Las volcanitas de Colipilli (37°S) y su relación con otras unidades paleógenas de la Cordillera

LLAMBÍAS, E.J.; RAPELA, C.W.

Revista Asociación Geológica Argentina

Asociación Geológica Argentina

Lugar: Buenos Aires; Año: 1989 vol. 45 p. 224 - 236

0004-4288

The Paleogene igneous rocs from ColIipilIi (37°5O’S; 70°20’ belong to the Neuquen-Mendoza volcanic province (PVNM) of similar age, which extends from 34°S to 38°30’S. These rocks are described as Molle Group and consist of two formations: 1) Cayanta Formation which consists of hornblendic andesite lava flows, volcanic brecias and volcanic aglomerates; 2) Collipilli Formation, which is composed predominantly of concordant intrusive bodies like sills and lacoliths. They range in composition from hornblendic andesites to diorites and quartz diorites. SmaIl dacite intrusive bodies are also included in this formation. Most common minerals are: zonal plagioclase (An), homblenda, and apatite and magentite as accesories. The lava flow have, besides the minerals mentioned, clinopyroxene phenocrists. In the intrusive rocks clinopyroxene is uncommon and sometimes it is found as a core in the hornblenda phenocrysts. In some cases hornblenda phenocrysts are replaced by a fibrous amphibole (tremolite-actinolite?). Crystallization of amphibole and apatite along joints are also rather common. Igneous rocks from Collipilli are calc-alkaline, with K-normal and metaluminous characteristics. Comparison with the Andean-Patagonian volcanic province (PVAP) results in significative differences, the most important ones being: 1) Hornblende is a common mineral in the PVNM; in the PVAP it is rare; 2) Several porphyry copper type deposits and base metal veins are related to the igneous rocks of the PVNM, while they have not been recognized in the PVAP up to now; 3) the Ba/ Nb ratio is higher in Collipilli than in the PVAP, with the exception of the Cerro Ventana area, Bariloche, which is similar, AU the differences referred are consistent with the hypothesis that the segmentation of the Cordillera proposed for the Upper Cenozoic had also ocurred during Lower Cenozoic times.