Allochthonous terranes in northwestern Ecuador

Thournout, F. Van; Hertogen, Jan; Quevedo, L.

doi:10.1016/0040-1951(92)90427-8

Cited by 30 publications

(20 citation statements)

References 23 publications

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“…The geochemical data suggest that footwall volcanic rocks, hangingwall volcanic rocks, and dolerite were not cogenetic and suggest that they were emplaced in a primitive arc environment (transitional to tholeiitic), possibly during a rifting stage (indicated by the MORB signature of the dolerite). This is in agreement with the island-arc environment of the Macuchi unit suggested by previous studies (e.g., van Thournout et al, 1992;BGS and CODIGEM, 1999;Chiaradia and Fontboté, 2001). …”

Section: Massive Jasper (Tri-41) B Laminated Jasper (Tri-12) C supporting

confidence: 93%

“…However, the weakly altered hanging-wall basalts and basaltic-andesitic flows have arc-tholeiitic affinities, whereas hanging-wall dolerite samples were derived from a MORB-type source. In agreement with previous studies on the Macuchi unit (e.g., van Thournout et al, 1992;BGS and CODIGEM, 1999;Chiaradia and Fontboté, 2001), these data suggest that the La Plata deposit was formed in a primitive island arc environment, possibly prior to or during an intra-arc rift event.…”

Section: Chiaradia Et Alsupporting

confidence: 92%

“…Various geodynamic models for the Western Cordillera of Ecuador have proposed the accretion of several oceanic terranes and multiple subduction events that produced islandarc sequences during the Early Tertiary (e.g., van Thournout et al, 1992;Jaillard and Soler, 1996;Jaillard et al, 1997;Reynaud et al, 1999;Chiaradia and Fontboté, 2001;Hughes and Pilatasig, 2002).…”

Section: Regional Geologic Settingmentioning

confidence: 99%

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Geologic Setting, Mineralogy, and Geochemistry of the Early Tertiary Au-Rich Volcanic-Hosted Massive Sulfide Deposit of La Plata, Western Cordillera, Ecuador

Chiaradia¹,

Tripodi²,

Fontboté³

et al. 2008

Economic Geology

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La Plata is, economically, the most important of several volcanic-hosted massive sulfide (VHMS) deposits of the Western Cordillera of Ecuador, with preliminary estimated reserves of 840,000 metric tons (t) at 4.8 g/t Au, 54 g/t Ag, 4.1 percent Cu, 0.7 percent Pb, and 4.2 percent Zn. Like the other VHMS deposits of the Western Cordillera, it is hosted by the Early Tertiary Macuchi unit, which represents an oceanic island-arc sequence, composed of submarine volcanic and volcaniclastic sequences of basaltic to andesitic compositions, accreted to the continent along the Chimbo-Toachi shear zone in the late Eocene.La Plata displays a typical VHMS vertical zonation, from bottom to top, of a pyrite stockwork, disseminated sulfides, massive sulfide lenses, and a thin red jasper bed with exhalative-sedimentary textures. The massive sulfide lenses are located within footwall andesitic volcanic and volcaniclastic rocks that are pervasively altered to a quartz-pyrite-sericite assemblage, and immediately below a hanging-wall sequence consisting of at least two basaltic to basaltic-andesitic lava flows, each capped by autoclastic breccia intervals. The hanging-wall rocks, especially the autoclastic breccias, are hematite altered up to 300 m above the ore level. Hanging-wall volcanic rocks have geochemical features typical of arc tholeiites. Dolerite dikes in the hanging wall have middle-ocean ridge basalt (MORB)-type geochemical features likely indicating deposit formation in an intra-arc extensional environment.The massive sulfide lenses are situated mainly on the eastern side of an antiform with footwall rocks at the core and hanging-wall rocks on both flanks. Lithologic contacts, volcanic strata, and axial planes of folds have a consistent north-south strike. Footwall rocks at the core of the antiform are locally affected by meter-thick, north-south-trending subvertical shear bands that result in an intense tectonic foliation. Deformation structures are absent in the hanging-wall rocks suggesting that the footwall rocks, due to a different rheology, were ductile during deformation. Centimeter-to decimeter-sized massive sulfide clasts are incorporated into a mylonitic matrix of altered footwall rocks within these shear bands that contain dextral movement indicators. The larger massive sulfide lenses (up to 100 m long) probably result also from tectonic dismembering, within the footwall rocks, of a former, larger, massive sulfide body. The deformation is probably related to the ChimboToachi dextral shear zone situated a few kilometers to the east of La Plata.The main ore minerals are pyrite, chalcopyrite, sphalerite, bornite, and galena, with minor tennantite and digenite. The gangue minerals are barite, quartz, and sericite. Mineralogical banding, defined by chalcopyriterich layers alternating with pyrite-and/or sphalerite-rich layers, is mostly the result of tectonic injection of ductile chalcopyrite, bornite, and galena within shear bands of cataclastic pyrite and sphalerite. Pyrite and sphalerite also show mild r...

