Ordovician Rocks from La Pampa Province, Argentina

Hugo TICKYJ1, Eduardo Jorge LLAMBÍAS2,3 and Ricardo Néstor MELCHOR1,3

Abstract : ORDOVICIAN ROCKS FROM LA PAMPA PROVINCE, ARGENTINA. The Ordovician rocks of La Pampa consist of two major units: 1) the crystalline basement of the southeastern region and 2) the limestones of the San Jorge Formation. Basement rocks are composed of very low to low-grade metasedimentary rocks and medium-grade gneisses, both with a main metamorphic event of Ordovician age, and granites and granodiorites of Late Cambrian, Silurian and Early Devonian ages. Its geological evolution is related to the Famatinian orogenic cycle. The San Jorge Formation consists of a 125 m-thick sedimentary member (San Jorge Sur) composed by carbonate microbial deposits with development of stromatolites and thrombolites, showing thin interbedded potassiumrich tuff levels; and a metamorphic member (Rogaziano) consisting of very low-grade marbles, showing tight, NE-verging folding. This Formation is considered the southernmost tip of the Cuyania/Precordillera terrane.

Resumen: ROCAS ORDOVÍCICAS DE LA PROVINCIA DE LA PAMPA. Las rocas ordovícicas de La Pampa comprenden dos unidades principales: 1) el basamento cristalino de la región sur-oriental y 2) las calizas de la Formación San Jorge. El basamento está compuesto por metasedimentitas de muy bajo a bajo grado y gneises de grado medio, ambos con un metamorfismo principal de edad ordovícica, y por granitos y granodioritas del Cámbrico superior, Silúrico y Devónico inferior. Su evolución geológica está vinculada con el ciclo orogénico Famatiniano. La Formación San Jorge comprende un miembro sedimentario (San Jorge Sur) compuesto de 125 metros de calizas microbialíticas con desarrollo de estromatolitos y trombolitos, que presentan delgados niveles de tobas potásicas; y un miembro metamórfico (Rogaziano) integrado por mármoles de muy bajo grado, que presentan un plegamiento apretado con vergencia hacia el noreste. Esta Formación es considerada la extensión austral del terreno Cuyania/Precordillera.

Keywords: Crystalline basement. Microbial limestones. Stromatolites. Thrombolites. La Pampa. San Jorge

Formation. Famatinian Cycle. Cuyania/Precordilleta terrane.

Palabras clave: Basamento cristalino. Calizas microbiales. Trombolitos. Estromatolitos. Formación San Jorge.

La Pampa. Ciclo Famatiniano. Terreno Cuyania/Precordillera.

Introduction

The Ordovician rocks from La Pampa (central Argentina) crop out in the south of the province, approximately between 36° 30’ and 39° 00’ S. In particular, two groups of rocks are distinguished: the crystalline basement of the south-eastern region and the sedimentary and metamorphic limestones from the central and south-western region. Previous workers have mentioned these rocks as part of the Cinturón Móvil Mendocino-Pampeano (Criado Roqué, 1972) or Provincia Sanrafaelino Pampeana (Criado Roqué and Ibáñez, 1979). Afterwards, Llambías and Caminos (1986) and Llambías et al. (1996) have included the crystalline basement within the Chadileuvú Block. These crystalline rocks are considered as the southern extension of the Sierras Pampeanas Occidentales (Linares et al., 1980; Tickyj, 1999). The Ordovician limestones and a Grenvillian basement (Sato et al., 2000) are part of the Cuyania/Precordillera terrane in La Pampa (Melchor et al., 1999b).

This chapter contains a summary of the current knowledge on Ordovician rocks from La Pampa, along with new information produced in the last years.

Ordovician crystalline basement of south-eastern La Pampa province

Hugo TICKYJ1 and Eduardo Jorge LLAMBÍAS2,3

Introduction

The crystalline basement of La Pampa province occurs in the eastern and central region, as small and isolated outcrops, from Lonco Vaca Range in the north to Colorado river in the south (Fig. 1). The first detailed work corresponds to Llambías (1975) and Linares et al. (1980). They distinguished three major units, mainly on the basis of structural and geochronological data: 1) Upper Proterozoic metamorphic rocks, composed by phyllites, schists, gneisses and amphibolites, 2) the Curacó Granite (Upper Cambrian – Lower Ordovician) and 3) the Lonco Vaca Granite (Lower Devonian). Taking into account these data the authors stated that the crystalline basement of La Pampa represents the southern part of the Sierras Pampeanas.

