Geochronological data from the Arroyo del Torreón Formation, Andacollo minning District, Neuquén, Argentina.

The Andacollo Mining district located in the Cordillera del Viento, Neuquén province, Argentina is known for its Au/Ag vein deposits. The host rocks are pre-Permian rhyolitic vitreous to lithic welded-tuff of Arroyo del Torreón Formation, coeval to the Carboniferous Dacita Sofía (327.9 ± 2.0 Ma, Suárez et al. 2008). The units are covered by rocks of the marine Huaraco Formation (Upper Carboniferous to Lower Permian) and intruded by Permian I-S types granites. To improve the age of the Arroyo del Torreón Formation, samples from a welded-tuff flow were dated using LA-ICP-MS U-Pb in zircon at the Multilab Laboratory, UERJ, Brazil. Eleven magmatic prismatic zircons produced a crystallization age of 322.0 ± 5.0 Ma, indicative of active magmatism during the Upper Mississippian-Lower Pennsylvanian. In the Arroyo del Torreón Formation, an age of 383 ± 6 Ma (Middle Devonian) was obtained in detrital zircons by Zappettini et al. (2012).  The Chachil Plutonic Complex has a Re-Os age of 313 a 314±1.6 Ma (Middle Pennsylvanian). In the Neuquén Cordillera and the western North Patagonian Region Pankhurst et al.(2006) reported ages of 321±2 Ma, 323±3 Ma, and 330±4 Ma in plutonic rocks. The results indicate the presence of a magmatic arc from the northern part of Neuquén to the central part of the Chubut provinces, more than 700 km long in N-S direction. In the Andes of Central Chile, granites yielded a U-Pb age of 328.1±2.8 Ma. Ages between 319 and 300 Ma (SHRIMP U-Pb on zircons) were determined in the Coastal Batholith of Chile. In addition, detrital zircons in the accretionary complex of central Chile show ages of 345 Ma and 330 Ma. The results evidence the existence of two pseudo-parallel magmatic arc, with the eastern one in a rear-arc position. Besides the Carboniferous ages, there are detrital zircon populations at 470 Ma, 1 Ga, 1.9 Ga and 2.6 Ga in the Arroyo del Torreón Formation.

The population at 470 Ma is represented by 8 rounded zircons, indicating a Lower Ordovician age. These Ordovician zircons were recognized by Zappettini et al., (2012) with ages between 465 ± 7 Ma and 482 ±5 Ma. There are a coeval magmatism in the Sierra Pampeanas and Famatina regions of central-northern Argentina, including also La Pampa province, with ages of 474±6 and 475±5 Ma. In the eastern North Patagonian region granitic rocks yielded ages of 475±6, 476±4 Ma and 476± 6 Ma, whereas in the central part give 467± 7 Ma, 470± 2, 454±2, 468±5 Ma.

Moreover, in the western North Patagonian region, unpublished U-Pb ages of inherited magmatic zircon in Permian granites indicate ages of 462.4 Ma and 480.5 Ma. The results show that the Famatinian magmatism extend more than 400 km E-W in this region, being possibly the source of the Ordovician zircons in the Andacollo area.

The Arroyo del Torreón Formation's detrital zircons exhibit peaks at 1 Ga, 1.9 Ga, and 2.6 Ga. These were identified within the Neuquén Cordillera, the eastern North Patagonian region, the Bloques de Las Matras and San Rafael, the Sierras Pampeanas, and the Río de La Plata and Kalahari cratons.

The results are indicative that the provenance area of the detrital zircons is predominantly eastern.

Suárez M, De la Cruz R, Fanning M, Etchart H., 2008. Carboniferous, Permian and Toarcian magmatism in Cordillera del Viento, Neuque´n, Argentina: First U–Pb Shrimp dates and tectonic implications. XVII Congreso Geológico Argentino, Actas, pp 906–907. S. S. de Jujuy

Pankhurst RJ, Rapela CW, Fanning CM, Márquez M., 2006. Gondwanide continental collision and the origin of Patagonia. Earth Sci Rev 76: 235–257.

Zappettini, E.O., Chernicoff, C.J., Santos, J.O.S., Dalponte, M., Belousova, E. and McNaughton, N., 2012. Retrowedge-related Carboniferous units and coeval magmatism in the northwestern Neuquén province, Argentina. Int J Earth Sci (Geol Rundsch), 101: 2083–2104.