Petrologic features of mesoproterozoic lamprophyric dykes from Montevideo ( Piedra Alta terrane , South Uruguay )

How to cite item Muzio, R., Martino, N., & Peel, E. (2021). Petrologic features of mesoproterozoic lamprophyric dykes from Montevideo (Piedra Alta terrane, South Uruguay). Earth Sciences Research Journal, 25(2), 157-168. DOI: https:// doi.org/10.15446/esrj.v25n2.89652 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 with 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. ABSTRACT Petrologic features of mesoproterozoic lamprophyric dykes from Montevideo (Piedra Alta terrane, South Uruguay)


Introduction
Lamprophyres comprise a set of uncommon hypoabissal maficultramafic rocks usually of restricted occurrence as sills or dykes, occasionally as dyke swarms or stocks. They are usually linked to lamproite, kimberlite and carbonatite petrogenesis and to deep sublithospheric mantle melts. This group of rocks is characteristic of volatile-rich magmas, with fast ascent and emplacement at crustal levels during regional stages of lithospheric relaxation (Rock, 1987(Rock, , 1991Mitchell, 1994;Tappe et al., 2008). Alkaline magmatism is frequent in within-plate settings (Whalen et al., 1987;Bonin, 1996), associated with extensional processes. 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. Mostly, mafic dykes are prominent extensional structures and are helpful tools for paleogeography reconstructions as markers of former conjugate margins that were separated probably in relation to mantle plumes (e.g., Bleeker and Ernst, 2006;Ernst and Buchan, 2001a, b;Halls and Fahrig, 1987).
Mafic dykes have been formerly reported for the Rio de la Plata Craton (RPC), both for the Uruguayan shield and for the Tandilia belt in Argentina by Bossi et al., 1993;Mazzucchelli el al., 1995;Teixeira et al., 1999;Iacumin et al., 2001;and Teixeira et al., 2002; among other references. According to geochemical and isotopic studies, they correspond to a basaltic-andesite composition, with tholeiitic and calk-alkaline chemical affinity, and Paleo-Mesoproterozoic ages Girardi et al., 2013).
Alkaline to sub-alkaline mafic magmatism has been found in southeast Uruguay, associated with the extensional events of the opening of the South Atlantic Ocean (Lustrino et al., 2005;Cernuschi et al., 2015).
The first references of lamprophyric dykes in Uruguay, that described the structural and petrographic features of mafic dykes located in the Montevideo coastal area, belong to Walther (1935;1948). Later, regional studies on the crystalline basement of the RPC characterized these dykes as "mafic alkaline dykes", tentatively assigning them a Neoproterozoic age according to their field relationships, since they crosscut Paleoproterozoic units (Coronel and Oyhantçabal, 1988;Masquelin et al., 2003;Oyhantçabal et al., 2003;Pascale and Oyhantçabal, 2010).
Early reports of lamprophyres in the Tandilia basement (Argentina) correspond to the work of Quartino and Villar Fabre (1967), who described these rocks as possible deformed amphibolites. More recently, Dristas et al. (2013) carried out a petrological characterization on an ultramafic lamprophyre, also complemented by isotopic studies and K-Ar (on biotite/phlogopite) dating, yielding a minimum age of 1928 ± 54 Ma and a calk-alkaline nature.
Therefore, this paper is focused on the petrological characterization of a set of lamprophyric dykes in Uruguay and the first 40 Ar-39 Ar (on biotite/ phlogopite) dating, in order to classify them and to constrain this magmatism in the context of the RPC. Almeida et al. (1973) originally defined the Río de la Plata Craton (RPC), including under this term the ''ancient cratonic areas'' of the southernmost South American platform already amalgamated by the upper Proterozoic.

