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Ver el documento (formato PDF)   Lagorio, Silvia Leonor.  "El volcanismo cretácico alcalino de la Sierra Chica de Córdoba: geoquímica, petrogénesis e implicancias geodinámicas"  (2003)
Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires
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Resumen:
Se realizó la caracterización petrológica, geoquimica y petrogenética del volcanismo básico alcalino del Cretácico Inferior de la sierra Chica de Córdoba. Éste forma parte de un complejo volcánico-sedimentario que constituye el relleno de sinrift depositado en hemigrábenes restringidos dentro de una cuenca transtensional, cuya formación está relacionada con el desmembramiento de Gondwana. En las diferentes áreas se reconocieron distintos grupos sobre la base del quimismo de los elementos mayores: ankaratritas, basanitas, suite basalto alcalino —traquifonolita/traquita y suite basalto transicional - latibasalto. El carácter general del volcanismo es potásico a potásico transicional; el grado de alcalinidad es moderado, sin verificarse litotiposperalcalinos. Las características geoquímicas junto a los datos paleomagnéticos de estudios previos para el norte de la sierra de los Cóndores, indican que los niveles lávicos se distribuyeron diacrónicamente a lo largo de toda la sucesión, con variaciones laterales entre las localidades, no resultando posible convalidar los dos ciclos volcánicos propuestos por autores precedentes. El rift tipifica como de baja volcanicidad, con actividad de tipo estromboliana. En las áreas en donde la sedimentación fue más importante se registran niveles brechosos de tipo peperítico. Episodios freatomagmáticos han tenido lugar en forma muy subordinada. Fenómenos de remoción en masa, de tipo debn's flow, han sido en cambio frecuentes. Los contenidos de Al2O3 y las composiciones normativas que presentan los magmas de tipo primario indican que la fusión del manto tuvo lugar a 35-70 km (13-18 kb). Las anomalías negativas para el K Rb obtenidas sugieren posible flogopita o anfíbol como fases residuales en la fuente mantélica portadora de granate. Los magmas de tipo primario de los diferentes grupos se caracterizan además por poseer diversas relaciones Th/La. La/Ta, La/Nb y contenidos de Zr, Hf y LREE. Los porcentajes de fusión (5 - 7%) a partir de un manto fuertemente enriquecido (con flogopita residual), son consistentes con la ubicación periférica respecto a la gran anomalía térmica (sobre la base del Geoide residual) que desencadenó los voluminosos derrames tholeíticos de Paraná. Los afloramientos ubicados al este (faja oriental), con mayor frecuencia de productos transicionales, podrían representar porcentajes ligeramente superiores a los de la faja occidental; no obstante, el comportamiento de algunos elementos traza revela anomalías consistentes con heterogeneidades en la fuente mantélica. Además, un distinto contenido en volátiles (H2O,C02) pudo haber condicionado la formación de magmas con distinto grado de saturación, particularmente en áreas de coexistencia de dichos productos.

Abstract:
A petrological and geochemical study of the Lower Cretaceous basic volcanism of the Sierra Chica of Córdoba has been done. It belongs to a volcanic-sedimentary complex, which defines a synrift fill in small half-grabens within a transtensional basin, due to tectonics which led to Gondwana break-up. In the different areas four groups of volcanic rocks were recognized from the major elements: ankaratrites, basanites, alkaline basalt - trachyphonolite/trachyte suite and transitional basalt - latibasalt suite. The general character of this volcanism is potassic transitional to potassic. Alkalinityis moderate, according to the absence of peralkaline types. Geochemical characteristics, with previous paleomagnetic data. indicate that lavas are distributed diachronically throughout the whole succession of the north of Sierra de los Cóndores, with lateral variations between the localities. So that the distinction of two volcanic cycles as was previously proposed by other authors, is not supported in this work. The Lower Cretaceous basin of Sierra Chica is a low volcanicity rift type, with strombolian activity. Several peperite type breccias were formed where the sedimentation was more important. Freatomagmatic events were very scarce. Instead, viscous mass flows, as debris flows, were frequent in the area. Al2O3 contents and normative compositions of the primary type magmas point out that melting took place under 35-70 km (13-18 kb). Negative anomalies in K and Rb suggest phlogopite or amphibole as a residual phase in the garnet bearing mantle source. Primary type magmas of the different groups also have diverse