Major-trace elements and Sr-Nd-Pb-B isotopes were measured in samples from a collecton of rocks erupted by El Reventador, Sumaco and Cerro Negro volcanoes, located in the rear arc zone of the Ecuadorian Andes (Northern Volcanic Zone NVZ). Despite being located at the same distance from the trench (~350 km), and only ~30 km far from each other, magmas erupted from these three volcanoes (NS aligned), exhibit different major-trace element compositons and isotopic signatures. The products of El Reventador actve volcano (the northernmost one) are mainly represented by porphyritc lavas, ranging from basalt to rhyolite, with basaltcandesite and andesite being the most abundant. They belong to the medium-K to high-K calcalkaline series, characterized by higher 87Sr/86Sr and lower 143Nd/144Nd with respect to those of Sumaco (the southernmost one) but well within the Sr-Nd isotopic range of the Ecuadorian NVZ [Ancellin et al., 2017; Gutscher et al., 1999]. Extrusives of the actve to semiactive Sumaco volcano consist of high-K alkaline, markedly SiO2-undersaturated (ofen with modal feldspathoids) rocks ranging from basanite to phonolite. The overall geochemistry of the samples is very different from that observed for most volcanoes of the NVZ. The narrow range of Sr-Nd isotope compositons and the major and trace elements distributons indicate that, besides relatvely low-degree of partal meltng of the mantle source, the Sumaco magmas differentated mainly through crystal fractionation with probably small contributon of crustal contaminaton, in accordance with the thick (>50 km) and old (Precambrian) contnental lithosphere beneath the volcanic edifice. Cerro Negro (a semidormant to dormant volcano), geographically located between the Sumaco and El Reventador is characterized by trachybasalts to trachydacites of the high-K calcalkaline series and shows geochemical features which partally overlap those of the other two volcanoes. The Sr-Nd isotope ratios show a quite large variation range (i.e. 87Sr/86Sr =0.7053-0.7070 and 143Nd/144Nd = 0.5124-0.5128) and an anomalous negative correlation. Although El Reventador, Cerro Negro and Sumaco volcanic products show the typical Nb-Ta-Ti negatve anomaly as the result of the common subduction-related environment, they exhibit completely different modal mineralogy and trace element geochemistry. Small enrichment in incompatible trace elements with silica increase are typical of El Reventador lavas, especially for LREE (e.g. La, Ce). By contrast, the Sumaco lavas are strongly enriched in all incompatible elements and show very low values of LILE/HFSE and LILE/REE ratos (e.g. Ba/Nb and Ba/La). Products of Cerro Negro distinctly cover the compositonal gap which is present between El Reventador and Sumaco for LREEs abundances and LILE/HFSE and LILE/REE ratios. The three investgated Ecuadorian volcanoes grew upon the same basement rocks and over a mantle wedge having the same characters. Different geochemical and isotopic footprints might be therefore driven by the presence of the southern limit of the flat-slab Carnegie ridge (i.e. the Grijalva Fault Zone) in the southernmost area of the three investigated volcanoes (close to Sumaco) that can produce a lithosphere tearing between the younger (Nazca) and the older (Farallon) subducting lithospheric plate. In fact, differences in the geometry and/or age of the subducted slab may be responsible for heterogeneous kind of fluids and melts which can induce different extent of metasomatism in the mantle wedge where the basic magmas of the active continental margin originate. The Sumaco basic magmas are consistent with very low degrees of partal melting due to the metasomatism of a supraslab mantle wedge by small amounts of 11B-depleted fluids released from subducted altered oceanic crust and sediments. In contrast, under the El Reventador volcano a carbonatic sediment component (with low 206Pb/204Pb and high d11B) is invoked as significant agent of mantle wedge metasomatsm. Cerro Negro (Pb and B isotopic analyses are ongoing) which is just located in a transitonal environment could represents the trait d’union between the two other volcanoes.