Regional framework
The RPC represents one of the major nuclei of Western Gondwana, surrounded by late Neoproterozoic-Cambrian orogenic belts (e.g., Almeida et al., 2000;Cingolani 2010;Rapela et al., 2011 and references therein). The main tectonic units exposed in the Uruguayan shield are represented by Piedra Alta (Bossi et al., 1993) and Nico Pérez (Bossi and Campal, 1992) terranes, which are separated by the Sarandí del Yí shear zone (Preciozzi et al., 1979). Both Piedra Alta and Nico Pérez terranes in Uruguay, together with Buenos Aires Complex (BAC) in Argentina (Marchese and Di Paola, 1975) and Taquarembó block in southern Brazil, constitute the RPC (Figure 1a), according to the original definition of Almeida et al. (1973). Recently, Oyhantçabal et al. (2011) redefined the RPC limits in Uruguay and excluded the Nico Pérez terrane and Taquarembó block from the craton, after new geochronological, isotopic and gravity data interpretations.
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. Both units are crosscut by pegmatitic and aplitic dykes and, by the mafic dykes of the present study.
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). They present structural trends of N280º to N70º of light-gray to pinkish granolepidoblastic rocks, composed of oligoclase, quartz, biotite, epidote, muscovite, apatite and microcline, and strongly crosscut by pegmatitic and aplitic intrusions. The only radiogenic age available for this unit, by Rb-Sr whole rock method, yielded and age of 1990 ± 32 Ma (Cingolani et al. 1997).
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).

Analytical procedures
Seven rocks with minimal visible secondary alteration and corresponding to two different set of dykes from the coastal area were sampled, examined in thin section and prepared for lithogeochemical analyses. Three polished petrographic sections were also prepared at the Instituto de Ciencias Geológicas for SEM-EDS and BSE analyses. These analyses were developed with the JEOL 5900 -Low vacuum equipment at the Faculty of Sciences, Uruguay. Analytical techniques that are routine for this type of equipment as well as standard mineral spectra from Severin (2004) and Reed (2005) were used.
All the chemical data interpreted here as well as the location of the dykes are shown in Table 1. All samples were carefully cleaned, crushed and further pulverized using an agate mill at the Instituto de Ciencias Geológicas (Montevideo). Major and trace elements were analyzed by ACME-Bureau Veritas Laboratories (Vancouver, Canada) following the LF-202 Litho-Research package methodology. Ba, Sr, Y, Sc, Zr, V, and Be were analyzed using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Trace-elements (including REE) were determined by inductively coupled plasma-mass spectrometry (ICP-MS). The analytical protocol at the ACME Laboratory includes the analysis of reference materials (STD-DS10; STD-GS311-1; STD-GS910-4; STD-OREAS45EA; STD-SO-18 and STD . The 40 Ar/ 39 Ar geochronology was carried out at the University of Nevada (Las Vegas), using the data reduction LabSPEC software written by B. Idleman (Lehigh University). Biotite crystals of one sample of a lamprohyre dyke (PG-01) were separated using conventional techniques and then irradiated at the U.S. Geological Survey TRIGA Reactor (Denver, Colorado) for 20 hours, along with the GA-1550 biotite standard together with CaF 2 and K-glass fragments. After a period of cooling, the irradiated sample was loaded for step heating in a Cu sample tray in a high vacuum extraction line and were fused using a 20 W CO 2 laser and measured using a Balzers electron multiplier mass spectrometer. The measured ( 40 Ar/ 39 Ar)K values were 4.1 (± 58.54%) x 10 -3 . The Ca correction factors were ( 36 Ar/ 37 Ar)Ca = 2.55 (± 3.50%) x 10 -4 and ( 39 Ar/ 37 Ar)Ca = 6.97 (± 3.69%) x 10 -4 . The J factors were determined by fusion of 4-8 individual crystals of GA-1550 biotite neutron fluence monitors, giving a reproducibility of 0.3% to 0.8% at each standard position. The measured 40 Ar/ 36 Ar ratios were 282.36±1.41%; thus, a discrimination correction of 1.0467 (4 AMU) was applied to the measured isotope ratios. An age of 98.50 Ma (Spell and McDougall 2003) was used for the GA-1550 biotite fluence monitor to calculate the ages of the samples. The criterion followed to define a plateau age is the identification of three or more successive steps overlapping with an error at 2σ level that together comprise >50% of 39 Ar released. All analytical data are reported at the confidence level of 1σ (standard deviation) and shown in Table 2.

