untitled EARTH SCIENCES RESEARCH JOURNAL Earth Sci. Res. J. Vol. 13, No.1 (June 2009): 16-29 MESOZOIC MAGMATISM IN EAST URUGUAY: PETROLOGICAL CONSTRAINTS RELATED TO THE SIERRA SAN MIGUEL REGION Rossana Muzio, Elena Peel, Ethel Morales, Gerardo Veroslavsky, Bruno Conti Departamento de Evolución de Cuencas, Facultad de Ciencias. Iguá 4225 C.P. 11400, Montevideo, Uruguay Corresponding author: Rossana Muzio, rossana@fcien.edu.uy ABSTRACT This work presents new results of a detailed geological and structural investigation focusing the easternmost Uruguayan Mesozoic magmatic occurrences related to the south Atlantic opening. Lithological descriptions, their stratigraphic relationships and complimentary lithochemical characterizations carried out in the San Miguel region (East Uruguay) are presented. Three volcanic/sub-volcanic units have been recognized. The fel- sic volcanic association is composed by rhyolitic - dacitic flows, mainly with porphyritic textures and sub-alka- line nature and related pyroclastic rocks. The felsic sub-volcanic association is characterized by granophyres of about 25 km2 of exposed area, cross- cut by mafic and felsic dykes. Finally, a mafic association has been identi- fied characterized by dykes and a small intrusion of gabbroic composition. All these units are Mesozoic in age (130 – 127 Ma) and according to their chemical nature they correspond to sub-alkaline to weak peralkaline magmas. Key words: petrography, geochemistry, Mesozoic, Uruguay. RESUMEN Este trabajo presenta nuevos resultados geológico-estructurales y geoquímicos relacionados con el magmatismo Mesozoico del extremo Este de Uruguay (Sierra de San Miguel, Departamento de Rocha), asociados a la apertura del océano Atlántico Sur. Fueron reconocidas para la región estudiada tres unidades volcánicas/subvolcánicas. La asociación volcánica félsica está compuesta por derrames riolíticos – dacíticos, con texturas dominantemente porfiríticas y naturaleza química subalcalina. Asociadas a estas riolitas ocurren 16 Manuscript receiver: January 12th, 2009. Accepted for publication: March 30th, 2009. niveles de rocas piroclásticas. La asociación subvolcánica félsica está compuesta por granófiros con una extensión superficial de aproximadamente 25 km2, cortados por diques máficos y félsicos. Fue identificada también una asociación básica compuesta por diques y un cuerpo de gabro. Todas estas unidades son de edad Mesozoico (130 – 127 Ma) y de acuerdo a su carácter químico corresponden a magmas de naturaleza subalcalina a débilmente peralcalina. Palabras clave: petrografía, geoquímica, Mesozoico, Uruguay. Introduction The Uruguayan geology is underlain by Precambrian terrains comprising a wide range of metamorphic rocks and granitic intrusions. Five sedimentary bas- ins rest on the cratonic basement, one Palaeozoic and the other four Mesozoic in age. Mesozoic magmatism tectonically related to the breakup of Gondwana is well represented in the Uru- guayan extension of the Paraná Basin and in the rift-type basins of Santa Lucía and Laguna Merin (Fig.1). Recently, in the offshore Punta del Este basin have been also recognized relicts of this magmatic event (Ucha et al., 2003). The Paraná Magmatic Province (PMP) is one of the largest continental magmatic provinces in the world (Peate, 1997). Although the main outcrop ar- eas are exposed in Brazil, some extensions are well developed in Argentina, Paraguay and Uruguay. Fur- thermore, Uruguay has a privileged position because of the presence of the southernmost extension of these magmatic exposures. During the early Cretaceous, a strong volcanic activity made up of basaltic and rhyolitic