The Lonco Vaca area (Fig. 1 inset) was studied by Parica (1986) who identified two regional metamorphic events of late Proterozoic age (724 ± 18 Ma, Rb-Sr, whole rock; 684 ± 30 Ma, K-Ar, hornblende) and three magmatic events dated at late Proterozoic (665 ± 25 Ma, K-Ar, muscovite), Early Ordovician (473 ± 8 Ma, Rb-Sr, whole rock) and Early Carboniferous times (320 ± 10 Ma, KAr, muscovite). Subsequently, its geological evolution was correlated with the Sierras Pampeanas de Córdoba.

The outcrops of basement from the southeastern region display a dominantly NW structural trend (Fig. 1), which have controlled the development and distribution of magmatic and sedimentary rocks during the Late Paleozoic (Llambías et al., 1996).

Tickyj (1999) and Tickyj et al. (1999a, b) defined a new stratigraphic scheme based on new petrological and geochronological data. Thus, the crystalline basement of south-eastern La Pampa province consists of Upper Cambrian (?) – Ordovician, very low to medium-grade metamorphic rocks (Las Piedras Metamorphic Complex) and Upper Cambrian – Lower Devonian, non-deformed, post-orogenic granitoids (Pichi Mahuida Group), which are considered to belong to the Famatinian orogenic cycle. In addition, granite orthogneisses of Late Palaeozoic age were included in the Cerro de Los Viejos Complex. Here we present a summary of geological and geochronological data of the Ordovician Las

Piedras Metamorphic Complex.

Las Piedras Metamorphic Complex

The Las Piedras Metamorphic Complex consists of 1) phyllites, muscovite-bearing schists and metasandstones exposed between Puelches and Cuchillo-Có, and along of the Colorado river valley; 2) gneisses and amphibolites exposed in Valle Daza and Paso del Bote. In both cases the structure is characterized by WNW S1 foliation, developed by a regional compressive D1 deformation associated with a metamorphic event of very low to medium-grade. A second deformation folded the S1 foliation in cm- to m-scale, open folds with axial planes oriented N290°-300°/35°-45°S.

Figure 1. Outcrop distribution of main Palaeozoic units in the south-eastern La Pampa province. 1 Las Piedras metamorphic Complex (Upper Cambrian? - Ordovician); 2 Pichi Mahuida Group (Upper Cambrian - Lower Devonian); 3 Cerro de los Viejos Complex (Upper Palaeozoic); 4 Carapacha Formation (Permian); 5 Gondwanian magmatism; 6 Salt lakes; 7 Foliation.

The best exposures of the Las Piedras Complex are located at Estancia Las Piedras, Estancia Antimán, Valle Daza and Paso del Bote.

In Estancia Las Piedras the most abundant rocks are metasandstones with thin layers of phyllites, with well-preserved sedimentary structures. They are polymetamorphic, with a low-grade paragenesis from a first regional metamorphic event (M1), followed by growth of non-oriented porphyroblastic biotite during a thermal metamorphic event (M2) dated at 254 ± 2 Ma (Tickyj, 1999). The typical mineral association for M1 is muscovite + quartz + biotite ± chlorite, with feldspars, opaque minerals, apatite and zircon as accessories. The S1 schistosity has a mean orientation of N71°W/ 61°NE with a mean mineral lineation of N41°E/20°NE.

In Estancia Antimán, phyllites alternate with muscovite-bearing schists of low metamorphic grade. The typical mineral association is muscovite + chlorite + quartz, with opaque minerals, tourmaline, apatite and zircon as accessories. The main structures are a pervasive schistosity oriented N49°W/81°SW and a mineral lineation with a mean orientation of N53°W/04°SE.

In Valle Daza porphyroblastic garnet-biotite gneisses are intruded by centimetre to metre-thick granite veins. The porphyroblasts are xenoblastic garnet (5-10 mm) with abundant inclusions. The matrix is composed by quartz, biotite, plagioclase, K-feldspar, apatite and zircon. A penetrative foliation oriented N16°W/78°E and a mineral lineation with a mean value of N01°W/40°N are the main structures. Folded quartz veins showing intrafolial folds are common.

In Paso del Bote, small bodies of amphibolites with a subvertical, N-S-trending foliation occur together with granitic to tonalitic orthogneisses. The amphibolites are coarse-grained rocks with nematoblatic texture. The principal minerals are hornblende, plagioclase and quartz; accesory minerals are titanite, apatite and opaque minerals.

