Ordovician Magmatism in the Sierras Pampeanas of Córdoba

Aldo BONALUMI and Edgardo BALDO

 

Abstract: ORDOVICIAN MAGMATISM IN THE SIERRAS PAMPEANAS OF CÓRDOBA. During the Ordovician period, the Sierras Pampeanas of Córdoba constitute the foreland of the Famatinian orogenic belt. The gneissic-migmatic basement and granitoids of lower Cambrian age was deformed by ductile shear zones and intruded by igneous rocks related to the Famatinian magmatic arc.In the lower to medium Ordovician period (499-460 Ma) little plutons and dikes of tonalitic, trondhjemitic, granodioritic, (TTG magmatism) and, to a lesser degree, granitic composition were emplaced. The plutons are elliptic or round-shaped, they cut the metamorphic foliations of lower Cambrian age, and, in contact with marbles, they form high to medium temperature metasomatic skarns. The plutons are mainly concentrated in the eastern side of the ranges (Sierra Chica, Sierra de Cuniputo and Sierras de San Marcos) and in this area they are of tonalitic-trondhjemitic composition with minor granodiorites. In the Sierra de Guasapampa, stocks of granitic-granodioritic composition of similar age occur. These geographical distribution suggests a systematic compositional variation of these plutons, with predominance of the tonalitictrodhjemitic varieties eastwards and granitic-granodioritic types westwards. In the Sierra Norte of Córdoba, rhyolite dikes of shallow level of emplacement and little plutons of granitic composition are recognised; The TTG magmatism of the Ordovician period in Córdoba is rich in Na2O ( 3.2 – 5.8%), CaO (3.4 – 2.6%), and Sr (800 – 400 ppm), very depleted in K2O (0.65 – 2.02%), FeOt (0.70 – 1.20%), Cs (0.2 –2.3 ppm), Y (1 – 23 ppm) and U (0.2 – 1.5 ppm). Besides, this magmatism has very low Rb/Sr ratios (0.015 – 0.12), positive or absent Eu-anomaly (Eu/Eu*= 0.9 – 2.4), [La/Yb]n ratios between 20 and 55, relatively low 87Sr/ 86Sr initial ratios (0.705 – 0.707) and eNd between + 1.5 and – 4.8. This geochemical signature suggests at least for some of the trondhjemitic units, a deep magmatic source and probably astenospheric mantle source.In the regional context, the Ordovician magmatism of Córdoba represents an inner magmatic arc, emplaced in the Pampean basement of the western Gondwana margin. This arc developed contemporaneously with the main Famatinian magmatic arc, located westwards; the latter gave rise to huge metaluminous “I-type”, metaluminous and calcic batholitic units that outcrop in the Chepes, Ulapes and Valle Fértil ranges.

Resumen: EL MAGMATISMO ORDOVÍCICO EN LAS SIERRAS PAMPEANAS DE CÓRDOBA. Durante el período Ordovícico, las Sierras Pampeanas de Córdoba constituyeron el antepáis del cinturón orogénico Famatiniano, el basamento gneisico-migmático de edad Cámbrica inferior fue intruido por rocas ígneas vinculadas al arco magmático famatiniano y deformado en fajas discretas de cizallamiento dúctil. En el Ordovícico inferior a medio (499 - 460 Ma) se emplazaron pequeños unidades plutónicas y diques de composición Tonalítica, Trondhjemítica, Granodiorítica y minoritariamente Granítica (magmatismo TTG). Los plutones presentan formas circulares o elípticas en planta, cortan las foliaciones metamórficas del Cámbrico inferior y generan aureolas metasómaticas de alta temperatura en contacto con los mármoles. Se concentran mayoritariamente en el sector oriental de las sierras, (Sierras Chicas, Sierras de Cunuputo y San Marcos ) y en estos sectores la composición es esencialmente Tonalítica a Trondhjemítica siendo menos frecuente las granodioritas. En la Sierra Norte de Córdoba se reconocen diques riolíticos de emplazamientos muy someros y plutones menores de composición granítica, en tanto que al oeste, en la Sierras de Guasapampa afloran plutones de composición granítica y granodiorítica de edad similar a los anteriores. La distribución geográfica sugiere una variación sistemática de la composición de estos plutones con un incremento de las variedades graníticas - granodioríticas hacia el oeste y de las variedades trondhjemíticas - tonalíticas al este. El magmatismo TTG del Ordovícico de Córdoba se caracteriza por ser rico en Na2O ( 5,8 - 3,2%), CaO (3,4 - 2,6%) y Sr (800 - 400 ppm ), muy deprimido en K2O (0,65 - 2,02%), FeOt (0,70 - 1,20%), Cs (0,2 -2,3 ppm), Y (1 - 23 ppm ) y U (0,2 - 1,5 ppm). Presentan además muy bajas relaciones de Rb/ Sr (0,015 - 0,12), ausencia de anomalías negativas de Eu (Eu/Eu*= 0,9 - 2,4), relaciones [La/Yb]n entre 20-55, relaciones iniciales 87Sr/ 86Sr relativamente bajas (0,705 - 0,707) y (Nd entre +1,5 a - 4,8. Esta signatura geoquímica sugiere, al menos para algunas de las unidades trondhjemíticas, una fuente magmática profunda y probablemente de vinculación con el manto astenosférico.