rocks oc- curred in south eastern Uruguay along the border of the Paraná basin. Particularly, these exposures are well represented in rift type basins located in the east- ern portion of Uruguay (Laguna Merín basin) and comprise from basic to acidic units on which this pa- per is concerned. Several petrologic works have been carried out in the south eastern portion of Uruguay, with particu- lar emphasis in the felsic volcanic/sub-volcanic units (Muzio, 2000; Kirstein et al., 2000; Kirstein et al., 2001; Muzio et al., 2002; Lustrino et al. 2003; Lustrino et al. 2005). As main results can be pointed out that the petrologic and isotopic features exhibited by these units differ completely from their temporal correlatives of the Paraná Province (Palmas and Chapecó rhyolites, according to Bellieni et al. 1986). Recent studies performed by Muzio et al. (2004) and Muzio et al. (2005) in the San Miguel region, al- lowed the identification of a gabbroic body which 17 MESOZOIC MAGMATISM IN EAST URUGUAY: PETROLOGICAL CONSTRAINTS RELATED TO THE SIERRA SAN MIGUEL REGION Cuenca de Paraná B R A Z I L A R G E N T I N A Sedimentary basins A - Paraná Basin B - Santa Lucía Basin C - Laguna Merín Basin Crystalline basement - 54º00´ - 31º 00´ A B C Exposed areas of Mesozoic magmatism Jaguarão N 0 100 km SaLAM (Santa Lucía - Aiguá Merín) Lineament Montevideo Sierra San Miguel -35º00’ Figure 1. Structural framework and location of the area, modified after Ucha et al. (2003). has been mapped at semi-detailed scale (Fig. 2). This is the first basic intrusive body that has been recog- nized out-cropping in the southern portion of the Laguna Merin basin, closed to an important gravimetric anomaly (+90 mGa; Reytmair, 2001). According to Rossello et al. (2000), this gravimetric anomaly results from the development or reactiva- tion of deep parallel fractures, trending N70° to E-W, which affected the crust during Gondwana fragmen- tation. These fractures, recognized through strong structural, magnetic and gravimetric lineaments, be- came mantle feeding structures that allowed the em- placement of basic/mafic rocks which caused the excess of gravity in the region (Rossello et al, 2000; Veroslavsky et al., 2002). The aim of this paper is to present the petrologic and structural characterization of the units exposed in the Sierra San Miguel region, corresponding to the southernmost border of the Laguna Merín Basin (East Uruguay), with particular emphasis in the gabbroic intrusion. Geologic setting and petrography The Sierra San Miguel Complex is located in the De- partment of Rocha (East Uruguay), 340 km eastwards Montevideo, the capital of the country. It represents an important geomorphologic feature, of about 25 km length and regional trend EW within the wetlands and Quaternary sediments of the southern extreme of the Laguna Merin basin (Fig. 2). The structural framework controlling the devel- opment and evolution of the Santa Lucía and Laguna Merin basins is underlain by the SaLAM (Santa Lu- cia – Aiguá – Merin Lineament, after Rossello et al.; 2000). This structural feature constitutes a transtensive tectonic corridor of the strike-slip faults (ca. 450 km), cross-cutting the Río de la Plata craton. As it has been described by Rossello et al. (2001), ei- ther two pull-apart depocentres (Santa Lucia and Laguna Merín basins) at its extremes and the em- placement of Mesozoic volcanic/sub-volcanic rocks have been controlled by this structure. According to these authors, the SaLAM compounds two main tec- tonic phases: (i) a Jurassic to Early Cretaceous extensional stage, represented by intraplate mag- matism synchronous with the PMP, and (ii) an Aptian – Albian dextral transtensive stage. In this context, the tectonic development of the Sierra San Miguel Complex is controlled by the Aiguá - India Muerta – Chuy fault, one of the components of the SaLAM lin- eament that conditioned the opening and extensional development of the Laguna Merín Basin (Rossello et al., 2000). The pioneer geological works in the Sierra San Miguel correspond to Walther (1927), Caorsi & Goñi (1958) and Bossi & Fernández (1963) and it has been described as a volcanic/sub-volcanic association composed by granophyres and, subordinated rhyo- lites. These lithologies were later integrated to the Arequita Formation by Bossi (1966). The only avail- able radiometric data by K/Ar systematic (whole rock analyses) yielded ages between 125 – 130 Ma (Bossi, 1969). After recent geological studies devel- oped by Muzio et al. (2004), three petrographic asso- ciations have been identified, each-one related to different tectomagmatic stages (Fig.2). Unit 1 – Felsic sub-volcanic association (FSA) The FSA is located mainly in the east and central por- tions of the Sierra San Miguel which occurs as an ir- regular body along an EW to NE axis. It is composed by strongly fractured granophyric rocks, cross-cut by sub-vertical basic and acidic/felsic dykes with struc- tural trend N65º and EW respectively. The grano- phyres are progressively covered by rhyolitic lava flows. The fracture system (sub-horizontal and vertical unfilled fractures) outlining a roughly polyhedral pattern can also be observed. These granophyres are mineralogical and petrographycally homogeneous. The mineral association of these rocks is essentially anhydrous. They are medium to fine grain-sized, with equigranular and occasionally porphyritic hypidiomorphic textures (up to 10% of plagioclase phenocrysts). It can be observed the following miner- alogical assemblage: quartz - alkali feldspar inter- growths (micrographic/granophyric texture) and 18 ROSSANA MUZIO, ELENA PEEL, ETHEL MORALES, GERARDO VEROSLAVSKY, BRUNO CONTI 19 MESOZOIC MAGMATISM IN EAST URUGUAY: PETROLOGICAL CONSTRAINTS RELATED TO THE SIERRA SAN MIGUEL REGION E st e ro d e laP u n ta N e g ra D el Tig re C a ñ a d a R in có n Br av o S an M ig ue l w et la nd s V IG IA H IL L R od eo V ie jo cr ee k C A R B O N E R A H IL L S a nM ig u e l S an M ig ue lF or t S ar an dí cr ee k 18 de Ju lio S a ra n d í B R A Z IL 70 0 70 2 70 4 70 6 70 8 5 3 º4 1 ’ 3 3 º3 2 ’ 71 0 71 2 70 2 70 4 70 6 70 8 71 0 71 2 70 0 69 8 69 6 69 4 69 2 69 0 68 8 69 8 69 6 69 4 68 8 69 0 69 2 27 2 27 0 26 8 26 6 26 4 27 2 27 0 26 8 26 6 27 0 LO S D IF U N TO S H IL L O M B Ú H IL L 1 9 4 K m . 1 K m . 2 K m . 3 K m . 0 1 N Lo ca l r ou te R oa d / l an e C re ek s In te rn at io na l b or de r V ill ag e H ill s R hy ol ite s G ab br o G ra no ph yr es F au lts / f ra ct ur es C en oz oi c de po si ts C re e k C re ek Cr ee k U R U G U AY Br az il Argentin a S tu di ed ar ea M on te vi de o D e la Pl at a R iv er At lan tic oc ea n F ig u re 2 . G eo lo g ic al sk et ch o f th e S ie rr a S an M ig u el re g io n , af te r M u zi o et al . (2 0 0 4 ). partially altered clinopyroxene (augite) often exhibit- ing uralitization processes. Apatite and opaque min- erals are frequent as accessory minerals. In the central-north part of the area, the granophyres are hardly affected by faulting with trends N70º. Also, a synmagmatic foliation N250º - 260º can be observed. This structural trend is coincident with the emplace- ment of mafic and felsic dykes. Unit 2 – Mafic association (MA) Two different types of basic rocks have been recog- nized, according to their geological setting. The first