Abstract
Mafic dykes of lamprophyric affinity cropping out along the coastal area of Montevideo city are described. These dykes trend N75º-85º and crosscut 2.1 Ga Paleoproterozoic metamorphic units of the Rio de la Plata craton. They show mainly porphyritic textures with phlogopite and clinopyroxene macrocrysts in a groundmass composed of carbonates, phlogopite, augite, and feldspathoids. Ocellar structures filled by leucite, carbonates and fibrous alkaline amphibole are present. The mineralogical assembly allows their classification as lamprophyres (minettes), but according to their chemical nature, they can be classified as alkaline lamprophyres. A crystallization age of 1.42 Ga, by Ar-Ar method (on biotite/phlogopite) was obtained.
Highlights
Lamprophyres comprise a set of uncommon hypoabissal maficultramafic rocks usually of restricted occurrence as sills or dykes, occasionally as dyke swarms or stocks
According to Rock (1991), chemical affinities define calk-alkaline, alkaline and ultramafic lamprophyres; the first usually being related to convergent settings, whereas the others two can be potentially related to large igneous provinces
According to the petrographic features described previously, the analyzed dykes can be classified as lamprophyres
Summary
Lamprophyres comprise a set of uncommon hypoabissal maficultramafic rocks usually of restricted occurrence as sills or dykes, occasionally as dyke swarms or stocks. The study area is located in the southern portion of the PAT, along the coastal margin of Montevideo city (Figure 2) This area consists of medium grade San José metamorphic belt (Bossi et al 1993; Oyhantçabal et al 2003), which includes the Montevideo Formation and the Punta Carretas Orthogneissic Unit. The Punta Carretas Orthogneissic Unit was defined by Oyhantçabal et al (2003), who identified the orthogneiss as resulting from metamorphic recrystallization of calk-alkaline granitoids It represents the most widespread unit of the crystalline basement in Montevideo, intercalated with rocks of the Montevideo Formation and associated granitoids (Pascale 2013). Previous work, involving some petrographic and structural features of the lamprophyric dykes, inferred that they represent at least the last magmatic event affecting the Montevideo Formation (Pascale and Oyhantçabal 2010; Pascale 2013)
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