stripes
The Mitu Group, which is of Permian and/or Triassic age, is widespread in southern and central Peru. It consists of coarse clastic sediments and volcanic rocks, of which a small but unknown proportion are alkaline. In the Junin area lavas and sills of teschenite and analcime basanite are found (Harrison, 1943 and 1956), and similar rocks are thought to occur in the overlying Mesozoic of the Cuzco vicinity. Audebaud et al. (1980) distinguish six phases of Permian magmatism, of which two include analcime-bearing lavas. At least 50 km2 of peralkaline rhyolites occur within the Mitu Group northeast of Ayacucho and may be very much more extensive (Noble et al., 1978). However, classification is based on trace element chemistry, because while Na is less than Al, it is considered that Na was lost from the rocks during primary crystallization, fine grained iron oxides having replaced original sodic amphibole.
AUDEBAUD, E., VIVIER, G. and VATIN-PERIGNON, N. 1980. Magmatic evolution of southern Peru, Part 1. Abstracts, 26th International Geological Congress, Paris, 1: 13.
HARRISON, J.V. 1943. The geology of the central Andes in part of the province of Junin, Peru. Quarterly Journal of the Geological Society of London, 99: 1-36.
HARRISON, J.V. 1956. La geologia de la region Casca-Junin: la geologia del Valle del Rio Mantaro. Boletim, Ministerio de Fomento y Obras Publicas Instituto Nacional de Investigacion y Fomento Mineros, 15: 1-55.
NOBLE, D.C., MILES, L.S., MEGARD, F. and BOWMAN, H.R. 1978. Comendite (peralkaline rhyolite) and basalt in the Mitu Group, Peru: evidence for Permian-Triassic lithosphere extension in the central Andes. Journal of Research, United States Geological Survey, 6: 453-7