Th/La, La/Ta and La/Nb ratios and Zr, Hf and LREE contents. Melting degrees (5 - 7%) from an enriched mantle source (with residual phlogopite) are in accordance with the peripherical position related to the great thermal anomaly (from the residual Geoid) which triggered the voluminous flood tholeiites of Parana. Eastern outcropts of Sierra Chica, of more frequently transitional character, could represent slightly higher melting degrees than the western ones; nevertheless, certain anomalies in trace element compositions point out heterogeneities in the mantle source. Also, a diverse volatile content (H2O,C02) could have favoured magmas with distinct silica saturation, particularly where coexistence of those products has taken place. Trace element diagrams suggest that the more evolved rocks (volumetrically subordinated) were not genetically linked to the primitive magmas by fractional crystallization, so that they must have evolved from diverse parental ones. Likewise, textures involving resorption and reaction with the liquid showed by some clinopyroxene and plagioclase phenocrysts, specially from the more evolved rocks, and the slight but rather general increase in the Mg contents of clinopyroxene from the rim of the phenocrysts to the microlites of the groundmass, indicate frequent recharge with more primitive magmas. This also must have promoted mixing between diverse batches, so that an open-system behaviour is characterized, in which crustal contamination must have also taken part, though it does not seem to have been significant from the geochemical data. Differentiation processes would have taken place under low pressure, in accordance with Al^VIof the clinopyroxene phenocrysts. From the comparison with other alkaline localities also peripherical respect to Parana flood basalts, the Sierra Chica volcanism is coeval and lithologically similar to that of central-eastern Paraguay, also of potassic character. Nevertheless, while basic rocks from Córdoba are high-Ti, those from Paraguay are low-Ti and show striking Ta-Nb-Ti anomalies. The magmas of Sierra Chica are comparable hence to the younger ones from southeastern Brazil, for example those from Alto Paranaíba, though these have higher LILE, LREE contents and (La/Yb)n ratios, consistent with the ultrapotassic rocks that prevail in that area. Chemical similarities reflect therefore analogies between both sources, that could be due to resemblances in the metasomatic events. While low-Ti magmas reflect a metasomatic event during the Late Archean to the Middle Proterozoic in the Rio de la Plata craton, high-Ti ones represent a mantle enrichment episode that occurred in Middle to Late Proterozoic related to mobile belts, according to the model ages obtained by other authors. Hence, it seems probable that mantle beneth Sierra Chica, as that of Alto Paranalba, must have verified metasomatism during Brasiliano orogeny. The presence of low-Ti flood basalts and low-Ti potassic magmas from Paraguay, to the east and northeast of Córdoba, seems therefore to reflect the transition from the oldest to the younger metasomatic events, consistent with terrane accretion in the western margin of Gondwana. Tectonic models geologically supported by the presence of Cambrian calcalkaline granitos in the region, do no correlate with Cretaceous alkaline Chemistry. Ba/Nb vs. La/Nb, Ta/Yb vs. Th/Yb, Ba/La ratios and the absence of a Nb-Ta depletion on a normalized multielement plot do not reveal metasomatism by fluids or melts derived from a downgoing slab in the past. It seems probable that melting must have affected portions of the mantle that were not contaminated by the subduction processes. This is consistent with the heterogeneous nature of the lithospheric mantle, whose non-convective geodynamics, would have preserved the heterogeneities over long periods of the geological time.

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Registro:
Título : El volcanismo cretácico alcalino de la Sierra Chica de Córdoba: geoquímica, petrogénesis e implicancias geodinámicas    
Autor : Lagorio, Silvia Leonor
Director : Remesal, Marcela Beatriz
Año : 2003
Editor : Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires
Filiación : Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales
Departamento de Geología
Grado obtenido : Doctor en Ciencias Geológicas
Ubicación : Preservación - http://digital.bl.fcen.uba.ar/gsdl-282/cgi-bin/library.cgi?a=d&c=tesis&d=Tesis_3634_Lagorio
Idioma : Español
Area Temática : Geología / Geoquímica
Geología / Petrología
Palabras claves : CORDOBA; SIERRA CHICA; CRETACICO; BASALTOS ALCALINOS; GEOQUIMICA; CRETACEOUS; ALKALINE BASALTS; GEOCHEMISTRY; CORDOBA; SIERRA CHICA
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