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Section: Massive Jasper (Tri-41) B Laminated Jasper (Tri-12) C supporting

confidence: 93%

Section: Chiaradia Et Alsupporting

confidence: 92%

Section: Regional Geologic Settingmentioning

confidence: 99%

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Geologic Setting, Mineralogy, and Geochemistry of the Early Tertiary Au-Rich Volcanic-Hosted Massive Sulfide Deposit of La Plata, Western Cordillera, Ecuador

Chiaradia¹,

Tripodi²,

Fontboté³

et al. 2008

Economic Geology

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“…This hypothesis also is not satisfactory considering that most of the basement from the western Cordillera is composed of MOR basalts of the Piñon Formation, and calc-alkaline rocks of the Macuchi Formation [Van Thournout et al, 1992]. Both types of rocks are characterized by high concentrations in HREE and Yttrium (ex : Y > 28 ppm).…”

Section: Interaction With the Lower Continental Crust ?mentioning

confidence: 99%

Slab melting and slab melt metasomatism in the Northern Andean Volcanic Zone : adakites and high-Mg andesites from Pichincha volcano (Ecuador)

Bourdon

Eissen²,

Gutscher

et al. 2002

Bulletin De La Société Géologique De France

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Abstract. -Situated in the fore-arc of the Northern Volcanic Zone (NVZ) of the Andes in Ecuador, Pichincha volcano is an active edifice where have been erupted unusual magmas as adakites and high-Mg andesites. The particular geodynamic setting of the ecuadorian margin (i.e. the flat subduction of the Carnegie Ridge) suggests that thermo-barometric conditions for the partial melting of the oceanic crust are accomplished beneath this volcano. Pichincha adakites possess all the geochemical and isotopic characteristics of slab melts described in various other arc settings. High-Mg andesites with geochemical characteristics close to those of adakites present strong enrichments in MgO that suggest that, once they were produced by ca. 10 % partial melting of the downgoing subducted slab, some adakites en route to the surface strongly interacted with the peridotitic mantle wedge. Adakitic magmas could then represent, as in many other arcs where slab melting occurs, the principal metasomatic agent of the mantle in the NVZ in Ecuador. Résumé. -Situé dans la zone volcanique nord des Andes en Equateur, le volcan Pichincha est un édifice actif entré ré-cemment dans un nouveau cycle éruptif. La position exacte de la plaque plongeante sous l'Equateur est difficile à dé-terminer en raison de l'absence de sismicité intermédiaire dans la zone de subduction. Gutscher et al. Les adakites et les andésites riches en[1999] ont cependant récemment proposé qu'il puisse exister sous la marge une portion de plaque horizontale portée par la ride de Carnegie de faible densité qui est actuellement en subduction. D'autre part, Gutscher et al.[2000] ont observé à l'échelle du globe une relation étroite entre les portions de plaque subductée horizontale et la production de magmas adakitiques. Cette association suggère que l'horizontalisation de la plaque subductante permet une modification importante de la structure thermique du coin de manteau et le maintien de la lithosphère subductante dans les conditions de pression et de température compatibles avec la fusion de la croûte océanique. Un échantillonnage du volcan Pichincha a permis de mettre en évidence que les produits émis sont constitués d'andésites et de dacites moyennement potassiques possédant toutes les caractéristiques des adakites (c'est-à-dire des produits de fusion directe de la croûte océanique) telles que des enrichissements en Na 2 O, de fortes teneurs en Sr, mais de faibles concentrations en yttrium et en terres rares lourdes. Par ailleurs, on trouve parmi ces laves des andésites magnésiennes possédant à la fois les caractéristiques des adakites mais également des concentrations en magnésium éle-vées. Minéralogiquement, les laves du Pichincha contiennent essentiellement des cristaux de plagioclase, d'amphibole, d'orthopyroxène, de clinopyroxène et de magnétite. L'amphibole est généralement absente dans les andésites magné-siennes où le pyroxène devient le minéral dominant.Une modélisation géochimique montre qu'il est parfaitement possible d'expliquer la formation des adakites ...

