Syn-rift volcanism of the Neuquén Basin, Argentina ... - Sernageomin

Abstract
Syn-rift volcanism of the Neuquén Basin, Argentina: relationships with the
Late Triassic – Early Jurassic evolution of the Andean margin. The Neuquén
Basin constitutes one of the most conspicuous features of the Andean margin of
Gondwana during Mesozoic times. The initial syn-rift infill (Upper Triassic—Lower
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, Sañicó 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 volcanic 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
Preliminar
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 a “silicic cold