Rocks from La Pampa Province, Argentina
TICKYJ1, Eduardo Jorge LLAMBÍAS2,3
and Ricardo Néstor MELCHOR1,3
1 Universidad Nacional de La Pampa, Av. Uruguay 151, L6300CLB Santa Rosa, La Pampa, Argentina
2 Centro de Investigaciones Geológicas, calle 1 N° 644, 1900 La Plata, Buenos Aires
ROCKS FROM LA
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.
DE LA PROVINCIA DE LA
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
Crystalline basement. Microbial limestones.
Stromatolites. Thrombolites. La Pampa. San Jorge
Famatinian Cycle. Cuyania/Precordilleta
cristalino. Calizas microbiales. Trombolitos. Estromatolitos. Formación San
Pampa. Ciclo Famatiniano. Terreno Cuyania/Precordillera.
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).
chapter contains a summary of the current knowledge on Ordovician rocks from La
Pampa, along with new information produced in the last years.
crystalline basement of south-eastern La Pampa province
TICKYJ1 and Eduardo Jorge LLAMBÍAS2,3
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.
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
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).
(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
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.
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.
best exposures of the Las Piedras Complex are located at Estancia Las Piedras,
Estancia Antimán, Valle Daza and Paso del Bote.
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.
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.
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.
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.
The age of the main deformational event of the Las Piedras Complex is poorly constrained by
ages. Here, we present the last geochronological data obtained by the Ar-Ar and
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,
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.
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%.
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.
phyllite yielded a total-gas date of 504 ± 1 Ma and an imperfect plateau age of
523 ± 3 Ma.
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.
it is concluded that these ages represent a maximum estimate for the true age of
the metamorphism of Estancia Antiman phyllite.
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.
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).
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).
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.
Jorge Formation Limestones
Néstor MELCHOR and Hugo TICKYJ
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
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).
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).
Jorge Sur Member
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
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.
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 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).
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).
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
C., 1994. Thrombolitic-stromatolitic
cycles of the Cambro-Ordovician boundary sequence, Precordillera Oriental basin,
western Argentina. In: Bertrand-Safarti, J. and Monty, C. (Eds.), Phanerozoic
Stromatolites II: 421-441, Kluwer Academic Publishers.
C., Cabaleri, N. and Valencio, S., 1996. Modelo paleoambiental de la Formación
La Flecha (Cámbrico Superior) en el área de Jáchal, Provincia de San Juan. Revista
de la Asociación Geológica Argentina, 51:165-176.
R., Benedetto, J.L. and Vaccari, N.E. 1995. The early Paleozoic evolution of the
Argentine Precordillera as a Laurentian rifted, drifted, and collided terrane: A
geodynamic model. Bulletin Geological Society of America, 107:253-273.
Z., 2001. Refinement of isotopic analysis techniques for trace Pb and Sr and
their selected applications. Ph.D. dissertation, The Ohio State University, 356
Roqué, P. and Ibáñez, G., 1979. Provincia Geológica Sanrafaelino-Pampeana. In:
Turner, J.C.M. (Ed.) Segundo
Simposio de Geología Regional Argentina: 837-869, Academia Nacional de
Roqué, P., 1972. Cinturón Móvil Mendocino-Pampeano. In: Leanza, A.F. (Ed.),
Primer Simposio de Geología Regional Argentina: 283-303, Academia Nacional
de Ciencias de Córdoba, Córdoba. Foland, K.A., Fleming, T.H., Heimann, A. and Elliot,
D.H., 1993. Potassium-argon dating of fine-grained basalts with massive Ar loss:
Application of the 40Ar/39Ar
technique to plagioclase and glass from the Kirkpatrick
Antarctica. Chemical Geology (Isotope Geoscience Section), 107: 173-190.
Foland, K.A., Linder, J.S., Laskowski, T.E. and Grant, N.K., 1984. 40Ar/39Ar
dating of glauconites: Measured 39Ar
recoil loss from well-crystallized specimens. Isotope Geoscience, 2: 241-264.
P.D., Poiré, D.J. and Varela, R., 2002. Hallazgo de trazas fósiles en la
Formación El Jaguëlito y su relación con la edad de las metasedimentitas,
Macizo Nordpatagónico Oriental, provincia de Río Negro. Asociación Geológica
Argentina, Revista, 57: 35-44.
