Volcanismo de sin-rift de la Cuenca Neuquina, Argentina: relación con la evolución Triásico Tardía-Jurásico Temprano del margen Andino

the neuquén 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, catán lil, chacaico, sa？icó y piedra del águila) 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 mélanges 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 dacitic-rhyodacitic compo