Los Alisos
A near vertical, sheet-like intrusion extends intermittently over 10 km and has an outcrop width of 20-80 m. The rock, formerly described as kimberlite, is now considered an alnoite.
A near vertical, sheet-like intrusion extends intermittently over 10 km and has an outcrop width of 20-80 m. The rock, formerly described as kimberlite, is now considered an alnoite.
A peralkaline granite pluton covering 7x3 km cuts and hornfelses Ordovician sediments of the Chiquero Formation northeast of Cobres on the Sierra de Los Cobres. The granite consists of microcline, albite, riebeckite, aegirine and accessories, including pyrochlore.
In the area north of San Miguel de Tucuman is a province of dykes, sills and chimneys predominantly of alkali olivine basalts but containing abundant zeolite and some nepheline.
A dyke intrudes Precambrian granodiorite on the northern side of Cerro Aspero and consists of amphibole phenocrysts up to 5 mm long, which with matrix amphibole comprises 50% of the rock, with nepheline (20%), pale green pyroxene (10%), magnetite and accessories including an isotropic mineral (?
Alkaline rocks appear to be widespread in the 'Precordillera' (approximately between 39°-33°30'S and 68-69°W) but detailed distributions and descriptions are not available.
A small plug lying just to the south-southeast of Las Chacras, and four others in the vicinity, cut Precambrian granites and gneisses. They are described as basalts but plagioclase occurs in only one of six specimens described by Pandolfi (1943).
Nepheline basalts form the plugs of La Leoncita, La Madera, La Piedra and La Garrapata, all having diameters of about 1 km, while the plug of Cerro Piedra is much smaller. They cut Precambrian micaceous gneisses and form prominent hills.
East of Las Salinas a gabbro occurs which contains abundant, interstitial nepheline.
Among the voluminous volcanics of the Cordillera Principal alkaline rocks are scarce. However, tephrites and basanites, essexites with nepheline, analcime and biotite, trachytes and trachydolerites and rocks described as bostonites, but containing sodalite and nepheline, all occur.
This volcanic province extending for 70 km north-south and 20 km east-west comprises flows and plugs of basanite and olivine basalt, with some pyroclastics, and plugs and necks of nephelinite, tephrite and phonolite.