Abstract

COLOMBO, Fernando; LIRAI, Raúl; O´LEARY, María Sol  and  TASSINARI, Colombo C. G. The comenditic dikes from Papachacra (Catamarca): peralkaline magmatism in Eastern Sierras Pampeanas. Rev. Asoc. Geol. Argent. [online]. 2010, vol.66, n.1-2, pp.194-205. ISSN 0004-4822.

In Papachacra (Catamarca Province, NW Argentina) there is a pluton composed of several different A-type granitoids that show alkaline affinities. The latest rocks are peralkaline rhyolites (comendites), which form dikes up to 50 cm thick cross-cutting the genetically unrelated El Portezuelo granite. They have porphyritic texture and are composed of K feldspar (Or_97-98 Ab_2-3) and aegirine (Ac_94-98) phenocrysts in an aphanitic groundmass of quartz, K feldspar, albite, aegirine and Li-bearing amphibole chemically related to fluor-ferro-leakeite. Minor accessory phases include ilmenite, magnetite, pyrochlore-group minerals, REE-rich epidote, zircon (interstitial), monazite-(Ce) and an unidentified Nb-Y-Fe oxide. Their SiO2 content ranges between 68.28 and 69.33%, with very low Mg, Ca, Ba, Sr and Eu, and high concentrations of Nb (399-409 ppm), Ta (16-34 ppm), Th (46-84 ppm), Y (101-192 ppm), Zn (280-320 ppm), Ga (42-47 ppm) and especially Zr (2324-3000 ppm). The comendites were intruded during a distensive magmatic event. A reconnaissance Rb-Sr isotopic study performed on related rocks indicates an age of 295 + 8 Ma, with an initial ⁸⁷Sr/⁸⁶Sr ratio of ca. 0.7041, pointing to a source from the sublithospheric mantle or the base of the lower crust. These rocks belong to the A1 class of A-type granites, suggesting an origin related to rifting or hot spots, and their trace element ratios show affinities with melts related to oceanic island basalts. In this respect, they are different from all the post-Devonian A-type granites described in Eastern Sierras Pampeanas.

Keywords : Anorogenic granites; A-type granites; Rifting; Oceanic island basalts.

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