En el contexto regional, el magmatismo Ordovícico de las Sierras de Córdoba representa un arco magmático interno, emplazado sobre el basamento pampeano del margen occidental de Gondwana, el que se desarrolló contemporáneamente con el arco magmático principal famatiniano, ubicado mas al oeste, y que dio l ugar a la formación de las grandes unidades batolíticas de tipo ¨I¨, metaluminosos, como los aflorantes en las sierras de Chepes - Ulapes y Valle Fértil.

Keyword: Ordovician magmatism. Córdoba. Argentina

Palabras clave: Magmatismo Ordovícico. Córdoba. Argentina.

Introduction

The Sierras Pampeanas of Córdoba (SPC) has a complex tectonothermic and magmatic history initiated in the lower Cambrian – Upper Proterozoic, with the formation of a calc-alkaline magmatic arc, emplaced on continental crust and followed by a cortical thickening with development of a high grade metamorphism and anatexis, that formed migmatitic complexes with Cdr + Grt + Sil + Kfs (minerals symbols after Kretz, 1983) and “S-type” granites of 520 Ma. (Rapela et al., 1998, Sims et al., 1998 and references therein). This evolution is assigned to the Pampean Orogenic Cycle proposed by Aceñolaza and Toselli, (1972) and in the south-east sector is characterized by a short life ( 25-35 Ma) and fast exhumation of the metamorphic complex (Baldo et al., 1999).

In the lower to middle Ordovician period, the Cambrian basement of the SPC has been the foreland of a new orogenic cycle (Famatinian Orogenic) and the magmatic activity is manifested in the development of small and isolated plutons, distributed in the Sierra Chica, Sierra Grande and Sierra Norte (Fig. 1). Seventeen main plutons are adscript to the famatinian magmatic event, which have been studied by several authors and with different degree of detail and that are assigned to the Ordovician magmatism through geochronologic dating or geochemical correlation. These plutons are generally of reduced dimensions (3 to 10 km of diameter) and its composition is mainly trondhjemitic – granodioritic for the ones located in the oriental sector of the SPC, granodioritic in the central sector and granitic – granodioritic in the occidental sector. In the central and northern sector of the Sierra Chica and in the Sierra of Cuniputo, swarms of tonalitic dikes are observed, with N-S directions and subvertical dip (>70°). On the other hand, in the Sierra Norte of Córdoba the plutons and subvolcanic rocks are of granitic and rhyolitic composition.

The main tectonometamorphic event assigned to the Ordovician of Córdoba is the formation of important milonitic belts in the Green schists facies, with N-S directions and high grade dip to the east, that represents inverses faults with top to the west (Martino et al., 1993 a-b, Simpson et al., 2001). The age of this deformation is not well constrained and the location in the Ordovician time is on accounts of cut relations with the plutonic events and in general restricted to the upper Ordovician, which allows primary to correlate it with the Ocloyic tectonic phase of the Famatinian cycle.

The Tres Arboles shear belt, in the southwestern of Córdoba, is a main milonitic shear zone of SPC and probably represent the principal tectonic contact between Pampean and Famatinian Orogenic Belt (Whitmeyer and Simpson, in press). Recent observations carried on by Fagiano et al. (2002), in the milonitic belt Guacha Corral, state that the dominating planar fabric occur after the thermal peak and is determined by two retrograde associations, one in amphibolites facies (Bt + Sil) and the other

 

one in green schists facies (Chl+ Ms + Ser). These events would have been produced during the Famatinian Cycle. However, Martino et al., (2002) propose an Upper Cambrian ages for the milonitic belt of Los Túneles, in the west sector of SPC.

This paper is a review and a compilation of the petrographic, geochemical and geochronological data of the main plutons and dikes assigned to the Ordovician in the Sierras of Córdoba, and its integration in the regional context is discussed.

