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The Petrogenesis of the ophiolitic mélange of Central Cuba: origin and evolution of oceanic litosphere from abyssal to subduction and suprasubduction zone settings

AuthorsButjosa Molines, Lidia
AdvisorProenza, Joaquín A.; Garcia Casco, Antonio
KeywordsEstructura interna de la Tierra
Manto terrestre
Internal structure of the earth
Mantle of the earth
Metamorphism (Geology)
Issue Date12-Jan-2018
PublisherUniversidad de Barcelona
CitationButjosa Molines, Lidia. The Petrogenesis of the ophiolitic mélange of Central Cuba: origin and evolution of oceanic litosphere from abyssal to subduction and suprasubduction zone settings. Universidad de Barcelona. 2018
Abstract[EN] This PhD is about the serpentinitic matrix, mafic crust and exotic ultramafic blocks of the Villa Clara serpentinitic mélange (VCSM) in central Cuba in an attempt to evaluate the evolution of oceanic lithosphere. Each of these units bears witness of mantle source composition, origin of oceanic crust and its relation to the mantle, ocean floor metamorphism, metasomatism and fluid flux in the subduction zone. The study of the serpentinitic matrix allowed the identification of two peridotite protoliths. Group A, with fertile compositions (high Al2O3 and low Cr# in pyroxene and spinel and enriched in heavy rare earth elements) and group B, displaying refractory compositions (low Al2O3 and higher Cr# in pyroxene and spinel compositions and depleted in middle and heavy rare earth elements). Group A can be related to typical abyssal/fracture zone peridotite, whereas group B is typical of forearc peridotite. Melting modelling shows that, group A resulted from low melting degrees (c. 4-8%) of a depleted mantle source, whereas group B reached up to c. 14-22% melting upon a two-stage melting of a depleted mantle followed by melting of a protolith similar to group A. The studied mafic crust includes rocks of sub-volcanic (diabase and microgabbro) and plutonic origin (cumulate gabbro and olivine gabbro). The sub-volcanic unit attests for two types of mafic magma: group 1 displays forearc basalts signature (FAB; low Ti/V ratio) and group 2 island arc tholeiite composition (IAT; medium Ti/V ratio), both with positive Th and negative Nb anomalies in comparison to N-MORB compositions. These compositions as well as the isotopic signature of the plutonic unit (low 143Nd/144Nd and low 87Sr/86Sr) point to a subduction-related imprint. Geochemical evidence supports a genetic relationship between the protoliths of the serpentinitic matrix rocks and the sub-volcanic mafic crust. An indirect evidence is the light rare earth element enrichment of group B peridotites, which is commonly interpreted as a result of re-equilibration with percolating basaltic melts like those represented by the sub-volcanic mafic crust. Also, melting modelling of primitive melts of the mafic crust (group 1-FAB related) results in c. 8-10 % melting of an abyssal mantle source like group A peridotites that produced a residue like group B peridotites. On the other hand, ocean floor metamorphism affected the mafic crust, which displays greenschist to amphibolite facies assemblages that attest for low pressure/low to medium temperature at shallow depths. This process had an impact on the concentration of mobile elements. A different metasomatic/enrichment process is recorded by trace element geochemistry and stable and radiogenic isotopes (B, Nd, Sr and Pb) in the serpentinitic matrix. The isotopic relations point to a slab fluid formed after devolatilitzation of the subducting plate as the source of metasomatic agent. The slab fluid is composed of diverse proportions of altered oceanic crust fluid (AOCF), global subducting sediment fluid (GLOSSF) and terrigenous fluid (TERF). The combination of these three isotopic reservoirs with an already serpentinized mantle related to ocean floor hydration reproduces the isotopic signature of the serpentinitic matrix of the VCSM. This result is in agreement with evidence provided by the high-pressure exotic ultramafic block of the VCSM, which showssimilar isotopic composition indicating interaction with a similar fluid in a context of subduction. The petrological and geochemical characteristics of the exotic ultramafic block allow distinguishing two types of serpentinite: i) antigorite-serpentinite and ii) dolomite-bearing antigorite serpentinite. Both represented a subducted peridotite that derives from a peridotite protolith locally CaO-enriched as a result of infiltration of a H2O-CO2 fluid mixture. Fluid infiltration in the subduction channel triggered serpentinization /carbonation and formation of tremolite veins and associated blackwalls developed in host antigorite-serpentinite. Mineralogical and chemical zoning in the blackwall (Atg + Chl + Tr towards the host serpentinite and Chl + Tr towards the vein) attest for metasomatic changes in fluid composition during fluid-rock interaction. The differences in chemical composition between blackwall and antigorite-serpentinite show that the infiltrating fluid was enriched in Ca, Al, LILE and LREE. Pseudosection modelling in the vein structure indicates that their formation took place at c. 4S0QC and c. 10 kbar. All these findings allow constraining the geodynamic evolution of the Villa Clara serpentinitic mélange in the context of the Caribbean realm. The serpentinitic matrix attests for two contrasting geodynamic settings. Group A peridotites formed at an abyssal/fracture zone setting in the Proto-Caribbean oceanic basin, whereas group B peridotites, exotic ultramafic block and mafic crust are pointing to a forearc setting. Both scenarios are reconciled in a geodynamic model of Upper Jurassic-Lower Cretaceous oceanic lithosphere formation upon break-up of Pangea followed by subduction initiation, likely at a fracture zone setting, during the early Cretaceous, and further development of a serpentinized forearc mantle and associated subduction channel during Lower-Upper Cretaceous time until final emplacement of the ensemble (serpentinitic mélange) during the latest Cretaceous-Eocene.
Publisher version (URL)http://hdl.handle.net/2445/122449
Identifiersuri: http://hdl.handle.net/2445/122449
Appears in Collections:(IACT) Tesis
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