Petrographic features
Mafic dykes are gray to black, fine to medium grained, 0.3 to 2.0 meters width and tabular in shape. They often show irregular contacts with PAT units and are affected by two sets of NW and N-NW vertical to subvertical fractures. Sometimes they show an anastomosed pattern (Figure 3a), with sharp contacts and thin chilled margins (Figure 3d). No evidence of contact metamorphism has been found.
They are concordant with the regional foliation trend (N70º-N80º). Even the smaller dykes with widths of a few centimeters often intrude through lateral fractures with a NW direction. Double intrusion processes or "dyke in dyke" features are observed (Figure 3c), which has been well documented as a common process for lamprophyres worldwide (Rock 1987). They have porphyritic texture with increasing grain size toward the dyke center. Occasionally, the central zone is porphyritic and vesiculated, with biotite/pyroxene macrocrysts and globular structures.
These globular structures (ocelli; Phillpotts 1990) are almost spherical but frequently show coalescence, developing irregular shapes ( Figure 3b). They vary in size between 0.1 cm to 1.5 cm, and they are filled mainly by carbonates, zeolites with halos of opaque minerals and arfvedsonite (Figure 3g).
Mafic and felsic xenoliths from the surrounding crystalline basement are frequent, represented by ortho-gneiss and amphibolite (Figures 3e and 3f).
SEM-EDS studies were developed to better characterize the mineral composition of the groundmass. The spectra obtained were compared to standard mineral spectra from Severin (2004) and Reed (2005) in order to identify part of the minerals of the dykes. Some of these representative minerals and their spectra are presented in Figure 5.
According to the TAS diagram (Le Bas et al. 1986; Figure 6) the samples can be classified as phonotephrites/tephriphonolites. All the samples have total alkalis close to 10 wt% and K 2 O/Na 2 O ratios approximately 4, a relation that points to a possible ultrapotassic character/shoshonitic nature. Using the Al 2 O 3 versus CaO classification diagram from Foley et al. (1987) and the K 2 O-MgO-Al 2 O 3 ternary diagram from Rock (1987), the samples can be classified as lamprophyres and, according to their alkali values the samples overlapped the fields lamprophyres/lamproites ( Figure 7b).
As shown in Table 1, the samples have high alkali values (mean K 2 O + Na 2 O > 8, mostly close to 10 wt.%), with K 2 O/Na 2 O > 3. In order to constrain the alkaline affinity of the dykes and to discard a possible shoshonitic nature the K 2 O -Na 2 O diagram was used, showing as a result an ultrapotassic character for all the samples (Figure 8).  Foley et al. 1987, after Turner et al. 1996. Symbols are the same as in Figure 6.
The REE patterns normalized to chondrite (Boynton 1984) Rock (1987). Symbols are the same as in Figure 6.
References Fig. 7a: I -Lamproites; II -Kamafugites (low SiO 2 , high Cao); III -Orogenic areas (high CaO and Al 2 O 3 ) and IV -Transitional group. The dykes have high Nb/Ta ratios, similar to the primitive mantle. Also, high Nb and Zr values with relatively low values of Hf and Y are commonly observed in alkaline rocks from within-plate settings (Green 1995).

Ar/ 39 Ar results
The biotite-phlogopite analysis from the center of one dyke (Nº PG-01, site PG; Figure 2) produced relatively undisturbed spectra and a well-defined plateau age ( Figure 10, Table 2) of 1425.40 ± 9.12 Ma (1σ). The plateau is defined by the release of at least 99% of 39 Ar gas in fifteen consecutive steps out of sixteen.
Symbols are the same as in Figure 6.

