Nature et évolution metamorphiques des terrains océaniques en Equateur: Conséquences possibles sur la genèse des magmas adakitiques
Résumé
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.
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.
L'Equateur est localisé le long de la marge nord-ouest du continent sud américain. D'est en ouest, l'Equateur comprend le bassin d'Oriente, les cordillères andines et la bordure côtière pacifique. Les Andes d'Equateur sont formées par deux cordillères séparées par une vallée inter-andine. La Cordillère occidentale et la partie côtière sont composées de terrains océaniques d'âge Crétacé à Eocène. Ces terrains sont constitués de plateaux océaniques et localement d'arc insulaires, qui se sont accrétés à la marge ouest d'Equateur. Plus tard, à l'Eocène supérieur, un arc continental s'est développé, d'abord dans la partie ouest de la marge, puis il a migré vers l'est. Pendant le Miocène moyen à supérieur, plusieurs corps intrusifs se sont mis en place le long la cordillère ouest. A l'Oligocène, des failles kilométriques transpressives dextres se sont développées. Ces failles ont permis l'exhumation des roches magmatiques et métamorphiques. Les roches métamorphiques exhumées sont constituées d'amphibolites, de granulites et métapélites à grenat et quartz. Les roches magmatiques sont des basaltes, dolérites, gabbros, pyroxènites et peridotites. Les roches mafiques, les amphibolites ainsi que les granulites montrent des affinités géochimiques de plateau océanique et d'arc insulaire. Les roches ultramafiques représentent des fragments du manteau appauvri. Toutes ces roches forment la racine de la Cordillère occidental d'Equateur. Les métapélites et les métabasites ont enregistré des histoires termo-barométriques très différentes. Alors que les métabasites sont marquées par un gradient métamorphique BP – HT, les métapélites enregistrent un métamorphisme HP – BT. Les corps intrusifs échantillonnés le long de la Cordillère occidentale peuvent être classés en trois types : intrusifs de nature calco-alcaline, intrusifs d'affinité adakitique et intrusifs adakitiques. La mesure et l'étude des compositions isotopiques (Pb, Nd, Sr) des intrusifs miocènes, couplées à celles des terrains océaniques accrétés à la marge équatorienne ont permis d'apporter des contraintes sur l'origine de ces intrusifs. Les intrusifs calco-alcalins sont probablement issus de la fusion du coin du manteau. En revanche, nous proposons que les terrains océaniques accrétés aient participé à la genèse des intrusifs adakitiques par fusion lors de l'épisode métamorphique BP-HT enregistré par ces roches métabasiques.
Domaines
Géochimie
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