type corresponds to basic dykes which vertically cross-cut the granophyres. As it was mentioned pre- viously, they are porphyritic basic dykes with struc- tural trend N65-70, 75º. They have porphyritic texture with plagioclase phenocrystals (up to 10 %) in a subophitic to intersertal groundmass. The min- eral assemblage is composed by plagioclase, orthopyroxene and clinopyroxene (augite), opaque minerals and apatite. The second type of basic rocks is represented by an intrusive body – named as San Miguel Gab- bro (SMG). The main outcrop area is located around the central-north region of the Sierra San Miguel (S33º43’; W53º38’), and it can be de- scribed as a sub-horizontal intrusion cross-cutting the grano- phyric rocks. It is strongly fractured with sub-horizontal and sub-vertical joints. Typi- cal spherical structures due to alteration processes, ranging in diameter from 40 to 80 cm, can be ob- served. According to the IUGS classification criteria (Le Maitre, 1989) this intrusive body can be de- scribed as a leucocratic gabbro/dolerite with mas- sive structure, with equigranular and subophitic texture, medium to coarse grain sized, composed by plagioclase (An55-45), altered olivine, orthopy- roxene, clinopyroxene (augite), opaque minerals and apatite. Occasionally amphibole - as deuteric al- teration – is present. Other small gabbroic intrusions appear as wedges or secondary dykes; all injected through the granophyres. Euhedral to subhedral plagioclase crystals surrounded by micrographic/ granophyric intergrowths are present. This petro- graphic feature is also observed as an interstitial granophyric intergrowth, and occurs plagioclase and clinopyroxene crystals with intergranular ar- rangement, in association with K-feldspar and quartz. According to Shelley (1992) this kind of texture is consistent with residual crystallization under eutectic conditions and corresponds either with the emplacement of shallow intrusions or with fast de- gassing processes. Unit 3 – Felsic Volcanic Association (FVA) They correspond mainly to pinkish - grey colored rhyolites and compound at least three lava flows softly dipping to the North. They are distributed along the north, northwestern and western region of the Sierra San Miguel. These lithologies are grouped in the Arequita Formation (Bossi, 1966) and can be separated, according to their petrographic and struc- tural features in: a) Rhyolitic lavas, occasionally with porphy- ritic textures (approximately 10 to 15% of alkali feldspar and/or quartz phenocrysts), with vesicular levels filled by calcite, zeo- lite and quartz. Most of these vesicles are stretched according to flow directions such as N170º to EW. Microscopically, spheru- litic texture and flow structures formed dur- ing the cooling of the rhyolitic lava can be observed. Some of the porphyritic varieties present abundant glass phenocrysts, round shaped and brown to dark black colors. Some volcaniclastic facies in association with the rhyolites have been found and ac- cording to McPhie et al. (1993) descriptive criteria they correspond to volcanic lithic breccias and autoclastic breccias (Fig. 3). b) Rhyolitic dykes with porphyritic texture trending N60-70º and cross-cutting all the lithologies are present in the study area. They present quartz and K-feldspar pheno- crysts in an aphyric matrix composed by 20 ROSSANA MUZIO, ELENA PEEL, ETHEL MORALES, GERARDO VEROSLAVSKY, BRUNO CONTI quartz, K-feldspar (orthoclase?) and clino- pyroxene. Their stratigraphic relationship with the mafic dykes was not observed. Lithogeochemistry Chemical composition of ten samples (one granophyre, three gabbros, one mafic dyke, three