Sienita alcalifeldespática parcialmente fenitizada asociada al plutón El Portezuelo Occidental (Granito Papachacra), Catamarca, Argentina.

COLOMBO, FERNANDO; LIRA, RAÚL; DORAIS, MICHAEL J.

La Plata

Congreso; XVI Congreso Geológico Argentino; 2005

Asociación Geológica Argentina

Hypersolvus alkali-feldspar syenite forms a body elongated in an E-W direction within a pluton of biotite granite called El Portezuelo Occidental, in Catamarca Province (NW Argentina). The approximate coordinates of the alkali-feldspar syenite are 27º 1´ 15´´ S, 66º 54´ 50´´ W and it covers an area of ~0.95 km2. This rock is composed of 90.48% mesoperthite (albitized), 0.90% plagioclase, 1.30% quartz, 1.06% zircon, 0.10% amphibole (unidentified species) and 6.16% aegirine + riebeckite. It hosts many enclaves of more basic rocks. Perthite forms an interlocking aggregate of subhedral grains that show crystal faces when they project into cavities. Ordered microcline (content of Al in the t10 site: 0.97) is heavily clouded by Fe oxides. It has an end-member composition. Plagioclase occurs as magmatic subhedral crystals and as veinlets due to unmixing of a homogeneous (K,Na) precursor. There is also an unidentified primary brown amphibole. The syenite is fenitized. Metasomatic albite rims and penetrates perthite, and forms patches of twinned plagioclase inside microcline. Quartz fills cavities. Zircon occurs as prismatic crystals of both magmatic and hydrothermal origin. Aegirine (close to pure NaFe3+Si2O6) is partially replaced by riebeckite. Fibrous amphibole found enclosed in quartz ranges from ferrorichterite to arfvedsonite.