Figura 1. Geological map of the Sierras de Córdoba. Locations and outcrop.

Location, petrographic and geochronological data

I- Sierra Chica Sector

1. Güiraldes Trondhjemite: (30°57.5’ LS - 64°27.1’LW). This pluton is located to the northern sector of Sierra Chica (Fig.1) and it was described by Perez et al., (1996 a and b), Lyons et al. (1997) and Rapela et al. (1998). It has a roughly elliptic shape of about 10 km of length, it is lengthened in NW direction and intruded in a discordant way to the metamorphic basement, It carry xenoliths of schist, amphibolites and marbles, producing on the latter an epidotic skarn. It has also microgranular enclaves with Pl + Qtz + Bt + Ms (tonalitic) and enclaves of dioritic rocks with Pl + Hbl (>50%). It has a rough foliation, but penetrative, which accentuates towards the pluton’s margins. Is a semibritlle deformation that produce the orientation of the muscovite and quartz crystals, which anastomosely surround the euhedric crystals of plagioclase, the foliation has an average direction of N 310°/ 65 E.

The magmatic texture is granular and is composed by 40-45% of Pl (An 20-25) + 35-40% Qtz + 10% Ms ± Bt ± Kfs, as accessories Ep, Zr, Ap, Mnz and Chl + Ep2 as Bt alteration, the color index is < than 8, which allows to be classified as trondhjemite.

Rapela et al., 1998 obtained for this magmatic unit an age of 499 ± 5Ma (U-Pb SHRIMP in magmatic zircon), an age of 496 ± 2 Ma (U-Pb traditional in zircon) and an age of 438 ± 9 Ma (UPb SHRIMP in monazite).The firsts represent a crystallization ages and is the most ancient registered for this type of magmatic unit in the SPC. The age of 438 ± 9 Ma in monazite could represent the postcrystalline deformational event that affected the Güiraldes trondhjemite.

2. San Agustín Trondhjemite: (31° 58’38’’LS - 64° 23’57’’LW). Located in the southern sector of the Sierra Chica, it is the minor of a group of similar bodies of this sector and it is constituted by discontinuous and elliptic outcroppings with a inner deformation of fragile type, that produces a very pronounced foliation of NNW direction. It is a white grayish rock, of thick granular texture and compound by Pl (andesine) + Qtz + Bt + Ms and ± Kfs. Rapela et al., (1998) characterize it geochemically and define it as a very characteristic trondhjemite, fact through which the authors infer an Ordovician age.

3. Calmayo Trondhjemite: (32° 01’46’’LS - 64° 27’29’’LW). The Calmayo pluton, located to the south of the previous body, is one of the most representative, it presents an approximately circular shape and it has about 3.5 km of diameter. The petrological features were described by Martino and Fagiano (1982) and Martino et al., (1995). Has been emplaced discordantly in the metamorfic complex of the southern Sierra Chica, it has altered xenoliths of mafic rocks, gneisses and marbles, with a metasomatic alteration on the latter. It is also observed a marked development of fractures produced by an overimposed brittle deformation event. It is a leucotonalite with >> Pl (An 20) + Qtz + Bt ± Ms ± Kfs, as main accessories Ap, Zr and Ep and sericite as subsolid alteration products. Sinmagmatic aplitic intrusives and variation of the grain size towards the contacts are distinguished inside the pluton.

San Agustín trondhjemite as well as the Calmayo one do not present isotopic dating, nevertheless the composition and the field relations allow that these are incorporate in to the magmatic event of Ordovician age.

4. El Hongo Trondhjemite: (32° 11’48’’LS - 64° 23’21’’LW). El Hongo pluton is located in the environs of the locality of Embalse de Rio Tercero. It is emplaced, as well as the Calmayo pluton, in the Metamorphic Complex of south of Sierra Chica. It presents a slightly rectangular shape of about 4 km of length and it is oriented in ENE-WSW direction. It was described by Martino and Fagiano, (1982) and more recently by D’Eramo et al., (1999).

The contacts with the country metamorphic rocks is tectonic and the pluton is densely cracked, mainly in the oriental sector where the rock presents an evident cataclastic deformations with development of tectonics breccia.