fel- sic dykes and two rhyolites) of the Sierra de San Miguel Complex was analyzed. Major and trace ele- ments were obtained by ICP – AES techniques whereas REE by using ICP – MS, performed at ACME Laboratories Inc., Vancouver, Canada. The analyti- cal data are listed in Table I and illustrated in Fig 3-7. They were plotted in different classification and vari- ation diagrams and they were also compared with similar rocks performed by Kirstein et al. (2001) and Lustrino et al. (2005) in order to evaluate the data for the region. The SiO2 variation diagrams (Fig.4) show no ex- tremely high contents in the felsic volcanic samples. However, they present high contents of L.O.I. (>3%). The analyzed samples have SiO2 ranges from 71.1 to 48.49 wt.%; TiO2 from 1.48 to 0.58 wt.%; Al2O3 from 16.22 to 11.6 wt.%; Fe2O3 from 14.2 to 5.01 wt.%; MgO from 9.11 to 0.23 wt.%; CaO from 12.04 to 0.82 wt%; Na2O from 3.33 to 2.19 wt.% and K2O from 4.89 to 0.2 wt.%. When plotted against SiO2, smooth trends are showed. TiO2, Al2O3, Fe2O3, MgO and CaO decrease with increasing SiO2 content. Na2O does not show a trend at all, while K2O increases towards the rhyolitic terms. Further- more, Na2O and K2O present an inverse behavior when compared with Lustrino et al. (2005) data. Trace element contents present a short range when they are compared with published data (Lustrino et al., 2005; Kirstein et al., 2000). Rb varies from 184.4 to 5.4; Zr from 387.1 to 58; Y from 76.6 to 22.2; Nb from 24 to 3.7; Ba from 721.1 to 111 and Sr from 207.8 to 85.4 (Fig. 5, Table I). The ratios be- tween incompatible elements (Fig. 6) vary within narrow ranges (Zr/Nb= 15.03±1.05; Zr/Y=4.11±1.54; Y/Nb= 4.39 ± 1.72). These values are very similar to the Treinta y Tres magma type pointed out by Turner et al. (1999); Kirstein et al. (2000) and Lustrino et al. (2005). The rock samples have been classified using the R1-R2 diagram (De la Roche et al., 1980) and TAS diagrams (Le Maitre et al., 1989) (Fig. 7a and 7b). Two samples of the plutonic rocks plot in the gabbro - norite field of the R1-R2 diagram. The volcanic rocks plot mainly in the rhyolitic and dacitic fields and one sample in the basaltic-andesitic field. All samples show a sub-alkaline affinity according to Irvine and Baragar (1971) diagram (Fig. 7c). How- ever, the analyzed data are not enough to establish any petrogenetic link among the samples. The agpaitic index calculated for the samples (A.I. = mo- lar (Na+K)/Al) ranges from 0.24 to 0.74. The rhyolites range from 0.71 to 0.74; dacites present val- ues of 0.66 and andesites around 0.58. The lower val- ues correspond to the gabbro – norite samples. None of the samples show a peralkaline character as de- scribed for the Arequita Formation by Muzio et al. (2002). Hence, they have been classified on the basis 21 MESOZOIC MAGMATISM IN EAST URUGUAY: PETROLOGICAL CONSTRAINTS RELATED TO THE SIERRA SAN MIGUEL REGION Figure 3. Lithic breccias and autoclastic breccias associ- ated with the felsic volcanics in Sierra San Miguel region. 22 ROSSANA MUZIO, ELENA PEEL, ETHEL MORALES, GERARDO VEROSLAVSKY, BRUNO CONTI 40 50 60 70 80 0.0 1.6 3.2 4.8 6.4 8.0 SiO2 K 2O 40 50 60 70 80 2 3 4 SiO2 N a O 2 40 50 60 70 80 0 1 2 SiO2 T iO 2 40 50 60 70 80 0 1 2 3 4 5 6 7 8 9 10 SiO2 M g O 40 50 60 70 80 0 10 20 SiO2 C a O 40 50 60 70 80 10 11 12 13 14 15 16 17 18 19 20 SiO2 A l O 2 3 40 50 60 70 80 0 10 20 SiO2 F e 2 O 3 t Figure 4. Major and minor element diagrams for samples from the San Miguel region. Symbols: � felsic dykes; � felsic lavas; � gabbros; � basic dyke; � granophyre. 