Abstract
The Neuquen Basin constitutes one of the most conspicuous features of the Andean margin of Gondwana during Mesozoic times. The initial syn-rift infill (Late Triassic-Early Jurassic) of this ensialic extensional basin involved a profuse magmatic activity (Precuyano Cycle). Outcrops of the southern depocentres of the basin (Chachil, Catan Lil, Chacaico, Sanico y Piedra del aguila) were studied in order to analyze the characteristics of this magmatic cycle. Shallow intrusive, lava flow and pyroclastic accumulation units of the syn-rift sequence conform up to 70% of the total infill. The syn-rift sequence shows a particular spatial-temporal evolution from mainly intermediate volcanic rocks at the base to mainly acidic pyroclastic rocks to the top. Volcanic feeding-systems were characterized by shallow and multiple-injection magmatic chambers. Eight main alteration facies related to syn- and post-volcanic stages were determined. Trace element data show that the lava and pyroclastic products share a co-magmatic trend, dominated by intermediate to acid products. Rocks belong to the sub-alkaline series, mostly calc-alkaline and transitional, and in a lesser extent tholeiitic, with orogenic signature. Nevertheless, the stratigraphical and compositional features (>50% of acid volcanic products) suggest some differences with typical arc-series. Recent models to explain the magmatic systems in complex convergent margins were discussed (i.e., intra-crustal and extra-crustal models), being chosen an extra-crustal model to justify the compositional and stratigraphical features of the Precuyano Cycle. According to this model the partial melting of an underplating ‘silicic cold plume’ (metasediment-oceanic crust melanges derived from subducted slab) would explain the wide distribution transversal to the margin of the Precuyano volcanism, whereas the degree of partial melting of the ‘plume’ would explain the prevailing composition of the cycle: andesitic and daciticrhyodacitic compositions. The onset of extension made possible the rising of the initial andesitic volcanism, whereas during either the rifting climax or along the sites with major crustal extension, the high viscosity acid magmas would have been preferentially risen. Finally, an Andean segmentation at 40°S would explain the development of an extensional segment towards the north, where the Neuquen basin was developed, and the magmatic arc towards the south, represented by the Subcordilleran Batholith.
Highlights
La evolución tectónica del sector andino norpatagónico durante el intervalo Pérmico-Jurásico Inferior ocurrió dentro de un escenario complejo en el que se registran varios procesos yuxtapuestos y relacionados entre sí
Recent models to explain the magmatic systems in complex convergent margins were discussed, being chosen an extra-crustal model to justify the compositional and stratigraphical features of the Precuyano Cycle
According to this model the partial melting of an underplating ‘silicic cold plume’ would explain the wide distribution transversal to the margin of the Precuyano volcanism, whereas the degree of partial melting of the ‘plume’ would explain the prevailing composition of the cycle: andesitic and daciticrhyodacitic compositions
Summary
La evolución tectónica del sector andino norpatagónico durante el intervalo Pérmico-Jurásico Inferior ocurrió dentro de un escenario complejo en el que se registran varios procesos yuxtapuestos y relacionados entre sí. La etapa de sin-rift de la cuenca constituye el mayor registro estratigráfico derivado del marco tectónico asociado al margen occidental de Gondwana en tiempos mesozoicos. 2008; Schiuma y Llambías, 2008; Pángaro et al, 2009; Muravchik et al, 2011; D’Elia et al, 2012), las características del volcanismo asociado a la etapa de sin-rift de la cuenca no han sido sometidas a un análisis integrado. El objetivo de este trabajo apunta a una caracterización estratigráfica, litológica y geoquímica del volcanismo de la etapa inicial de la Cuenca Neuquina a partir del estudio y muestreo de las secuencias aflorantes en los depocentros australes (i.e., depocentros de Chachil, Catán Lil, Chacaico, Sañicó y Piedra del Águila; Fig. 1), en conjunto con el análisis de los controles sobre los sistemas de alteración, el sistema magmático y su vinculación con el ambiente geodinámico. El estudio de esta secuencia Triásico-Jurásica dentro de la cuenca adquiere un valor fundamental, ya que posibilita el análisis de un ciclo magmático desarrollado en un contexto extensional que abarca cientos de kilómetros en sentido paralelo y transversal al margen Andino de Gondwana
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