Cambrian paleogeography at the western Gondwana ...

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Cambrian paleogeography at the western Gondwana margin: U-Pb ages and provenance areas of detrital zircons of the Mesón Group (Upper Cambrian), Northwest Argentina Christopher J. Adams1, Hubert Miller2, Guillermo F. Aceñolaza3, & Alejandro J. Toselli3 1

GNS Science, PO Box, Lower Hutt , New Zealand ([email protected]) LMU, Department für Geo- und Umweltwissenschaften, Luisenstr. 37, 80333 München, Germany ([email protected]) 3 Universidad de Tucumán, Facultad de Ciencias, Miguel Lillo 205, 4000 S. M. de Tucumán, Argentina ([email protected]) 2

KEYWORDS : Argentina, Cambrian, Mesón Group, U-Pb, zircons

Introduction In northwest Argentina, sedimentation of the Puncoviscana Formation (uppermost Neoproterozoic-Early Cambrian) finished with folding, metamorphism, and granitoid magmatism of the Pampean Orogeny in midCambrian times. Above a pronounced angular unconformity, the turbidites of the Puncoviscana Fm. are overlain by siliciclastic sedimentary rocks of mostly sandstone, partly conglomerate, siltstone and mudstone grain size, the Mesón Group that is divided into 3 formations: Lizoite, Campanario, and Chalhualmayoc. The Mesón Group is the basal unit for the sedimentation of the Famatinian (Ordovician-Devonian) orogenic cycle in northwest Argentina. The siliciclastic rocks of the Mesón Group were deposited in shallow, coast-near tide-dominated environments in the form of sand bars (Sánchez & Salfity, 1999, Aceñolaza, 2003, 2005). Generally, the age of the Mesón Group has been considered Cambrian. On paleontological evidence, Sánchez & Salfity (1999) and Aceñolaza (2003, 2005) restricted the age to “Middle to Upper Cambrian”. The presence of late Early Cambrian zircons in part of the underlying Puncoviscana Formation (Adams et al., 2008), and the early to mid Cambrian zircon ages of the Santa Rosa de Tastil and Cañaní granitoids intruding the Puncoviscana Formation (513 Ma: Adams, oral com.; 514 - 519 - 536 Ma: Bachmann et al., 1991), indicate that sedimentation of the Mesón Group did not begin before the Middle Cambrian. The Mesón Group as a lithological unit terminates

before

the

Ordovician,

whereas

quite

similar

siliciclastic

facies

continues

through

the

Cambrian/Ordovician boundary into the lowermost part of the Tremadocian Santa Rosita Fm. (Aceñolaza, 2005). In order to define the geotectonic position of the Mesón Group within the Gondwana Pacific margin, its relation to the underlying Puncoviscana Formation and the Pampean and Famatinian orogeny (Pankhurst & Rapela, 1998), and to recognize the relation of the provenance area of its sediments to those of the Puncoviscana Formation, samples have been taken from outcrops close to the Puncoviscana Formation. The aim of this work was to know, • if the provenance areas of sediments of the Gondwana margin have changed since deposition of the Puncoviscana Formation and the Pampean orogeny, • if erosion of the Puncoviscana Formation and its metamorphic equivalents has much contributed to the sediments of the Mesón Group, or

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• if there was an important new input from the Brazilian shield, similar to that of the time of deposition of the Puncoviscana Formation, or from anywhere else.

Figure 1. Left: Occurrence of the Mesón Group (Upper Cambrian) in northwest Argentina. A = Sample N° JJ2A, B = Sample N° SAL1, C = Sample N° PMXX3. Right: Frequency diagrams of U-Pb ages of detrital zircons. For discussion see text. Note the numerous grains from 2200 to 2000 Ma, nearly absent in the underlying Puncoviscana Formation.

