The geology and tectonic setting of the Velasco Alkaline Province, eastern Bolivia

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Abstract
The Velasco Alkaline Province of Mesozoic age is situated near to the Bolivian-Brazilian border, and is enclosed in Precambrian gneisses of the Brazilian Shield. It consists of an early volcanic suite, and a series of 14 interfering circular, ring and elliptical plutons, and is associated with a silicified carbonatite complex. The combined length of the province is nearly 80 km and it outcrops over an area of 440 km 2 . The plutonic rock types range from foyaite to aegirine granite but each individual pluton may be classified as either over- or under-saturated. The province was built up by a series of plutonic events which for the most part were aligned along a NE-trending lineament. Normal faulting was active during the development of the province. Although the Velasco Alkaline Province lies over 1500 km from the eastern continental margin it is deduced that it defines a plume-generated triple point initiated at the same time as the break-up of the South American-Africa plate.