The tonalite presents a light gray color, in part whitish, it has medium to coarse grain and it is composed by >> Pl (An 21-24) + Qtz + < 10% Bt ± Kfs. The texture is granular hypidiomorphic with the plagioclase in irregular crystals and with notably concentric zoned texture. In the central sector of the pluton, it has a more basic facies with Hb (D’Eramo et al., 1999). The same authors cited an age of 434 ± 22 Ma (K-Ar in biotite). Considering that the Bt is a little stable mineral for the K-Ar system and that its closing temperature is relatively low, it is probable that it does not reflect the age of crystallization, but the age of cooling or of some other posterior tectonic event. These biotite K-Ar age is very similar to the monazite age of 438 ± 9 Ma in the Güiraldes pluton.

5. Tonalitic dikes: In the central part of Sierra Chica and specially along the Quebrada del Río Suquía, it is possible to observe outcroppings of little tabular bodies that have variable thickness from only 0.5 m to 40 m as maximum, which form swarms of parallel dikes. These intrusive were firstly described by Gordillo, (1958) and Gordillo and Lencinas (1979) and more recently by Toselli (1999).

The dikes have a very regular directions almost N-S and subvertical dipping, they cut with low angle the metamorphic foliation S2 of general directions 320º/60E. They always present a sharp contact with the country rock, they can show a chilled margins and angular metamorphic xenoliths are common in the next to the contact. These dikes are late in correlation with the other intrusive rocks of the same area, they are only cut by the granitic pegmatites, of probable affinity with the Achala magmatism (Upper Devonian).

Gordillo, (1958) defined three main types:

a) Biotitic Tonalites: 45-48 % Pl (An 25-30) + 25-30 % Qtz +15-20 % Bt ± Kfs ± Hbl (Ttn + Ap + Mag + Zrn). They have a fine to medium granular or porphyritic texture with phenocryst of euhedric plagioclase and frequently with oscillatory concentric zoning and a thin external zone of albitization (less than 0.01 mm). The biotite is the only mafic mineral present and it is generally abundant, with zircon inclusions and pleochroic halos. This type forms the most wide dikes.

b) Hornblendic Tonalites: Pl + Qtz + Hbl ± Bt (Ttn +Ap + Ep). This rocks are textural and chemically very different to the previous ones, they do not have porphyric texture and they are of fine grain (0.1 to 0.2 mm) and the biotite is substitutes by hornblende.

c) Tonalitic Porphyries: Compositionally are similar to the previous ones, but they present a well marked porphyric texture with aphanitic matrix. It has two generations of plagioclase, one it is a basic oligoclase with concentric zonal structure and that forms euhedral phenocrysts, and the other, it is of less size and more albitic. The quartz is totally in the fine matrix. The mafic mineral can be Bt as well as Ho. Ep, Ap, Py and Ti in less proportion complete the minerology. In general, these are dikes of small thickness.

A Trondhjemitic variety of these dikes is recognizes by Toselli (1999) and it has light gray color, with granular or porphyric textures with 60% of Pl + 34% Qtz + 6% of Bt ± Ms and Ap, Zr and Op as accessories. They have fluidal textures and an important postcrystalline deformation.

The textural and field evidences suggest a lower emplacement of these intrusives rocks, which would have emplaced in a relatively could and rigid basement. The age of the tonalitic dikes is not well known in the Quebrada del Rio Suquia, nevertheless, On the similar dike of Las Tunas (environs of Cosquín city), Gromet (com. Pers.) obtained an age U-Pb in zircon of 460 ± 7 Ma (upper intersection of the concord over three points). If this represents a crystallization age, these intrusives would represent the closing of the Famatinean magmatic event in the SPC. The cooling age of 428 ± 12 Ma (40Ar - 39Ar in Bt) for pseudotachylytes in the Rio Suquía sections (Northrup et al., 1998) probably marc the upper limit for the ductile deformation and for the emplacement of these intrusives rocks.

6. Piedra Grande Tonalite: (31° 05’23’’ LS - 64°31’56’’LW). In the place Alto de la Piedra Grande, located in the eastern sector of the “El Perchel- Pampa de Olaen block¨, outcrop intrusives rocks of tonalitic to granodioritic composition, associated with aplitics and pegmatitics.

The Piedra Grande Tonalite is the most representative pluton, it has 1,200 m of length by 500 m of wide (Pastore, 1932; Gomez, 1983) and it has a NNE-SSW directions, intrudes in a subconcordant way to La Falda metamorphic complex and carry xenoliths of this late of centrimetrical sizes. A little marked foliation is recognized. It is classified as a Tonalite of dark gray color, composed essentially by Qtz + Pl + Bt and scarce Kfs and Ms, the texture is granular with fine to medium grain and change to porphyric in the marginal facies with phenocrysts of plagioclase. Though there are not precise dating of these rocks, this type of bodies, not very deformed, post metamorphic and of tonalitic composition, are similar to the bodies of the Ordovician age of the region.

