Uruguayan dyke swarms

The Uruguayan dyke swarms consist of three groups of dykes of Precambrian age that intrude Río de la Plata Craton and Brasiliano Cycle continental crust in Uruguay. The dykes – including the Florida dyke swarm, the Nico Perez dyke swarm, and the Treinta y Tres dyke swarm – are of mafic to intermediate composition and each group lies in a separate tectono-stratigraphic terrane. The rocks of the Florida dyke swarm have been quarried since the 1960s and are used in the construction industry as black dimension stone of very high quality, being marketed as "black granite".

The Florida dyke swarm (also known as Piedra Alta dyke swarm and Uruguayan dyke swarm) crops out in the Piedra Alta Terrane of Río de la Plata Craton west of Sarandí del Yí Shear Zone. The strike of the dykes has an arcuate pattern: it changes from west to east from NE-SW to E-W and then near the shear zone to NW-SE. The Florida dykes have dips between 90° and 70°. Common thicknesses are of 20–30 metres (66–98 ft), with occasional dykes reaching thicknesses of 50 metres (160 ft) and 80 metres (260 ft) and the smaller dykes being 0.5 metres (1 ft 8 in). The dykes are made up of basaltic andesite and andesite of the tholeiitic magma series. The andesites have higher titanium dioxide contents and are usually darker and more massive, two characteristics that make them more valuable as decorative stone. The basalts instead have lower titanium dioxide contents and grey colors. The dykes made up of basalt might be slightly younger than the andesite dykes.

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