The Uruguayan dyke swarms consist of three groups of dykes of

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

age that intrude

Río de la Plata Craton The Rio de la Plata Craton (RPC) is a medium-sized continental block found in Uruguay, eastern Argentina and southern Brazil. During its complex and protracted history it interacted with a series other blocks and is therefore considered importan ...

and Brasiliano Cycle continental crust in

Uruguay Uruguay (; ), officially the Oriental Republic of Uruguay ( es, República Oriental del Uruguay), is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast; while bordering ...

. The dykes – including the Florida dyke swarm, the Nico Perez dyke swarm, and the Treinta y Tres dyke swarm – are of

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

intermediate composition In igneous petrology, an intermediate composition refers to the chemical composition of a rock that has 5263 wt% SiO2 being an intermediate between felsic and mafic compositions. Typical intermediate rocks include andesite, dacite, and trachyandes ...

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 Construction is a general term meaning the art and science to form objects, systems, or organizations,"Construction" def. 1.a. 1.b. and 1.c. ''Oxford English Dictionary'' Second Edition on CD-ROM (v. 4.0) Oxford University Press 2009 and com ...

industry as black

dimension stone Dimension stone is natural stone or rock that has been selected and finished (e.g., trimmed, cut, drilled, ground, or other) to specific sizes or shapes. Color, texture and pattern, and surface finish of the stone are also normal requirements. A ...

of very high quality, being marketed as "

black granite In the construction industry, black rocks that share the hardness and strength of granitic rocks are known as black granite. In geological terms, black granite might be gabbro, diabase, basalt, diorite, norite, or anorthosite Anorthosite () is ...

Florida dyke swarm

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 In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear ...

to NW-SE. The Florida dykes have dips between 90° and 70°. Common thicknesses are of , with occasional dykes reaching thicknesses of and and the smaller dykes being . The dykes are made up of

basaltic andesite Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central Ameri ...

and

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomi ...

of the

tholeiitic magma series The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma ...

. The andesites have higher

titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insolubl ...

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. Despite having some characteristics similar to those of

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...

s formed near

subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

s, the Florida dyke swarm rocks are unlikely to have formed in such environment. Instead, the magma that formed the dykes is thought to be derived from the partial melting of garnet peridotite in a parcel of the Earth's

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

that was metasomatized without the direct influence of subduction. These dykes are of Late Paleoproterozoic age. Nearby rapakivi granites of the Illescas Batholith are related to the dyke swarm. By the time of intrusion of the Florida dyke swarm, Piedra Alta Terrane was experiencing

extensional tectonics Extensional tectonics is concerned with the structures formed by, and the tectonic processes associated with, the stretching of a planetary body's crust or lithosphere. Deformation styles The types of structure and the geometries formed depend on ...

Nico Perez dyke swarm

Tholeiitic basalt and basaltic andesite are the main rock types of Nico Perez dyke swarm. Before solidifying as dykes the magmas fractionated minerals at relatively shallow (upper crust) magma chambers. The dykes of the swarm have geochemical heterogeinities that indicate they did not originate from the same magmas. Despite having geochemical characteristics typical of

magmatism the dykes are judged unlikely to have formed in such environment and are instead considered an example of

intraplate In geology, anorogenic magmatism is the formation, intrusion or eruption of magmas not directly connected with orogeny (mountain building). Anorogenic magmatism occurs, for example, at mid-ocean ridges, hotspots and continental rifts. This contrast ...

magmatism.

Treinta y Tres dyke swarm

The Treinta y Tres dyke swarm intrude Brasiliano Cycle

granitoid A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz ...

s that crop out east of Sierra Ballena Shear Zone. The dykes are about 750 million years old ( Neoproterozoic). Compared to the two other Uruguayan dyke swarms the Treinta y Tres dyke swarm originated from lower degrees of partial melting in the mantle, but of similar garnet peridotite sources.

References

{{Major South American geological formations Dike swarms Geology of Uruguay Neoproterozoic geology Neoproterozoic South America Paleoproterozoic geology Precambrian South America