W.D., Bergström, S.M., Kolata, D.R., Cingolani, C.A. and Astini, R.A., 1998. Ordovician
K-bentonites in the Argentine Precordillera: relations to Gondwana margin
evolution. Geological Society, London, Special Publication, 142:159-179.
J.H. and James, N.P., 1986. Thrombolites and stromatolites: Two distinct
types of microbial structures. Palaios, 1:492-503.
E., Llambías, E.J. and Latorre, C.O., 1980. Geología de la Provincia de La
Pampa, República Argentina y geocronología de sus rocas metamórficas y
eruptivas. Asociación Geológica Argentina, Revista, 35: 87-146.
E.J. and Caminos, R., 1986. El magmatismo neopaleozoico de Argentina. In: Archangelsky,
S. (Ed.): El sistema carbonífero de la República Argentina, pp. 239-264. Córdoba.
E.J., 1975. Geología de la provincia de La Pampa y su aspecto minero. Dirección
de Minas de la provincia de La Pampa, 38 p. Unpublished report, Santa
Rosa, La Pampa.
E.J., Melchor, R.N., Tickyj, H. and Sato, A.M., 1996. Geología
del Bloque del Chadileuvú. 13º Congreso Geológico Argentino, Actas V:417-425.
R.N., Cheng, Z. and Foland, K., 1999a. Isotopic dating of San Jorge Fm.
limestones (Early Paleozoic): Preliminary results from a Pb/Pb isochron and
87Sr/86Sr ratios. 2°
Simposio Sudamericano de Geología Isotópica, Actas: 414-417..
Villa Carlos Paz.
R.N., Sato, A.M., Llambías, E.J. and Tickyj, H., 1999b. Confirmación
de la extensión meridional del terreno Cuyania/Precordillera en la provincia de
La Pampa, Argentina. 14° Congreso Geológico Argentino, Actas I:
R.N., Tickyj, H. and Dimieri, L.V., 1999c. Estratigrafía, Sedimentología y
Estructura de la Fm. San Jorge (Paleozoico Inferior), La Pampa, Argentina. 14°
Congreso Geológico Argentino, Actas I: 389-392. Salta.
R.J., Rapela, C.W., Fanning, C.M., 2001. The Mina Gonzalito Gneiss: Early
Ordovician metamorphism in Northern Patagonia. III South American Symposium on
Isotope Geology, Extended Abstract Volume (CD), 604-607, Sociedad Geológica
de Chile, Santiago, Chile.
P.D., 1986. Petrología y geocronología del sector central de la sierra de
Lonco Vaca, La Pampa. Asociación Geológica Argentina, Revista 41:270-289.
A.M, González, P. and Llambías, E.J., 2002. The Ordovician of Sierra de San
Luis: Famatinian magmatic arc and low to high-grade metamorphism. In: Aceñolaza,
F.G. (Ed.), Aspects on the Ordovician System of Argentina. Serie
A.M., Tickyj, H., Llambías, E.J. and Sato, K., 2000. The Las Matras
tonalitic-trondhjemitic pluton, central Argentina: Grenvillian-age constraints,
geochemical characteristics, and reigonal implications. Journal of South
American Earth Sciences 13:587-610.
H., 1999. Estructura y petrología del Basamento Cristalino de la región
centro-sur de la provincia de La Pampa, Argentina. PhD Thesis. Universidad
Nacional de La Plata, 228 p. (unpublished).
H., Basei, M.A.S., Sato, A.M. and Llambías, E.J., 1999b. U-Pb and K-Ar ages of
Pichi Mahuida Group, crystalline basement of SE La Pampa province, Argentina. II
South American Symposium on Isotope Geology, Extended Abstract pp. 139-142,
Carlos Paz, Argentina.
H., Llambías, E.J. and Sato, A.M., 1999a. El basamento cristalino de la región sur-oriental de la
provincia de La Pampa: Extensión austral del Orógeno Famatiniano de Sierras
Pampeanas. 14° Congreso Geológico Argentino, Actas I, pp. 160-163.
R., 1928. Contribución a la geología de los departamentos Chical-Có y Puelén,
de la parte occidental de La Pampa Central. Publicación de la Dirección
General de Minas, Geología e Hidrogeología, 40. Buenos Aires.
22 de Octubre de
Aceptado: 4 de Diciembre de 2002