Different geochemical and isotopic footprints of magmas erupted in the rear arc zone of Ecuador characterized by slab segmentation: insights from El Reventador, Sumaco and Cerro Negro volcanoes
Marco Taussi
;Michele Mattioli;Matteo Puerini;Filippo Ridolfi;Andrea Gorini;Alberto Renzulli
2020
Abstract
Major-trace elements and Sr-Nd-Pb-B isotopes were measured in samples from a collecton of rocks erupted by El Reventador, Sumaco and Cerro Negro volcanoes, located in the rear arc zone of the Ecuadorian Andes (Northern Volcanic Zone NVZ). Despite being located at the same distance from the trench (~350 km), and only ~30 km far from each other, magmas erupted from these three volcanoes (NS aligned), exhibit different major-trace element compositons and isotopic signatures. The products of El Reventador actve volcano (the northernmost one) are mainly represented by porphyritc lavas, ranging from basalt to rhyolite, with basaltcandesite and andesite being the most abundant. They belong to the medium-K to high-K calcalkaline series, characterized by higher 87Sr/86Sr and lower 143Nd/144Nd with respect to those of Sumaco (the southernmost one) but well within the Sr-Nd isotopic range of the Ecuadorian NVZ [Ancellin et al., 2017; Gutscher et al., 1999]. Extrusives of the actve to semiactive Sumaco volcano consist of high-K alkaline, markedly SiO2-undersaturated (ofen with modal feldspathoids) rocks ranging from basanite to phonolite. The overall geochemistry of the samples is very different from that observed for most volcanoes of the NVZ. The narrow range of Sr-Nd isotope compositons and the major and trace elements distributons indicate that, besides relatvely low-degree of partal meltng of the mantle source, the Sumaco magmas differentated mainly through crystal fractionation with probably small contributon of crustal contaminaton, in accordance with the thick (>50 km) and old (Precambrian) contnental lithosphere beneath the volcanic edifice. Cerro Negro (a semidormant to dormant volcano), geographically located between the Sumaco and El Reventador is characterized by trachybasalts to trachydacites of the high-K calcalkaline series and shows geochemical features which partally overlap those of the other two volcanoes. The Sr-Nd isotope ratios show a quite large variation range (i.e. 87Sr/86Sr =0.7053-0.7070 and 143Nd/144Nd = 0.5124-0.5128) and an anomalous negative correlation. Although El Reventador, Cerro Negro and Sumaco volcanic products show the typical Nb-Ta-Ti negatve anomaly as the result of the common subduction-related environment, they exhibit completely different modal mineralogy and trace element geochemistry. Small enrichment in incompatible trace elements with silica increase are typical of El Reventador lavas, especially for LREE (e.g. La, Ce). By contrast, the Sumaco lavas are strongly enriched in all incompatible elements and show very low values of LILE/HFSE and LILE/REE ratos (e.g. Ba/Nb and Ba/La). Products of Cerro Negro distinctly cover the compositonal gap which is present between El Reventador and Sumaco for LREEs abundances and LILE/HFSE and LILE/REE ratios. The three investgated Ecuadorian volcanoes grew upon the same basement rocks and over a mantle wedge having the same characters. Different geochemical and isotopic footprints might be therefore driven by the presence of the southern limit of the flat-slab Carnegie ridge (i.e. the Grijalva Fault Zone) in the southernmost area of the three investigated volcanoes (close to Sumaco) that can produce a lithosphere tearing between the younger (Nazca) and the older (Farallon) subducting lithospheric plate. In fact, differences in the geometry and/or age of the subducted slab may be responsible for heterogeneous kind of fluids and melts which can induce different extent of metasomatism in the mantle wedge where the basic magmas of the active continental margin originate. The Sumaco basic magmas are consistent with very low degrees of partal melting due to the metasomatism of a supraslab mantle wedge by small amounts of 11B-depleted fluids released from subducted altered oceanic crust and sediments. In contrast, under the El Reventador volcano a carbonatic sediment component (with low 206Pb/204Pb and high d11B) is invoked as significant agent of mantle wedge metasomatsm. Cerro Negro (Pb and B isotopic analyses are ongoing) which is just located in a transitonal environment could represents the trait d’union between the two other volcanoes.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