Discussion
According to the petrographic features described previously, the analyzed dykes can be classified as lamprophyres. Based on the latter, the mineral assembly (biotite-phlogopite + augite ± aegirine augite ± altered olivine ± dolomite ± amphibole ± chlorite ± feldspars and feldspathoids, in the groundmass) and the high values of K 2 O, P 2 O 5 and Ba pointed to their classification as minettes (Rock 1991;Le Maitre et al. 2002). Although the presence of feldspathoids in minette type lamprophyres is not provided by the IUGS classification, it has been reported by several authors (e.g., Esperanza and Holloway 1987;Wallace and Carmichael, 1989;Bhowmick, 2000;Gupta 2015). In addition to the general mineralogical features, other chemical criteria can be taken into account to constrain their classification (Mitchell and Bergman, 1991).
In this sense, the dykes from Montevideo are not only ultrapotassic (K 2 O/ Na 2 O > 3) but also peralkaline (K 2 O+Na 2 O)/Al 2 O 3 > 1) and perpotassic rocks (K 2 O/Al 2 O 3 > 1), with CaO and Fe 2 O 3t ≤ 10 wt. % and a high Ba (>5.000 ppm), TiO 2 (~3 wt. %), Zr (~650 ppm) and Sr (~1500 ppm). All these are typical characteristics of lamproites according to Foley et al. (1987) andLe Maitre et al. (2002). Nevertheless, the typical lamproite mg# values are higher than 70, and the La content is higher than 200 ppm whereas in the studied dykes they are between 62-67 and approximately 182, respectively. The dykes present a mean value of 3.7 for the Nb/Y ratio, which characterizes them as alkaline (Winchester and Floyd, 1977). Alkaline lamprophyres usually have Cr, Co and Ni contents of 97, 38 and 65 ppm, respectively. In contrast, the lamprophyric dykes of Montevideo have these values of approximately 480, 56 and 172 ppm, close to those representatives of ultrapotassic lamprophyres (Mitchell and Bergman, 1991).
Alkaline magmatism is frequent in within-plate environments, related to extensional processes (Whalen et al. 1987) and is characterized by high concentrations of HFSE and low values of LILE. Alkaline rocks showing strong enrichment in LILE and LREE are commonly associated with anorogenic or post-orogenic settings (Plá et al., 2011).
Alkaline ultrapotassic magma shows high incompatible element content regardless of the tectonic environment supporting an origin by partial melting of lherzolites (low rates <1%) from an enriched mantle source (Lloyd et al. 1985;Foley 1992;Foley and Peccerillo 1992;Ionov et al. 1997). Furthermore, high concentrations of LREE have been observed for other ultrapotassic within plate rocks, such as lamprophyres and lamproites as well as post-collisional lamprophyres like minettes (Foley 1992;Gibson et al. 1992). The high concentrations of Y and Zr, and relatively lower concentrations of Hf, Nb, Ta and HFSE are consistent with a within plate environment (Foley and Peccerillo, 1992). The Nb/U ratios for lower and upper crust are 21 and 9, respectively; rocks affected by crustal contamination present Nb/U values between 9 and 40 ppm (Gregoire et al., 2000). The dyke mean values of 43 ppm suggest that crustal contamination during the magmatic ascent/ emplacement is not obvious.
Several authors (e.g., Masquelin et al. 2003, Oyhantçabal et al., 2003, Pascale and Oyhantçabal, 2010 have attributed a Cambrian age to these dykes because other mafic dykes located to the North of Montevideo city crosscut the La Paz granite (585 ± 4 Ma, Cingolani et al. 2012). Martino (2017) analyzed these dykes and found that they correspond to lamprophyres of calk-alkaline affinity, and according to their petrographic features they were classified as espessartites. Interesting features that support their contemporaneity of emplacement with the La Paz granite are evidence of magma mingling processes. Therefore, these calk-alkaline lamprophyres are clearly different and later than the dykes discussed in this paper. Mafic dykes of Mesoproterozoic age have been mentioned for the RPC, but in general, with different mineralogy and/or chemical affinity Teixeira et al. 2013;Dristas et al. 2013, among others). An interesting fact reported by these authors is that according to the isotopic data from the Tandil dykes, an enriched original source could be a characteristic feature of the mantle of the Rio de la Plata Craton in Paleoproterozoic time. Lamprophyric dykes were only reported by Dristas et al. (2013), with a minimum age of 1.92 ± 54 Ga obtained by the K-Ar method on phlogopites. Teixeira et al. (2013) proposed the idea that the Columbia supercontinent started a major break up at 1.59 Ga, based on geochronological and paleomagnetic data. Therefore, the emplacement of the lamprophyric dykes studied in this paper may be related to the initial stages of a later major extensional event.
• According to the mineralogical criteria established by the IUGS the dykes can be classified as minettes (lamprophyres) and from a chemical point of view as alkaline/ ultrapotassic lamprophyres.
• The chemical features point to a deep/enriched mantle source for the origin of this magma related to a within-plate tectonic setting.
• The crystallization age of 1.42 Ga determined by the Ar/Ar method on biotite (phlogopite), points to an extensional event for the Rio de la Plata craton at that time.