23 MESOZOIC MAGMATISM IN EAST URUGUAY: PETROLOGICAL CONSTRAINTS RELATED TO THE SIERRA SAN MIGUEL REGION Table 1. Geochemical data of selected samples from the Sierra San Miguel region. Samples SM50-1 SM50-2 SM50-3 SM008-7 SM008-8 SM008-9 SM51 SM13 SM27 SM29-1 SiO2 (wt.%) 65,01 64,24 65,1 58,71 48,98 48,49 56,41 69,72 71,1 69,58 TiO2 0,88 0,92 0,87 0,89 0,58 0,59 1,48 0,76 0,63 0,7 Al2O3 12,58 12,38 12,38 13,99 16,22 16,13 12,67 12,74 11,6 12,81 Fe2O3 5,87 6,43 5,66 8,89 9,53 9,8 14,2 6,15 5,01 5,66 MnO 0,14 0,14 0,13 0,17 0,15 0,15 0,2 0,11 0,16 0,13 MgO 1,3 1,44 1,28 3,28 9,11 8,95 2,85 0,3 0,67 0,23 CaO 3,95 4,18 3,99 6,44 11,78 12,04 6,28 1,71 1,75 0,82 Na2O 3,33 3,13 2,97 3,18 2,25 2,19 2,79 3,24 2,42 2,28 K2O 2,77 2,76 3,12 2,42 0,2 0,25 2,54 3,81 4,22 4,89 P2O5 0,26 0,23 0,24 0,28 0,09 0,11 0,19 0,2 0,17 0,16 Cr2O3 0,004 0,005 0,015 0,013 0,053 0,068 0,019 0,015 0,041 0,008 LOI % 3,8 3,9 4 1,5 1 1,2 0,3 1,1 2,1 2,6 TOT/C % 0,03 0,02 0,03 0,03 0,03 0,05 0,02 0,05 0,05 0,04 TOT/S % 0,01 <.01 <.01 <.01 0,01 0,06 0,01 0,01 0,01 0,01 SUM % 99,89 99,76 99,76 99,77 99,98 100 99,95 99,86 99,89 99,88 Rb (ppm) 135,8 127,3 142,5 89,9 7,3 5,4 88,8 150,7 158,8 184,5 Ba 625 599,2 583,9 458,1 111 112,6 365,4 721,1 654,6 712,6 Sr 136,8 146,3 146,3 207,8 133,3 139,1 123,1 125,9 92,8 85,4 Zr 301,1 276,4 289,6 237,1 72,2 58 189,4 387,1 293,5 327,9 Y 57,6 56 56,9 56,4 22,2 22,6 50,2 76,6 64,4 58,1 Ga 15,9 16,1 15,8 19,4 14 14,5 19,7 18 15,9 17 Hf 8,6 8,1 8,5 7,2 2,2 1,8 5,7 11 8,7 9,3 Nb 21,6 19,5 20,8 16,6 4,6 3,7 13,4 24 18,7 20,4 Ta 1,4 1,4 1,5 1,2 0,4 0,3 1 1,7 1,4 1,5 Th 16 14,2 15,4 7,4 1,8 1,2 8,1 14,2 14,4 14,7 U 2,5 2,3 2,6 1,4 0,4 0,3 1,3 1,7 2,5 2,7 Ni 29,7 18,8 67,5 31,5 110,5 118,1 65,2 43,9 152,1 39 La 38,8 37,5 40,1 28,7 7,3 7,3 23,1 46,6 42,4 40,7 Ce 96,4 91,8 99,4 68,7 16,9 16,1 54,2 107,2 99,6 102,2 Pr 10,71 10,06 10,77 8,41 2,24 2,18 6,47 13,1 11,38 10,17 Nd 44,3 41,3 43,6 36,8 10,2 9,9 27,3 54,8 45 41 of the Alumina Saturation Index (A.S.I. = molar Al/ (Na+K+Ca)) yielding values from 0.63 to 1.24. Most of the samples present a metaluminous character (A.S.I. <1), and only three of them present a weak peraluminous character (A.S.I.>1). The samples present moderate light to heavy REE fractionation in chondrite normalized plot [(La/Yb) Í = 5.02 to 2.25]. In one hand, the rhyolites and the acidic to basic dykes have very strong negative Eu anomalies (Eu/Eu* = 0.23 to 0.19), and a flat to slightly concave pattern of HREE. On the other hand, the gabbro – norite samples show a flat pattern when REE normalized against chondrite C1 (La/Yb) N = 5) and the highest values for Eu/Eu* (from 0.26 to 0.28) yielding less marked negative Eu anomalies (Fig. 8a). The rocks analyzed do not show typical patterns in the primitive mantle normalized spidergram when compared with the early Cretaceous Santa Lucía type and Aiguá type magmas (Turner et al., 1999; Kirstein et al., 2000). Negative peaks of Ba, Nb, Sr and Pb are present. Only the rhyolitic samples do not show Pb troughs (Fig. 8b). The plutonic rocks show almost a flat pattern in primitive mantle normalized diagram. Enrichment in Ba, Th and U is observed. Discussion and Conclusions Field relationships, petrography and geochemistry indicate a bimodal character for San Miguel Com- plex and reflect the general character of extensional magmatism. This Complex is associ- ated with a regional fault system responsible for the opening and extensional development of the Laguna Merín Basin. 