The Choiyoi Group (lower-middle Permian) in the Cordillera Frontal of Calingasta. San Juan, Argentina: arc volcanism associated to extension
The southern portion of the Cordillera Frontal of San Juan (western Argentina) comprises some of the best exposures of the Choiyoi Group, the eruptive counterpart of a large silicic plutono-volcanic province of Permian-Triassic age developed on the southwestern margin of Gondwana. Stratigraphic, petrologic, and geochronologic studies allowed to recognize aspects of the physical volcanology, eruptive history, and petrogenetic evolution of this unit. The most common rocks are welded rhyolitic ignimbrites and lava flows. Phreatomagmatic deposits, subvolcanic intrusions and volcaniclastic sediments are less common. The eruptive volumes increased gradually from the emplacement of andesitic flows, rhyolite domes and ignimbrites intercalated between relatively thin sequences of volcaniclastic sediments, to almost uninterrupted successions of voluminous rhyolitic flows and thick ignimbrites related to volcano-tectonic collapse events. The volcanic activity began at ca. 290 Ma following the San Rafael orogenic phase as suggested by previously reported paleontological and radiometric data. A new U-Pb SHRIMP zircon age from the upper levels of the sequence with a value of 265 ± 2 Ma suggests that the duration of volcanism was about 25 m.y. Geochemically the succession shows a restricted compositional spectrum. All units have intermediate to silicic composition (>65% SiO2) with a clear acidification trend toward increasingly younger terms. It corresponds to rhyolites, dacites and traquidacites, mostly of high-K calc-alkaline and peraluminous affinities. Major and trace element contents suggest that magmas were emplaced in a non-thickened crust and evolved mainly through fractional crystallization processes. Trace element patterns show enrichment in large ion lithophile elements (Cs, Ba, Rb, K) in relation to high field strength elements (HFSE) and depressions in Nb, Ta and Ti, typical features of orogenic series. Progressive enrichment in HFSE (Nb, Ta, Y) with age suggests an increase in the within-plate affinity. This compositional change would relate to increasingly voluminous and differentiated magmatic pulses generated as a consequence of the arc system maturation and to a subduction related, progressively extensional tectonic regime. A plausible explanation to the observed variations is the retreating of the subducted slab (slab rollback) in the final stages of Gondwana orogeny.