic Association: A TEM study

Abstract Transmission electron microscopy (TEM) has been used to study submicroscopic particles in spinifex- like textured olivine from secondary harzburgites collected from the Cerro del Almirez locality in the Mulhacén Complex (Betic Cordillera, SE Spain). Three main types of submicroscopic oxides have been identified: (1) equidimensional Fe-rich spinel (magnetite), with average grain size in the order of 1-2 μm; (2) elongated Cr-bearing spinels (Fe-chromite to Cr-magnetite) with sizes ranging from 0.2 to 1 μm long and 0.01 to 0.1 μm thick; and (3) equidimensional Ti-rich particles from <0.01 to 0.1 μm and compositions ranging from ilmenite to Fe-Cr-Ti oxide. Chromite and ilmenite particles form parallel, chromite-rich and ilmenite-rich bands, extending along the a-axis of the host olivine. Both phases show a fixed orientation relationship with olivine, with the approximately hexagonal close-packed oxygen planes being parallel in both structures. These textural relationships indicate that both chromite and ilmenite were formed during a common exsolution process. Magnetite particles also are orientated preferentially relative to the olivine, but these particles are homogeneously distributed within the olivine, suggesting either a primary origin or an exsolution process that was not contemporaneous with formation of chromite and ilmenite. Chromite particles commonly are accompanied by lamellae of talc and/or enstatite, both showing a consistent orientation relationship with olivine. Talc lamellae are twice as thick as the associated chromite crystals, whereas enstatite lamellae show a greater thickness and, moreover, form single enstatite particles, which consist of clino- and orthoenstatite intergrowths. Talc formation may be explained by exsolution, together with spinel, from olivine containing OH-groups, probably related to incomplete dehydration of serpentine during olivine formation. On the basis of these results and previously reported petrological data, we have concluded that exsolution of chromite-silicate and ilmenite occurred during the retrograde stage that followed the climax of the eo-Alpine metamorphic event.

The evolution of the magmatism in the external zones of the Betic Cordilleras during the mesozoic

AbstractThe Mesozoic igneous rocks from the External Zones of the Betic Cordilleras extend for some 300 km along the Subbetic Zone. They are poorly differentiated basic rocks which, altogether, correspond to a transitional series containing tholeiitic and sodium-alkaline terms. They crop out as small ophite stocks and dykes intruded into middle- and upper-Triassic rocks, or as submarine flows and sills interlayered with Jurassic materials.Geological and radiometric evidence points to an upper-Triassic-Liassic age for the ophite-generating magmatism, while the fissure volcanism began locally in the early-Liassic and extended throughout the Dogger. It reached its climax in the Tithonian and ceased abruptly in the lowermost Cretaceous.The magmas that generated the two groups of rocks originated within the mantle. During the ascent through a continental crust they were contaminated by deep-crust granitoid rocks and by the assimilation of pelites from the basement. The chemical composition and fractional-cryst...

Vermiculite-like minerals in low-grade metasediments from the Coastal Range of central Chile

Abstract Mixed-layer minerals with optical properties similar to metamorphic vermiculite were identified in rocks belonging to a Palaeozoic and a Triassic formation separated by an angular unconformity and exposed in the Coastal Range of central Chile. Both formations are affected by low-grade metamorphism. The mixed-layer minerals were studied by optical microscopy, X-ray diffraction (XRD), electron microprobe (EMPA), and transmission/ analytical electron microscopy (TEM/AEM). Two types of phyllosilicates were identified: chlorite-vermiculite and mica-chlorite, which are present in the Palaeozoic and Triassic rocks respectively. Chlorite-vermiculite mixed layers form packets with well-defined boundaries and mainly show 1:1 ordered sequences. On the contrary, mica-chlorite mixed layers show, in most cases, random sequences evolving laterally toward chlorite. The AEM data indicate compositions close to that of chlorite in the ternary Si-Al-(Fe+Mg+Mn) diagrams for both types of mixed-layer phyllosilicates. Relative to the coexisting chlorite, they have lower (Fe+Mg) contents, and a higher Si/Al ratio. They are interpreted as products of the transformation of chlorite, developed during prograde metamorphism, and probably represent intermediate, metastable phases, in the chlorite to biotite transformation.

Microstructures and interlayering in pyrophyllite from the Coastal Range of central Chile: evidence of a disequilibrium assemblage

Abstract Pyrophyllite from a Triassic sedimentary formation from the Coastal Range of Chile has been investigated by transmission/analytical electron microscopy (TEM/AEM). The mineral assemblage includes pyrophyllite, muscovite, paragonite, a kaolin mineral, boehmite, rutile and hematite. The textures indicate that the protolith was a volcanoclastic rock. Petrographic evidence, chemistry, and the mineral assemblage suggest the intense leaching of the parent rock by a weathering process, before the metamorphic episode, to create the protolith for the pyrophyllite. Pyrophyllite always grows from the kaolin mineral, and both phases show close orientation relationships. The presence of parallel intergrowths of pyrophyllite and muscovite indicate that muscovite also grew from the kaolin mineral. Nevertheless, the composition of muscovite suggests that this phase must also form from another precursor, probably Al smectite. The AEM data and textural relationships between pyrophyllite and muscovite reveal the presence of two generations of muscovite and suggest that Na-rich muscovite recrystallized into a Na-free muscovite and paragonite.

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

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 A 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).