24 ROSSANA MUZIO, ELENA PEEL, ETHEL MORALES, GERARDO VEROSLAVSKY, BRUNO CONTI Samples SM50-1 SM50-2 SM50-3 SM008-7 SM008-8 SM008-9 SM51 SM13 SM27 SM29-1 Sm 9,1 8,9 9 8,5 2,7 2,5 6,1 11,7 10,3 8,6 Eu 1,87 1,86 1,79 2,26 0,77 0,79 1,55 2,35 2,02 1,84 Gd 9,35 9,08 9,41 9,03 3,08 3,24 7,4 11,97 10,31 9,06 Tb 1,8 1,7 1,75 1,78 0,64 0,66 1,41 2,36 2,01 1,81 Dy 9,85 9,57 9,84 9,68 3,58 3,92 8,07 12,87 10,92 10,08 Ho 1,94 1,95 2,05 1,95 0,75 0,81 1,76 2,53 2,16 1,99 Er 6,08 5,95 6,18 5,91 2,32 2,37 5,25 7,59 6,55 6,06 Tm 0,98 0,91 0,92 0,87 0,36 0,36 0,82 1,16 0,96 0,94 Yb 5,86 5,72 5,83 5,69 2,33 2,19 5 7,09 6,17 5,82 Lu 0,87 0,85 0,88 0,85 0,35 0,33 0,75 1,06 0,94 0,84 Zr/Nb 13,94 14,17 13,92 14,28 15,70 15,68 14,13 16,13 15,70 16,07 Zr/Y 5,23 4,94 5,09 4,20 3,25 2,57 3,77 5,05 4,56 5,64 Y/Nb 2,67 0,46 0,49 0,38 0,10 0,09 0,47 0,39 0,54 0,56 (La/Yb)N 4,75 4,70 4,93 3,62 2,25 2,39 3,31 4,71 4,93 5,02 Eu/Eu* 0,20 0,21 0,19 0,26 0,27 0,28 0,23 0,20 0,20 0,21 Rb/Sr 0,99 0,87 0,97 0,43 0,05 0,04 0,72 1,20 1,71 2,16 Agpaitic Index 0,673 0,657 0,667 0,561 0,241 0,24 0,579 0,742 0,736 0,705 A.S.Index 0,82 0,801 0,813 0,728 0,643 0,629 0,684 1,034 1,007 1,239 25 MESOZOIC MAGMATISM IN EAST URUGUAY: PETROLOGICAL CONSTRAINTS RELATED TO THE SIERRA SAN MIGUEL REGION Thus, the Sierra San Miguel morphological fea- ture shows a clear correspondence with the main faults of the basin and the Aiguá – India Muerta – Chuy lineament, trending N60º which is also discor- dant with other regional structures of the Punta del Este basement. The studied area shows structural concordance between the conspicuous foliation (N60º – 70º) ex- posed in the granophyres and the brittle deformation trends (E-W and N70º) affecting all the Mesozoic rocks, including the regional trend of the Sierra San Miguel The stratigraphic array observed for the sub-vol- canic/volcanic lithologies (gabbro, granophyres and rhyolites) shows that the fault system had important vertical components synchronous with the magmatic events in order to formulate a close spatial-temporal relation between these rocks. The San Miguel Gab- bro should be linked with the tectomagmatic event responsible of the opening of the Laguna Merín basin 0 100 200 300 400 20 30 40 50 60 70 80 Zr Y 0 100 200 300 400 0 10 20 30 Zr N b 0 100 200 300 100 200 300 400 500 600 700 800 Sr B a 45 50 55 60 65 70 75 80 0 20 40 60 80 100 120 140 160 180 200 SiO2 N i 0 5 10 15 20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 Th T h /N b Figure 5. Trace element diagrams for samples from the San Miguel region. Symbols: � felsic dykes; � felsic lavas; � gab- bros; � basic dyke; � granophyre. 26 ROSSANA MUZIO, ELENA PEEL, ETHEL MORALES, GERARDO VEROSLAVSKY, BRUNO CONTI 0 100 200 300 400 0 10 20 30 Zr N b (A) 0 100 200 300 400 20 30 40 50 60 70 80 Zr Y (B) 0.001 0.01 0.1 1 10 40 50 60 70 80 Rhyolite Rhyodacite-Dacite Andesite TrAn Sub-Ab Bas-Trach-Neph Com/Pan Trachyte Phonolite Zr/TiO2*0.0001 S iO 2 ( C) Figure 6. Trace element diagrams and trace element ratio diagram for samples from the San Miguel region. Sym- bols: � felsic dykes;� felsic lavas; � gabbros; � basic dyke; � granophyre. 16 14 12 10 8 6 4 2 0 40 50 60 70 80 SiO2 N a O + K O 2 2 (B) (C ) 35 40 45 50 55 60 65 70 75 80 85 0 2 4 6 8 10 12 14 16 18 20 Alkaline Subalkaline SiO2 N a 2 O + K 2 O Gabbronorite Olivine gabbro Alkali gabbro Syenogabbro Syenodiorite Essexite Nepheline syenite Syenite Quartz syenite Alkali granite Syenogranite Monzogranite Granodiorite Diorite Gabbro Tonalite Quartz monzonite Monzonite Monzonitediorite M on zo ga bb ro 400 400 800 800 1200 1200 1600 1600 2000 2000 2200 2400 2800 0 0 200 600 1000 1400 1800 R1 R 2 (A) Figure 7. Classification diagrams. (A) Multicationic clas- sification diagram (De la Roche et al. 1980) ; (B) TAS classi- fication diagram (Le Maitre et al. 1989); (C) TAS classification diagram (Irvine and Baragar, 1971). Sym- bols: � felsic dykes; � felsic