Internal Ossa-Morena Zone Ophiolitic Sequences: geodynamic implications for the evolution of the SW branch of the Iberian Variscan Chain

The Internal Ophiolitic Sequences correspond to allochthonous oceanic crust fragments that outcrop near the SW boundary of the Ossa-Morena Zone (Evora-Beja Domain), in an internal position relative to the Beja-Acebuches Ophiolitic Complex. The Internal Ophiolitic Sequences occur as tectonic imbrications or klippen, in the Moura Phyllonitic Complex. Despite deformation and metamorphism (greenschist/amphibolite facies), they still preserve a typical ophiolitic internal stratigraphy identical to the one described for many LOT (“Lherzolitic Ophiolite Type”) ophiolites. Geochemical data indicates that the Internal Ophiolitic Sequences have variable tholeiitic chemistry, transitional between N-MORB and E-MORB. Petrogenetic modelling suggests that the observed geochemical variations reflect both mantle source heterogeneity and partial melting processes. The Internal Ophiolitic Sequences geochemistry indicates that their igneous protholites formed in an anorogenic tectonomagmatic environment, like the ocean ridge basins, without any influence of orogenic components related to subduction mechanisms. These features contrast markedly with those reported for the Beja-Acebuches Ophiolitic Complex, which have been interpreted as representing oceanic crust generated in a “back-arc” setting,. Thus, supporting the existence of two distinct oceanic basins (ocean ridge and back-arc) during the geodynamical evolution of the SW Iberian Variscan Chain. Therefore, the Internal Ophiolitic Sequences in the Ossa-Morena Zone reflect the early oceanization events during the Variscan Orogeny

OFIOLITOS E METAMORFISMO DE ALTA PRESSÃO

Artigo de síntese sobre o limite entre as Zonas de Ossa-Morena e Sul Portuguesa e sobre a sua interpretação como uma zona de sutura varisca

Relicts of Rheic Ocean in Southwest Iberia (Ossa-Morena Zone - Portugal)

The geodynamic evolution of Iberian Peninsula during the Variscan Orogeny is directly related with the Wilson Cycle of Rheic Ocean (Vera, 2004; Dias et al., 2006; Ribeiro et al., 2007). This relationship is underlined by major sutures zones and by several occurrences of high pressure metamorphic rocks (i. e. eclogites and blueschits) and ophiolites terranes (scattered slices) in the Iberian Terrane. The Ossa-Morena Zone (OMZ), the southernmost unit of the Iberian Autochthon Terrane in SW Iberia (Fig. 1), is bounded by two major tectonic lines: the Northern boundary which corresponds to the Tomar-Badajoz-Cordoba Shear Zone (TBCSZ), a Cadomian suture reworked in transpression during the Variscan Orogeny, and the Southern boundary, which corresponds to the South Iberian Variscan Suture Zone (Ribeiro et al., 2007). In the OMZ the Proterozoic basement is separate from the lower Cambrian rocks (felsic volcanics and conglomerates) by a major unconformity that corresponds to the beginning of the Variscan Cycle. The rifting stage is marked by the deposition of lower Cambrian limestones and by bimodal extensional magmatism (Mata & Munhá, 1990). The lower Palaeozoic sequence of OMZ shows strong variations of facies, corresponding to the existence of several local sedimentary basins. This feature is related with a near continuous extensional tectonics regime (Rheic opening), until Silurian times. The Silurian sequence denotes a deep sedimentary environment. At this time, Rheic becomes a wide ocean and the passive margin evolution during lower Palaeozoic is confirmed by the sedimentary record and magmatism chemistry (Mata & Munhá, 1990, Ribeiro et al, 1992; 1997). During lower Devonian the sedimentary record show the existence of tectonic instabilities probably related with the beginning of the Rheic oblique closure. This assumption is supported by the existence of (1) local flysch deposits with lower Devonian age, (2) the occurrence of reworked slices of Silurian rocks within lower Devonian formations and (3) the presence of a Tectonic Accretionary Complex which truncates the autochthonous sequence of OMZ, ranging from Precambrian to upper Silurian/Lower Devonian (Araújo et al, 2005; 2006). During middle to upper Devonian occurs the main tectono-metamorphic event related with the Rheic closure in the southern border of OMZ (Quesada et al., 1994; Fonseca et al., 1999): a subduction/obduction process, with northwards polarity, is responsible by the generation of a flake like geometry and by the occurrence of the Tectonic Accretionary Complex, related with the obduction in the Southern Iberia Variscan Suture, which includes slices of (1) autochthonous rocks, (2) ophiolites and (3) high-pressure rocks (blueschists and eclogites). Also related with the subduction/obduction process occurs the emplacement of the Paired Ophiolite Belt. This ophiolite belt is formed by the Beja-Acebuches Ophiolite Complex (BAOC), an external ophiolite that mark the South Iberian Variscan Suture Zone, and by the Internal Ophiolite Sequence (IOS) (Fonseca, 1995; Pedro, 2004; Ribeiro, 2006; Ribeiro et al., 2007) Also in Devonian times the orogenic magmatism starts with the emplacement of the first rocks of Beja Massif and goes on until upper Carboniferous with the emplacement of widespread intrusive rocks

