MAL PAÍS FORMATION: AN EVIDENCE OF SHALLOW SUBDUCTION DURING PERMIAN TIMES IN MENDOZA PROVINCE, ARGENTINA: A COMPARISON WITH EQUIVALENT MIOCENE ROCKS

Mal País Fm is integrated dominantly by brownish red volcanic rocks. It crop out in a NNE-SSW 42 km belt of 4 km average width with 100 to more than 1100 m thickness in the Precordillera, from 32°36xS 69°08xW to 32°57xS 69°11xW.

They are andesitic and dacitic lava flows and porphyries, ignimbrites, tuffs and volcanic debris flows, deposited unconformably over Lower Paleozoic marine metasediments and covered in the same way by Triassic continental rocks. A conglomerate is sometimes present at the base.

Andesites and dacites present typical characteristics of calc-alkaline series, with plagioclase phenocrysts, with amphibole ± biotite ± orthopyroxene ± clinopyroxene ±olivine (xenocrystals) as characteristic accessory minerals.

Volcanic levels mostly dip to the East, but at the northern outcrops, exhumed near horizontal partly eroded estratovolcanes associated to highly eroded necks or domes are recognized.

K/Ar determinations, as 280 ±14 My in biotite assign them a Lower Permian age. Trace contents confirm their linking with active continental margins, as Ta, Nb and Ti depressions in the common normalized diagrams and Th-Hf/3-Ta relationships. La/Ta ratios (48-81) are similar to those of Miocene volcanic rocks of the same region, indicative of arc environment more than of backarc. Some slight differences are: higher La/Sm ratios with La/Yb in the range of the biggest Miocene values, suggesting a thicker crust. Higher Th/Ta (15-18 vs 5-15), while Th/La (0.23-0.32) match and slightly overcomes the biggest Miocene values, associated to larger SiO2 contents, suggest important cortical participation in Permian magmas. It is supported by high Ba contents, since barite Ordovician deposits are known in the basement. On the other hand, olivine xenocrysts are indicatives of magma mixtures, a common mecanism of differentiation in arc magmas.

In conclusion, Mal Pais Fm represents the eastern outcrops of a stratovolcanoes chain related to a Lower Permian continental arc. This arc was developed under similar crustal and mantle conditions than the Miocene arc of the same region. The proved linking of the last one with a shallowing subduction zone, and the presence of Lower Permian arc rocks to the west (Portezuelo del Cenizo Fm, Chilean granitoids) suggest a comparable tectonic situation for both magmatic unities.