The only geochronological information that is disposed of this unit it is an age of 377 ± 15 Ma (K – Ar in Bt, Linares and Gonzalez 1990) and it probably represents a late deformation stage.

7. Granitic Pluton of Loma Ancha: The next of La Calera city, in the Quebrada del Río Suquía, outcrop a little pluton with aproximately elliptic shape of about 500 m long by 350 m wide and lengthened in NNO-SSE direction. It intrudes the cordieritic migmatites of La Calera and the high temperature milonites of La Estanzuela shears belt, it is affected by a brittle deformation that deforms the quartz crystals and produces “kink band” micro folds in the muscovite. This granite is intruded by the tonalitic dikes previously described.

It has a granular texture of coarse grain with big flakes of secondary muscovite (5-7 mm), the modal composition is: 35% Qtz + 21% Kfs + 37% Pl + 5-15% Ms Bt and Ap, Zr and Grt as accessories (Gordillo, 1958, Toselli, G.1999).

The only age of this ganite is a K-Ar age in Ms ( 447 ± 15 Ma, Linares and Gonzalez 1990) and probably it represent the stage of the blastesis of the secondary muscovite. The granite cut de migmatic fabrics of the cordieritic migmatite with a metamorphism age of 522 ± 8 Ma (U-Pb in monazite, Rapela et al., 1998), and is intruded by the tonalitic dikes of 460 Ma for which its age would be between these two events.

II- Sierra de Cuniputo - San Marcos and Ascochinga Sector

8. Cruz del Eje Granodiorite: This outcropping is a small lightly circular stock of granodioritic composition which dimensions do not overcome the 200 m of diameter. It outcrops in the environs of the Cruz del Eje dyke’s wall, its color is white grayish with dark spots as a consequence of the presence of biotite. It presents a granular holocrystalline of medium grain texture. Its mineralogy is represented by lightly zoned plagioclase, microcline, with a great development and poiquilitic texture, that includes almost all the mineralogy, the quartz presents a light wavy extintion, but neigther deformation nor pulverization are observed, the biotite and muscovite complete the mineralogy.

Though there are no specific geochronological data about this rock, this type of no-deformed bodies present compositional particularities, geomorphologicaly and structurally much alike to the ones attributed to the Famatinian magmatic cycle.

9. La Fronda Trondhjemite: (30°52’03’’LS - 64°38’18’’LW). This magmatic unit was described by Massabie, (1982), Caffe (1993), Caffe and Baldo, (1994) and Lyons et al., (1997) and geochemically characterized by Rapela et al., (1998). It outcrops to the south of the Quilpo quarry and to the west of Rio Pintos (Fig.1). This is a ovoidal pluton of 6 km of length by 4 km of wide, it is elongated in the same directions to the foliation of the metamorphic country rocks and partly interdigitated with this, but its relation is clearly discordant in the north and south contact. It intrudes gneisses, schists and amphibolites of the Cruz del Eje-La Falda Metamorphic Complex, remaining these as xenoliths and pendants inside the pluton. In the North contact, it intrudes the marbles of the Quilpo Formation, developing skarns of Grt-Wo. It has a penetrating foliation in the border of the body which diminishes towards the core, the foliation is given by the orientation of the secondary muscovite and biotitic schlieren.

It is a leucotonalite with granodioritic and granitic facies, has a light gray color, equigranular texture of coarse grain (2 to 10 mm), formed by 30-40 % Qtz + 35-50 % Pl + 2-4 % Kfs + 5-16 % Ms (primary and secondary) + 3-6 % Bt, as accessories Ap, Zr, Mnz, Op and also Ep secondary, Chl as alteration assemblages.

The first ages for this intrusive body were published by Massabie (1982), who obtains an age of 455 Ma ± 15 (K-Ar in Bt). More recently, Rapela et al., (1998) define a Rb-Sr isochrone of 474 ± 6 Ma (MSWD = 4.3).

10. Granitic Pegmatites: In different sectors of the Sierras de Córdoba swarms of pegmatitic that intrude the metamorphic basement are distinguished. These pegmatites has a similar features that in the rest of the Sierras Pampeanas. They are very rich in potassium feldspar and muscovite and they are concentrated in sectors which seems to indicate a deformational control in its emplacement. In the Ascochinga-Carape area and in the Sierra de Cunuputo with ages of 435±7 Ma and 447±7 Ma (K-Ar in Ms) were cited and these ones can represent the intrusion or deformation age (Rapela et al., 1998).