Geochronology

The age of the main deformational event of the Las Piedras Complex is poorly constrained by

isotopic ages. Here, we present the last geochronological data obtained by the Ar-Ar and K-Ar methods.

An Ar-Ar dating was performed on biotite of garnet-biotitic gneiss from Valle Daza and on a phyllite from Estancia Las Piedras. The Ar-Ar measurements were conducted at the Radiogenic Isotopes Laboratory at Ohio State University using normal procedures described in Foland et al. (1984, 1993). Each sample was analyzed by incremental heating to successively higher temperatures and by total-fusion analysis (Fig. 2 and Table 1). Ages were calculated with a total decay constant of 5.543x10-10y-1. Uncertainties are quoted at the one-sigma level. For increments of a step-heating analysis, the uncertainties do not include J value uncertainty. For total-fusion and plateau ages,

Figure 2. 40Ar/39Ar age spectra for the Las Piedras Metamorphic Complex. A. Biotite mineral separate from garnet-biotite gneiss (Valle Daza). B. Phyllite (Estancia Antimán). In the age spectra, the uncertainty for the individual temperature increments (width of the field) is 1s.

uncertainties reflect a relative uncertainty of ± 0.25% in J value. An overall systematic uncertainty of 1% is assigned to all ages. The monitor used was an intralaboratory muscovite with a 40Ar/39Ar age of 165.5 Ma and an assigned uncertainty of ± 1%.

The biotite of the garnet-biotitic gneiss yielded a total-gas date of 453 Ma and a good plateau age of 461 ± 2 Ma. The ages in the first few steps of the incremental heating (below 671°C) probably reflect an alteration or younger phase that release Ar at lower laboratory temperatures. Then, the plateau age is considered a more reliable date than the total gas age. It is interpreted as a cooling age for the main deformation event of the gneiss from Valle Daza.

The phyllite yielded a total-gas date of 504 ± 1 Ma and an imperfect plateau age of 523 ± 3 Ma.

The spectrum of Ar release shows a gradual increment toward the high temperature steps. This is probably related to an incomplete resetting of the phyllite during the low-grade metamorphism.

Then, it is concluded that these ages represent a maximum estimate for the true age of the metamorphism of Estancia Antiman phyllite.

An age of 467.1 ± 13.0 Ma was obtained by K-Ar method on hornblende of amphibolite from Paso del Bote (Table 2). K-Ar analysis was carried out at Centro de Pesquisas Geocronológicas, San Pablo (Brazil). This date is considered a minimum estimate for the age of the metamorphism.

Regional correlation

Based on the geochronological data, it is interpreted that the Las Piedras Metamorphic Complex has a similar age as the peak metamorphism of the Famatinian orogenic phase (Sato et al., this volume).

In La Pampa this cycle seems to have a more time-restricted development than at other regions located to the north, as is shown in San Luis Ranges where deformational events span until Early Devonian times and plutonic activity until Early Carboniferous times (Sato et al., this volume).

The continuity of the Famatinian activity south of La Pampa is doubtful, although Ordovician metamorphism has been reported on the Mina Gonzalito Gneiss at the Northpatagonian Massif (Pankhurst et al., 2001). A more clear similarity exists between the Atlantic Area of the Northpatagonian Massif and the south-eastern La Pampa (Tickyj et al., 1999a; González et al,. 2002), but more detailed studies are required to make precise correlations.

San Jorge Formation Limestones

Ricardo Néstor MELCHOR and Hugo TICKYJ

Introduction

Since its recognition by Wichmann (1928) this formation has received scarce attention. The unit was proposed by Criado Roqué (1972) and later studied by Criado Roqué and Ibáñez (1979) and Linares et al. (1980). Previous workers have approximated its age by regional lithological comparisons with early Paleozoic units from the San Rafael Block and the Precordillera of Argentina. Recent detailed studies, aimed to define its age, and to assess its geotectonic significance, recognized two lithostratigraphic members: a sedimentary member named San Jorge Sur and a metamorphic member designated as Rogaziano (Melchor et al. 1999a, b, c, Cheng, 2001). The formation crop outs as

scattered exposures that compose a NW-SE trending belt (about 5 km wide) developed between 36° 49' 38" S; 67° 19' 19" W and 37° 28' 49" S; 66° 25' 25" W in La Pampa Province (1 trough 4 in Fig. 3).

