Alkaline Rocks and Carbonatites of the World

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Las Chacras

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Occurrence number: 
008-00-008
Country: 
Argentina
Region: 
San Luis
Location: 
Longitude: -65.75, Latitude: -32.58

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). They consist essentially of olivine, sometimes serpentinized, pyroxene and magnetite, with biotite and analcime in four of the six rocks described, melilite in two of them and nepheline and natrolite in one. They appear to be related to the olivine nephelinites.

Age: 
Dating by K-Ar on rocks from two plugs gave 85±5 and 83±5 Ma (Valencio et al. , 1980, Table 1).
References: 
PANDOLFI, C. I. de 1943. Estudio petrografico y bosquejo geologico de la Region de Chajan (Cordoba). Boletin, Ministerio de Agricultura de la Nacion. Direccion de Minas y Geologia, 54‚ 1-46. PASTORE, F. & GONZALEZ, R. F. L. 1954. Descrption geologica de la Hoja 23g, San Francisco (San Luis). Argentina Ministerio de Industria de la Nacion. Direccion Nacional de Mineria, Boletin, 80‚ 1-63. VALENCIO, D. A., LOPEZ, M. G., SOLA, P. & VILLANI, C. 1980. El significado geologico de los resultados del estadio paleomagnetico de vulcanitas alcalinas de los provincias de San Luis y Cordoba. Revista, Asociacion Geologica Argentina, 35‚ 340-7.
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