Resumo

MARO, Guadalupe; CAFFE, Pablo J  e  JOFRE, Cynthia. Xenolitos ultramáficos en lavas máficas neógenas de la Puna norte. Rev. Asoc. Geol. Argent. [online]. 2016, vol.73, n.2, pp.280-291. ISSN 0004-4822.

This paper is the first mention of ultramafic xenoliths included in basaltic andesites of the Neogene monogenetic volcanism This paper is the first mention of ultramafic xenoliths included in basaltic andesites of the neogene monogenetic volcanism from northern Puna. They consist of angular fragments of green to black rocks, 1 mm to 5 cm in diameter and commonly showing adcummular texture. These xenoliths are constituted by clinopyroxene, orhopyroxene and olivine, with or without Cr-rich spinel (Cr# ~ 50) and inclusions of Fe-Ni rich sulphides. These xenoliths classify as pyroxenites due to the dominance of pyroxene in the modal assemblage. In every case, the magnesium number [100MgO MOL/(MgO+FeO)MOL] of the mineral fraction is high (Mg# 81 a 87) and compositions are overlapped with those observed in the crystals free in the host lavas. The application of two pyroxenes geothermobarometers indicates equilibrium temperatures between 969 and 1246 °C and maximum pressures of 8 kbar. This work proposes that these ultramafic xenoliths have a cumulatic origin. Firstly, they were formed in deep reservoirs, probably associated with hot zones of accumulation of primitive magmas in the middle to lower crust. Secondly, they suggest a fast ascent rate for the most primitive magmas erupted in the northern Puna.

Palavras-chave : Ultramafic xenoliths; Basaltic andesites; Altiplano-Puna plateau.

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