Petrology of spinel peridotite xenoliths from Santo Antão, Cape Verde Islands

Os xenólitos ultramáficos, existentes em lavas basálticas alcalinas/nefeliníticas da Ilha de Santo Antão (Cabo Verde), incluem uma série harzburgítica e uma série dunítica/wehrlítica; a série harzburgítica apresenta características texturais/mineralógicas complexas e é mais refractária (Fo = 90-92) do que a série dos dunitos/wehrlitos (Fo = 82-88). Os xenólitos duníticos e wehrlíticos são, essencialmente, acumulados de olivina e espinela, com clinopiroxena intercumulus. A química mineral e os dados geotermométricos sugerem que os xenólitos duníticos/wehrlíticos cristalizaram a ~ 1000 ºC a partir de magmas alcalinos e acumularam em câmaras magmáticas profundas, sob a ilha de Santo Antão. Os xenólitos harzburgíticos são constituidos por olivina + ortopiroxena + espinela ± clinopiroxena. De acordo com as relações texturais e mineralógicas, os xenólitos harzburgíticos foram divididos em 3 grupos: I - protogranular, II - protogranular metassomatizado e III - porfiroclástico. A evolução térmica complexa registada nestes xenólitos, os elevados valores de fO2 (?FNIQ = 0.7 1.9) e o desenvolvimento de abundantes inclusões fluidas ricas em CO2, são atribuídos à infiltração recente nos harzburgitos de melts, aprisionados/cristalizados no interior do manto; estas características e a natureza refractária da série harzburgítica, apoiam a interpretação de que estes xenólitos representam litosfera oceânica empobrecida, modificada pelo magmatismo associado à génese da Ilha de Santo Antão

Petrology and Geochemistry of Recent Lavas from Santo Antão (Cape Verde Islands)

ABSTRACT: Santo Antão Island is the northernmost island of the Cape Verde Archipelago that is located in the Atlantic Ocean between 17º 13’N-14º 48’N and 22º 42’W-25º 22’W. The genesis of this Archipelago is explained by the activity of a hot-spot, which began 25 My ago. The main stratigraphic sequences of Santo Antão island (Silva et al., 1994; 2004) have been dated by Plesner et al. (2003) and comprise, from base to top, the “Complexo Eruptivo Antigo” (7.57 ± 0.56 My) the “Formação Conglomerático-Brechóide”, the “Formação Eruptiva Principal” (3.24 ± 0.89 to 0.22 ± 0.03 My) (which is overlain by a white phonolitic deposit pumice) and the “Formação Eruptiva do Tope de Coroa” (0.17 ± 0.02 My to 0.09 ± 0.03 My). The “Complexo Eruptivo Antigo” includes chiefly basaltic flows. The “Formação Conglomerático-Brechóide” is a sedimentary unit formed during an intensive erosion period and was followed by an important volcanic event, represented by the “Formação Eruptiva Principal”, comprising nephelinitic, phonolitic, tephritic and basanitic lava flows. The “Formação Eruptiva do Tope de Coroa”, the most recent unit of the island, was divided (Silva et al. 2004) into three sub-units (figure 1): “Sequência Antiga”, “Sequência Intermédia” and “Sequência Superior”. The “Sequência Antiga” is composed by phonolitic and nephelinitic flows and scorias, the “Sequência Intermédia” comprises basaltic flows and scorias and the “Sequência Superior” includes basaltic, phonolitic and nephelinitic flows and scorias. Santo Antão recent lavas are silica undersaturated (carrying abundant feldspathoids, particularly hauyne), alkaline and considerably evolved (Mg#=13-53 wt%, Ni=0-252 ppm, Cr=0-434 ppm). They display a strong enrichment in incompatible trace elements, suggesting that their primary magmas were produced by low degrees of partial melting, from a heterogeneous, metassomatized, source (including residual garnet and phlogopite), with a significant HIMU component.info:eu-repo/semantics/publishedVersio

C-O-H isotopic evidences for fluid sources of granulites in Ribeira Belt, SE Brazil

Publicado em: Geochemica et Cosmochimica Acta, Vol. 72, issue 12, Suppl. 1, A7