lavas; � gabbros; � basic dyke; � granophyre. that also allowed the generation of a great depocentre filled by basic lavas named Puerto Gómez Formation (Bossi, 1966), based on the geological and structural framework. These evidences allow us to point out a continu- ous reactivation of the basement which conditioned the faulting and jointing style that affected all the Me- sozoic rocks. The structural trends presented by the dykes and the spatial arrange exposed by faults and fractures, most of them concordant with ancient planes of weakness along the basement, remark the presence of dilatants situations in agreement with the dextral sense of displacement proposed for the fault system Cebollatí – Merín and Aiguá – India Muerta – Chuy. This structural framework, according to Rossello et al. (2000) determines the development of the SaLAM lineament and was the main mechanism responsible for the genesis and tectomagmatic evolu- tion of the Laguna Merín basin. The spatial distribu- tion, type and volume of magmatism related to the Laguna Merín basin, allow us to suggest that it repre- sents a magmatic pull-apart type depocentre, sub par- allel to the Atlantic margin and developed in a pre-structured cratonic domain. According to this, the volcanic rocks exposed in the eastern region of Uruguay would represent dif- ferent magmatic pulses synchronous with the tec- tonic reactivation of the basement. This would explain the coexistence of intrusive/shallow and ef- fusive terms in a relative short time of emplacement. As a result of the drill campaign performed by pri- vate founds near Lascano city (www.ume.com.uy), the rocks described correspond to gabbros/dolerites and granites (granophyres), providing us valuable information close to the Sierra San Miguel region. The Lascano geophysical anomaly has been inter- preted as being created by gabbro to granitic (grano- phyres) intrusive rocks underlying a package of magnetic basalts and felsic volcanic rocks. What- ever be the trigger responsible for the genesis of this gravimetric anomaly, its development can be related to the tectonic evolution of the SaLAM lineament during the early stages of the South Atlantic open- ing (Rossello et al., 2007). According to the geological background and field relationships among the different units, the ba- sic and acidic magmatism are considered Mesozoic in age and related with the magmatic events of conti- nental scale predecessors of the opening of the South Atlantic Ocean (Lustrino et al. 2003). Further chemi- cal analyses should be carried out in order to explain petrogenetic correlations between these magmas and the main magma types recognized by other authors. Acknowledgments The Authors are grateful to the Comisión Sectorial de Investigación Científica for the financial support to 27 MESOZOIC MAGMATISM IN EAST URUGUAY: PETROLOGICAL CONSTRAINTS RELATED TO THE SIERRA SAN MIGUEL REGION S a m p le /P ri m it iv e M a n tl e S a m p le /C 1 C h o n d ri te 1 10 100 500 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu (A) 0.0001 0.001 0.01 0.1 1 10 100 1000 Cs Rb Ba Th U Nb Ta La Ce Pb Pr Sr Nd Sm Zr Hf Eu Gd Tb Dy Y Ho Er Tm Yb Lu (B) Figure 8. (A) REE patterns for samples from the San Miguel region normalized according to Taylor and Mc Lennan (1985). (B) Spider diagrams normalized according to Sun and Mc Donough (1986). Symbols: � felsic dykes; � felsic lavas; � gabbros; � basic dyke; � granophyre. the Project “Magmatismo Mesozoico de la región sureste de Uruguay”, grant 2005 - 2007. Special thanks to Dra. L.Sánchez and to the Editorial Board for the revision and helpful comments. 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