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Nature et évolution metamorphiques des terrains océaniques en Equateur: Conséquences possibles sur la genèse des magmas adakitiques

Abstract : Ecuador is located along the north-western margin of South American. From east to west, comprise three principal geological domains: 1. The Orient basin: represent the foreland basin of the Andean orogeny. 2. The Andean Cordillera: consist of two cordillera separated by inter Andean valley; the orient (arcs magmatic) and western (exotic terrains) cordillera. 3. The coastal zone: is made of accreted oceanic terranes.
The geology of Ecuador is very different from that of the Cordillera Andes, by the presence of accreted oceanic terranes. The build-up of the Andean range is linked to the subduction of the Pacific lithosphere beneath the South American plate since the lower Jurassic. The processes of the Andean orogeny change from south to the north of the range. While continental crustal shortening predominates in the central Andes, accretion and under-plating of exotic oceanic terranes (oceanic plateaus and island arcs) occurred in the north and likely form the crustal root of the northern Andes today.
Several crustal fragments of oceanic plateaus overlain by island arcs accreted to the passive margin of Ecuador between ~ 80 and 40 Ma and presently crop out in the Western Cordillera. Later, from the Late Eocene - Early Oligocene, a continent-based arc developed first in the Western Cordillera and then, farther east. Since the Oligocene, dextral transpressive kilometer-scale-faults affected the accreted oceanic terranes and provoked the exhumation of magmatic and metamorphic rocks.
This study aims to determine the origin and thermodynamic evolution of these exhumed metabasites
The Western Cordillera from Ecuador consists of Cretaceous crustal fragments of oceanic plateaus and associated insular arcs accreted to the north-western South American margin during the Late Cretaceous and Paleocene. Slices of amphibolites, granulites, garnet meta-sediments, lherzolites, pyroxenites, harzburgites, gabbros and basalts are exposed along Miocene to Recent transcurrent faults. The tonalite rocks are present too. This intrusive shows adakite melts characteristics.
Foliated amphibolites are formed of Mg-rich hornblende + bytownite + magnetite ± quartz. Their major and trace element chemistry is similar to that of oceanic plateau basalts (flat REE patterns, La/Nb = 0.86) or cumulate gabbros. Granulites are formed of Ca-rich plagioclase (An55-75) + enstatite + diopside + quartz and share with oceanic plateaus similar trace element chemistry (flat REE patterns, La/Nb < 1) and ΕNdi values (+7.6). Garnet meta-sediments are formed of chlorite + quartz + muscovite + garnet. Their major traces present depletion of heavy REE (La/Y = 4.8) and negative anomaly of Eu.
The foliated lherzolites and clinopyroxenites consist of serpentinized olivine + cpx + opx ± Ca-plagioclase. The trace element abundances of the ultramafic rocks are very low (0.1 to 1 times the chondritic and primitive mantle values). Lherzolites and clinopyroxenites are LREE depleted with positive Eu anomalies while the harzburgite displays an U-shaped REE pattern. The geochemical features of the ultramafic rocks are similar to those of depleted mantle. The basalts (Cpx, plagioclase, Ti-magnetite) show geochemical characteristics of oceanic plateau basalts (flat REE patterns La/Nb = 0.85). The gabbros (Ca-rich plagioclase + enstatite + diopside) differ from the basalts by lower REE levels, positive Eu anomalies and Nb and Ta marked negative anomalies.
The acid intrusives are formed of zoned phenocryst plagioclase and amphibole. The matrix is very silicic. The REE pattern is very depleted (La/Y = 7.6) and the relationship between Sr and Y is very high (Sr/Y = 72.8). This
indicates adakite melts
According to the equilibrated mineralogical phases and also to the preliminary thermobarometic results rocks, are deformed in between 630° to 850° at relatively low pressure (6-9 kbars). Thus, these rocks likely represent the metamorphosed remnants of the accreted oceanic crustal fragments and associated depleted mantle that form presently the roots of the Ecuadorian Andean Ranges. Also, the presence of Miocene tonalite with adakite features supports the hypothesis that the partial melting of oceanic fragments accreted and under-plated plays an import role in the genesis of adakites. This constitutes an alternative to the model that attributes the origin of recent adakites of Ecuador to a flat subduction or to the ridge of Carnegie fusion.
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Contributor : Andrea Ekaterina Amortegui Vera <>
Submitted on : Thursday, October 4, 2007 - 2:40:44 PM
Last modification on : Thursday, November 19, 2020 - 3:54:26 PM
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Andrea Amortegui Vera. Nature et évolution metamorphiques des terrains océaniques en Equateur: Conséquences possibles sur la genèse des magmas adakitiques. Géochimie. Université Joseph-Fourier - Grenoble I, 2007. Français. ⟨tel-00176720⟩

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