THE CHOIYOI GROUP AT ARROYO MANZANO AREA, MENDOZA PROVINCE, ARGENTINA: A MAGMATIC RETROARC SEQUENCE

Intermediate to acidic volcanic rocks has been recognized at Arroyo Manzano area unconformably deposited over a metasedimentary unit and/or Carboniferous sedimentary rocks. Plutonic bodies of similar composition than the volcanic rocks intruded the previous rocks.

Volcanic rocks have been divided in two sections: a basal andesite-dacite section composed by andesite breccias, andesites, scarce lenses of white tuffs and dacite subvolcanic bodies. A rhyolite to rhyodacite section mainly represented by reddish welded tuffs overlays these rocks. Geological characteristics are consistent with an explosive origin for these rocks.

Petrological and geochemical data of these rocks show calcalkaline affinities and display high K, Rb, Ba, U and Th contents similar to subduccion zone related magmas. Low Hf and Ta contents in relations to Th content, and the high La/Ta ratios (50 to 127) are also consistent with a magmatic arc environment.

Quartz-rich monzonite to granodiorites, granite to granodiorite porphyries, microgranites and rhylolite to quartz-rich trachyte dykes represent the plutonic assemblages. They also show a calcakaline trend and display metaluminous to weakly peraluminous affinities. Most of them have a LILE enrichment probably associated with crustal contamination or continental crustal partial melting. High La/Ta ratios (126-200) suggest a continental arc affinity. Quartz-rich monzonites and granodiorites of Arroyo Manzano show REE pattern, LILE enrichment and no Eu anomaly similar to Cordón del Portillo granites. LREE vs. REE ratios display a comparable behaviour than Montosa Unit in Chile suggesting a free-garnet source and a normal thick crust during the magma generation.

A monzogranite body with arc affinitiy intruded the basement unit developed chilled margins, recrystallized textures and randomly oriented biotite spots associated with a thermal event and suggest a shallow emplacement. This rock has a youngest model age (ca 1,0 Ga) than the country rocks (1,4 to 1,7 Ga) and has been interpreted by Basei et al. (1998) as derived from a different Proterozoic basement than the intruded sequence.

We interpreted these rocks as the basal sequence of the Choiyoi Group in the Cordillera Frontal of Argentina, which are related to the distal retroarc of the subduction zone during the Late Paleozoic times.