III- Sierra Norte sector

11. Oncan Rhyolites: Located in the northern extreme of the Sierra Norte of Córdoba and south of Santiago del Estero, they are a subvolcanic dikes that have up to 40 m of thick and they can reach 5 km of legth (Bonalumi, 1988). In the environs of Oncan, in Santiago del Estero province, they outcrops as extrusive rhyolitic bodies. They intrude the granodiorites and granites with Hbl + Bt of the magmatic arc of the lower Cambrian and they are rocks of pale pink color, of fine to very fine granulometry with porphyric texture in the hypabyssal bodies to microporphyric in the rhyolites themselves. The most abundant mineral is the quartz, that is observed very clean and not pressed (the rock reaches up to 78% of SiO2 in the most evolving terms), the potassium feldspar and the plagioclase, that is An (8-10), complete the minerology. The rock can be classified as leucorhyolite, the only mafic minerals are the scarce laminas of microcrystalline biotite, this fact is geochemically corroborated, since the FeO values do not overlap the 0.85% on these rocks an age of 494 ± 11 Ma was observed (Rb-Sr Isochrone, MSWD=2.2, Rapela et al., 1991) locating it in the Lower Ordovician (Tremadocian-Arenig).

12. El Cerro de Tulumba Granite: (30°23’66’’LS - 64°05’44’’LW). In the austral sector of Sierra Norte of Córdoba, at 3 km to the east of the locality of Villa Tulumba, there outcrops a small subcircular pluton of 1.5 km of diameter denominated El Cerro granite (Mazzieri and Baldo 1994; Baldo et al., 1999). It intrudes to a deformed porphyritic granite, belonging to the magmatic serie of the Lower Cambrian, showing a sharp contact that interrupts the foliation of the porphyric granite.

In the El Cerro granite neither enclaves or metamorphic xenoliths are observed, either a postcrystalline deformation is identified, which is very evident in the other granitic units of the sector, allowing to infer a late emplacement and genetically unbound of the main magmatic Cambrian event.

The composition of the pluton is homogeneous and it is a leucocratic monzogranite, with granular texture and medium grain size, composed by 34-38% Qtz + 23-34% Pl (albitic) + 23-30% Kfs (microcline perthitic) + 5-12% Ms and as accessory Ap, Zr and small garnet crystals. An isochron Rb-Sr suggest an age of 434 ± 39 Ma (MSWD = 0.2, Baldo et al., 1998), that would locate this granite in the Upper Ordovician.

13. Villa Albertina Granite: (30°41’33’’LS – 64°17’59’’LW). It outcrops to the north and to the south of Villa Albertina and it is a N-S elongated pluton that has up to 8 km of length by an average wide of 2.5 km. It was described by Gordillo et al., (1997) as a monzogranitic pluton (Qtz + Pl + Kfs + Ms + Bt), with granular texture an medium grain, which presents scarce deformation and it intrude the Ischilín mylonitic belt, including xenoliths of this one. It is important to highlight that the Sierra de Ischilín, where this granitoid is emplaced, occupies the meridional extreme of the Sierra Norte and it is essentially constituted by metaluminous granodiorites with Hbl + Bt + Ttn + Ep.

Even though there are not absolute ages about this granite, this type of non-deformed plutons, that cut of ductil shear zone, can be compared with the other Ordovician granites.

IV Sierra Grande Sector

14. Paso del Carmen Tonalite: (31°04’31’’LS – 64°55’01’’LW). In the north of the Sierra Grande, near to Paso del Carmen locality, it outcrops an important intrusive defined by Olsacher (1960) as “Paso del Carmen granodiorite”, and later investigated by Caminos and Cucchi (1990) and Lyons, et al. (1997). It is a body of lightly triangular shape oriented in E-W direction, of about 6.5 km of length by 2.5 km of wide. It intrudes discordantly cutting almost perpendicularly the NNW foliation of the country gneiss, which does not suffer any type of deviation in the proximities of the contact (Gromet and Simpson 1998). Internally it does not register foliation by deformation and it presents an isotropic texture, granular hypidiomorphic of medium to coarse granulometry. The mineralogic composition is: Qtz + Pl + Bt + Ep + Hbl + Ttn. It has an important kaolinic and sericitic alteration of the plagioclases and the chloritization and epidotization of the ferromagnesians minerals.

Compositional variations and minor syn-magmatic intrusive dikes are distinguished, specially in the southeast border of the pluton, where the dikes of porphyric tonalities, microdiorites of 2- 3 m of wide, and pegmatitic veins are frequent.