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“…Ecuador consists of several terranes, both with continental and oceanic affinity, which were accreted onto the Amazon craton from the Early Cretaceous to the Eocene (Feininger 1987;Mourier et al 1988;Aspden and Litherland 1992;van Thournout et al 1992;Litherland et al 1994;Jaillard and Soler 1996;Jaillard et al 1997;Reynaud et al 1999;Hughes and Pilatasig 2002;Mamberti et al 2003). From east to west the following domains are recognized ( Fig.…”

Section: Geological Setting Geodynamic Evolution and Magmatismmentioning

confidence: 99%

Cenozoic continental arc magmatism and associated mineralization in Ecuador

Chiaradia

Fontboté

Beate

2004

Mineralium Deposita

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Most of the economic ore deposits of Ecuador are porphyry-Cu and epithermal style gold deposits associated with Tertiary continental arc magmatism. This study presents major and trace element geochemistry, as well as radiogenic isotope (Pb, Sr) signatures, of continental arc magmatic rocks of Ecuador of Eocene to Late Miocene ($50-9 Ma, ELM) and Late Miocene to Recent ($8-0 Ma, LMR) ages. The most primitive ELM and LMR rocks analyzed consistently display similar trace element and isotopic signatures suggesting a common origin, most likely an enriched MORB-type mantle. In contrast, major and trace element geochemistry, as well as radiogenic isotope systematics of the whole sets of ELM and LMR samples, indicate strikingly different evolutions between ELM and LMR rocks. The ELM rocks have consistently low Sr/Y, increasing Rb/Sr, and decreasing Eu/Gd with SiO 2 , suggesting an evolution through plagioclase-dominated fractional crystallization at shallow crustal levels (<20 km). The LMR rocks display features of adakite-type magmas (high Sr/Y, low Yb, low Rb/Sr) and increasing Eu/Gd and Gd/Lu ratios with SiO 2 . We explain the adakitetype geochemistry of LMR rocks, rather than by slab melting, by a model in which mantle-derived melts partially melt and assimilate residual garnet-bearing mafic lithologies at deeper levels than those of plagioclase stability (i.e., >20 km), and most likely at sub-crustal levels (>40-50 km). The change in geochemical signatures of Tertiary magmatic rocks of Ecuador from the ELM-to the LMR-type coincides chronologically with the transition from a transpressional to a compressional regime that occurred at $9 Ma and has been attributed by other investigations to the onset of subduction of the aseismic Carnegie ridge.The major districts of porphyry-Cu and epithermal deposits of Ecuador (which have a small size, <<200 Mt, when compared to their Central Andean counterparts) are spatially and temporally associated with ELM magmatic rocks. No significant porphyry-Cu and epithermal deposits (except the epithermal highsulfidation mineralization of Quimsacocha) appear to be associated with Late Miocene-Recent (LMR, $8-0 Ma) magmatic rocks. The apparent ''infertility'' of LMR magmas seems to be at odds with the association of major porphyry-Cu/epithermal deposits of the Central Andes with magmatic rocks having adakite-type geochemical signatures similar to LMR rocks. The paucity of porphyry-Cu/epithermal deposits associated with LMR rocks might be only apparent and bound to exposure level, or real and bound (among other possibilities) to the lack of development of shallow crustal magmatic chambers since $9 Ma as a result of a prolonged compressional regime in the Ecuadorian crust. More work is needed to understand the actual metallogenic potential of LMR rocks in Ecuador.

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Allochthonous terranes in northwestern Ecuador

Cited by 30 publications

References 23 publications

Geologic Setting, Mineralogy, and Geochemistry of the Early Tertiary Au-Rich Volcanic-Hosted Massive Sulfide Deposit of La Plata, Western Cordillera, Ecuador

Geologic Setting, Mineralogy, and Geochemistry of the Early Tertiary Au-Rich Volcanic-Hosted Massive Sulfide Deposit of La Plata, Western Cordillera, Ecuador

Slab melting and slab melt metasomatism in the Northern Andean Volcanic Zone : adakites and high-Mg andesites from Pichincha volcano (Ecuador)

Cenozoic continental arc magmatism and associated mineralization in Ecuador

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