An additional occurrence of lithologically similar limestones located southeast of Lihuel Calel Hills (5 in Fig. 3) could also belong to this unit. Currently, this unit is considered as the southernmost end of the early Paleozoic carbonate platform of the Precordillera (Astini, 1995; Melchor et al., 1999b).

San Jorge Sur Member

The 125 m-thick sedimentary member is composed of carbonate microbial deposits with development of stromatolites and thrombolites arranged in meter-scale shallowing-upward cycles, showing thin interbedded potassium-rich tuff levels (probable K-bentonites). These deposits are composed by limestones and dolomitic limestones with micritic and peloidal texture (pelmicrite and pelsparite). They are arranged in shallowing-upward cycles, 2.5-7.5 m-thick, which commonly include from bottom to top the following facies (Fig. 4): (1) stratiform thrombolites with an encephalic or vertical fabric fabric and postrate to ameboidal tromboids. (2) Micritic or microsparitic limestones, which are massive or display a diffuse cryptoalgal fabric. (3) Planar and domal stromatolites with tepee structures, fenestral fabric and thin horizons of flat-pebble conglomerate. These cycles are interpreted as reflecting the transition from deposition in a subtidal setting (1), to a subacueos intertidal setting (2) to finish with sedimentation in an upper intertidal to supratidal setting with occasional subaerial exposure (3). The probable K-bentonites are thin (< 0.1 m), fine-grained vitric tuff levels, with high potassium content (K2O = 12,6 %). The sedimentary member is interpreted as representing deposition in a shallow, low-energy peritidal setting with significant development of microbial deposits. The depositional age of the sedimentary member have been constrained using geochemical techniques (Pb/Pb and U/Pb isochrons and their 87Sr/86Sr composition) to about 530

Figure 4. Outcrop views of the typical sedimentary facies of the sedimentary member of the San Jorge Formation Limestones, from San Jorge Sur hills. A. Stratiform thrombolites with vertical fabric. B. Thrombolites with encephalic fabric. C. Subhemispherical domal stromatolites. D. Planar stromatolites.

Ma or about 500 Ma (Melchor et al., 1999a; Cheng, 2001). These inferences were recently supported by the finding of moderately well-preserved conodonts by Dr. S. Bergstrom (Cheng, 2001). The fauna is of typical, tropical North American shallow-water type, and corresponds to the latest Tremadocian or the earliest Arenig. The fossils have a Conodont Color Alteration Index of 5, which indicates heating to around 300ºC (Bergström, personal communication in Cheng, 2001).

Rogaziano Member

The Rogaziano Mbr. is composed by blue-gray crystalline limestones affected by ductile deformation. Its structure is characterized by gently inclined, tight, NE-verging folding with a mean penetrative foliation of N30°W / 10°SW (Melchor et al., 1999b). The deformation produced massive recrystallization and partial destruction of the sedimentary fabric. The rocks are classified as calcitic marbles with a granoblastic poligonal texture and are composed by calcite, dolomite and traces of quartz. There is no evidence for neoformation of other metamorphic minerals. The features of calcite twinning suggest that the temperature during deformation ranged between 150° C and 300° C, which point to low –grade metamorphic conditions (Melchor et al., 1999c). Melchor et al. (1999c) attributed tentatively this deformation to the Precordillerana orogenic phase (late Silurian-early Devonian).

Concluding remarks

The San Jorge Formation is composed of early Ordovician microbialitic carbonate deposits and crystalline limestones, the later suffered ductile deformation associated with folding. These rocks represent the southern extension of the early Paleozoic platform carbonates of the Precorlillera terrane. In particular, its lithofacial characteristics and the presence of meter-scale shallowing-upward cycles make them comparable with the La Flecha Formation (cf. Armella, 1994; Armella et al., 1996).

Additionally, the presence of thrombolites and potassium-rich tuffs further constraint its age: 1) the occurrence of thrombolites is almost exclusively restricted to the Cambrian – Middle Ordovician interval (Kennard and James, 1986) and within the Precordillera they are older than Arenigian (Armella et al., 1996), and 2) K-bentonites found in the carbonate platform sequences of the Precordillera are not younger than Arenig (Huff et al., 1998). The metamorphic facies of this Formation are part of a deformed belt that probably marks the western boundary of the Cuyania/Precordillera terrane in La Pampa province (Melchor et al., 1999b).

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Recibido: 22 de Octubre de 2002

Aceptado: 4 de Diciembre de 2002