Gromet and Simpson, (1998) define an U-Pb zircon age of 474±5 Ma (upper interception of the concordia with MSWD = 1.6) indicating for this non-deformed pluton an emplacement in the Lower to Middle Ordovician.

V Sierra de Guasapampa Sector

15. Charquina Granodiorite: (30°40’13”LS - 65°53’50’’LW). In the northwestern border of the SPC, to the west of the La Playa, it outcrops a pluton of ovoidal shape with its major axis northsouth direction and of about 8 km of length by 3.5 km of wide. The country rocks, are gneisses and migmatites of the San Carlos Massif in the eastern sector and milonites and gneisses in the western sector. The pluton cut the milonitic foliation of the Los Túneles shear zone that has a direction of N 320° / 45° NE (Martino et al., 2002).

Gomez and Lira (1998) recognize for this pluton two main facies, the one of lesser development is a biotitic monzogranite of medium grain, outcropping in the eastern border and central part of the body, constituted by 36% Qtz + 32% Pl (An 08), 24% Mic. + 7% Ms + <1% Bt, as accessories Ap,Zr, Rt, Grt, Ep and as alteration Chl, Ms, Ep, Op and carbonates. The facies that occupy the greater part of the pluton is a biotitic muscovitic granodiorite of uniform medium grain (1 to 2 mm). The mineralogical composition is: 35% Qtz + 36% Pl (An 17) + 12% Mic + 8% Bt + 4% Ms, besides of Ap, Ep and Zr as accessories and Chl, Ep2, Ms and Ttn as alteration. It also presents numerous syn-magmatic apilitic and pegmatitic intrusions N-S and E-W oriented.

Pankhurst et al., (2002) report an age of crystallization of 478 ± 8 Ma for the granodioritic facies of the pluton La Playa (age U-Pb zircon SHRIMP) confirming the age Rb-Sr obtained by Rapela et al., (1998).

16. Cuesta de los Romeros Tonalite: (31°04’35’’LS - 65°18’07’’LW). This intrusive body is emplaced inside the San Carlos metamorphic-anatectic massif, more precisely in the western side of the Sierra del Coro (Bonalumi, et al. in press). It is an intrusive of tonalitic composition lightly elongated with NW direction and it has 6 km of length and 2.5 km of wide. The tonalite is of gray color with black spots of biotitic composition with granular xenomorphic texture. The primary mineralogy is represented by: Qtz-Pl-Bt-Ti-Ap-Zr-Ep1. A strong sericitation of the plagioclases is observed, while the chloritization of the ferromagnesian is incipient. The Ep2 is a secondary replaces of Pl, Bt and Ti. The pluton present a marked deformation of their primary mineralogy, the Qtz show strain shadows, the Bt has “kink” folds and the Pl has an incipient deformation.

Fernandez (1989), makes a detailed description of its emplacement and observes a concordant position with a deformation cycle of NNW-SSE direction, with gradational passages to the country rock, which are as well cut by pegmatites bound with the Mesa del Coro granite (see the next), those same pegmatites cut the tonalite, so Fernandez (1989) suggests an older age for the Cuesta de Los Romeros tonalite, in regards to Mesa del Coro granite (471 ± 58 Ma using K/Ar in Bt), for which it could be located in the Famatinian cycle.

17. Mesa del Coro Granite: (31°05’12.2’’LS - 65°17’44’’LW) this granite outcrops 2 km south of the locality of Cienaga del Coro and very near to the eastern extreme of the Cuesta de Los Romeros tonalite. Its main outcropping (Fernandez, 1989) is lengthen with NNW-SSE bearing, direction in which it has 2 km by a variable wide between 0.9 km in the south half and 0.5 km in the north. The contacts are generally pure. It is a rock of pink color with light gray tonalites, it presents a midium granulometry being its main mineral components: Qtz + Kfs + Pl + Ms + Bt(scarce), while the accessories that integrate it are: Grt, Crd, Ap, Zrn, Tur, Toz, Fl, which is indicating its strong peraluminous tendency and an interesting grade of specialization. Locally and near the pegmatites important Tur crystals. It is important to highlight that in some localized sectors (on calc-silicatic rocks) it has been produced a thermic metamorphism with hornfels formation. Petrographically it is a muscovite granite with hydrothermal overimpossed alteration. Geochemically it is located inside the peraluminous subalkaline granites. This type of body is considered an specialized granite, with abnormal values in W and Sn (average 10 ppm and 29 ppm respectively: n=11). It was dated in 471 ± 58 Ma., using K/Ar in biotites, (Fernandez, 1989).

