Cerro Manomo lies 20 km northeast of the Velasco Alkaline Province (022-00-007) and forms a treeless mesa of 4x6 km flanked by cliffs and talus slopes and rising 400 m above a jungle-covered peneplain of lateritized Precambrian gneiss.
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Alkaline rocks in Argentina occur essentially in the west of the country in a zone parallel to the Andean chain, and associated with voluminous but predominantly calc-alkaline and tholeiitic volcanics. Few detailed petrological accounts of the alkaline rocks are available, but a general review has been given by Quartino and Llambias (1964).
Covering 440 km2 and extending over 80 km, the Velasco Province consists of an early suite of volcanics, 14 overlapping ring and elliptical plutons and the Cerro Manomo carbonatite, which is some 20 km northeast of the plutons and is described above (022-00-006).
A suite of north-south-trending alkaline dykes occurs just south of the border with Bolivia near Santa Victoria and it may be related to an alkaline centre in Bolivia. It includes trachytes, porphyritic foyaites with analcime, basanite and limburgite.
A tinguaite dyke about 1 m wide occurs north-northwest of Iruya, intruding Precambrian schists.
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).