Geochemical Characterization

The main feature of the Ordovician magmatism of the SPC is the association of trondhjemitetonalite- granodiorite, (TTG magmatism of Rapela et al., 1998) which develops preferably in the Sierra Chica, Sierra de Cuniputo – San Marcos and some isolated plutons in the north of Sierra Grande and Sierra de Guasapampa (Fig.2). The only granitic pluton in the Sierra Chica could be the Loma Ancha granite, which has been tentatively included together with the others Famatinean plutons.

The TTG magmatism is of slight to moderately peraluminous tendency and enriched in Na2O, with values upper than 3% and in some cases close to 6% (Fig.3), it is also rich in CaO and Sr, specially in the tonalitics and trondhjemitics varieties and comparatively depleted in K2O (0.65 – 2.02%), FeOt, Cs (0.2 – 2.3 ppm) and other incompatible elements, being also specially low the relations Rb/Sr and the total Rare Earths contents (Fig.3).

Rapela et al., (1998) and Pankhurst et al., 2000 also indicate, for the TTG group, a positive or absent Eu-anomaly, relations [La/Yb]n of 2 –55, initial 87Sr/ 86Sr ratios relatively low (0.705 – 0.707) and values of the eNd between + 1.5 a – 4.8. these features are specially evident in the trondhjemitic units as the ones from Güiraldes and San Agustin, suggesting, at least for these ones, a deep and basic magmatic source probably of astenospheric mantle source.

In the Sierra Norte of Córdoba some notable differences in regards to the rest of the SPC are observed. The rhyolitic intrusives are varieties of high silica (> 75% of SiO2) moderately peraluminous, rich in K2O (4.5 - 5.8% K2O) and high relations of Rb/Sr (Fig.3). they have a strong negative Eu-anomaly (Table 1) and high initial relations of 87Sr / 86Sr (0.80 – 0.89 ), which suggests an important cortical participation in its genesis. (Rapela et al., 1994).

El Cerro Granite shows some geochemical similitudes with the rhyolitic porphyry, specially its high tenor in SiO2 (75 to 77%), its low peraluminosity, relatively high relations Rb/Sr (> 40) and a strong negative Eu-anomaly (Table 1). However, the relation [La/Y]n of 12-26, and eNd of –1.9 would be evidences of a more deep source for this granite (Baldo et al., 1998).

Final Remarks

The magmatism of lower to middle Ordovician age of the SPC is characterized mainly by the generation of small plutons, tabular dikes and subvolcanic manifestations. These ones have discordantly emplaced in the Lower Cambrian metamorphic or igneous basement, cutting the metamorphic foliations and carry small and angular xenoliths or blocks than these ones, they can also generate a metasomatism effect in contact with marbles and calc-schists. They never show a high temperature ductile milonitization and the only postmagmatic deformation that they register is of semi-fragile to fragile type. From this it is deduced that its emplacement has been at relatively low deep and in a previous rigid basement. The tabular dikes has porphyric texture and an aphanitic matrix, which also indicates a shallow emplacement.

According to the nowadays available geocronological information (see plutons’ description), the trondhjemites are the most ancient units, with a maximun age of 499 ± 5 Ma (Güiraldes trondhjemite), they are followed by the granodioritic units with ages of 474 – 478 Ma (La Fronda, La Playa and Paso del Carmen) and finally, the tonalitic dikes ( 460 ± 6 Ma).

The distribution of the different plutons, (Fig. 1) suggest same compositional control for the emplacement of this plutons, the trondhjemite + tonalite ± granodiorites plutons (TTG group) are located preferable to the east of the SPC, while the granodioritic and granitic are located to the west.

The Ordovician magmatism of the Sierra Norte de Córdoba has a similar aspect with respect to the levels of emplacement, but show important geochemical differences that suggest a greater cortical participation in the genesis of these ones.

In the context of Sierras Pampeanas, the Ordovician magmatic event of the SPC is coeval with the early “I” type metaluminous and calcic suite magmatism, located farther west and that forms a big batholitic unites of granodiorites with Hbl + Bt, these are representatives of the main subductions magmatic arc of the Famatinean Orogen (Pankhurst et al., 2000). In this context the TTG of the SPC would represent an inner magmatic arc, emplaced on the pampean foreland of the western margin of Gondwana.

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Recibido: 15 de Octubre de 2002

Aceptado: 12 de Diciembre de 2002

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