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Results JJ2A is an angular clast from local rock debris slopes of the Mesón Group, close to sample N° JJ-2 of the Puncoviscana Formation on the old road from Jujuy to Salta (Adams et al., 2008) (Fig. 1, A). It shows a youngest peak at 538 to 519, close to the youngest peak of the close lying sample of the Puncoviscana sample JJ-2 (555 to 530 Ma). Other minor peaks are around 700 to 600 Ma and from 1000 to 900 Ma. The provenance of 6 zircon grains between 2200 and 1980 Ma will be particularly considered later. PMXX3 is a sample from outcrop in the upper part of the Lizoite Fm., in the Quebrada de Humahuaca, north of Tumbaya, Jujuy province (Fig. 1, C). A pronounced peak from 700 to 560 Ma is noticeable. From 2200 to 1980 Ma eight grains are present. SAL 1 (La Pedrera) is from the road from Salta to La Quesera, south of the city of Salta (Fig. 1, B) immediately above the angular unconformity of the Pampean Orogeny. It shows a sharply pronounced peak at 502 Ma, a broad peak at 640 to 580 Ma, and 6 grains from 2200 to 2000 Ma.

Discussion The maximum age of the Mesón Group is defined by the age of the youngest parts of the underlying Puncoviscana Formation (523 Ma: Adams et al., 2008), and the age of the youngest granitoids beneath (Bachmann et al., 1991: 514 Ma, and Adams, oral comm.: 513 Ma). The upper limit is defined by fossil evidence of uppermost Cambrian age within the lowermost sector of the overlying Santa Rosita Fm. The youngest peak of detrital zircon grains in the Mesón Group at 502 Ma (Fig. 1) corresponds to the Middle/Late Cambrian boundary. We think that these young zircons are the product of volcanic activity at the beginning of the Famatinian magmatic activity in the neighborhood (Loewy et al., 2004, Sims et al., 1998, Saavedra et al., 1998). An older peak of 538 to 519 Ma is recognized in sample JJ2A (Fig. 1, A). The ages resemble very much the youngest ages from the Puncoviscana Fm. In all three samples a peak around c. 600 Ma is prominent, similar to most of the Puncoviscana Fm. samples (Adams et al., 2008). In two of the samples, a peak between 1000 and 900 Ma is also present. It is nearly identical with a common peak of the Puncoviscana Fm. that appears mostly between about 1100 and 1000 Ma (Adams et al., 2008). Other, Mesoproterozic, ages occurring sometimes in the Puncoviscana Fm., are not present in the Mesón Group samples. On the contrary, within all Mesón Group samples, distinctive ages of 2200 to 2000 Ma occur. Now the question is: Are the sediments of the Mesón Group mostly composed of recycled material of the underlying Puncoviscana Formation, or, are both lithological units composed of material proceeding from the same areas? For the Early Cambrian grains of sample JJ2A the youngest parts of the Puncoviscana Fm. are the most suitable provenance areas. For the late mid to late Cambrian zircons of sample SAL 1, a provenance from an early Famatinian volcanism is probable. For the few late and early Neoproterozoic grains, recycling of rocks of the Puncoviscana Fm. is just as possible as an original provenance from the Neoproterozoic Brasiliano orogen and from the neighboring Sunsás orogen, respectively. The explicit occurrence of early Paleoproterozoic zircons sharply limited to the time span of 2200 to 2000 Ma is unexpected, but significant for all samples. Rapela el al. (2007) recently found such ages in boreholes east of the Sierra de Córdoba within part of the Río de la Plata craton that also presents such ages in Uruguay and southeast Brazil. Rapela et al. (2007) suggest a former more northern position of the Río de la Plata craton, with

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translation to its present site by large-scale dextral strike-slip movement. There are no younger Paleoproterozoic and early Mesoproterozoic rocks in between the Río de la Plata craton and the “Pampean Cycle” orogen of central and northwest Argentina. This explains the absence of such zircons, except Early Neoproterozoic ones. These may have been derived from the Puncoviscana Formation by recycling, or from a prolongation of the West Brazilian Sunsás Orogen beneath the Chaco plain in north Argentina intervening between the Río de la Plata orogen and the Mesón Group deposition site. Although Sánchez & Salfity (1999) record a sediment transport to the Mesón Group from mostly western sources, this is not documented by the zircon grain ages. Loewy et al., 2004) have shown that early Proterozoic magmatic and metamorphic rocks of the ArequipaAntofalla terrane in the west of the Mesón Group deposition centres are defined by ages from 2.0 to 1.8 Ga, whilst the characteristic Paleoproterozoic ages of the Mesón Group detrital zircons are older: 2.2 to 2.0 Ga).

Conclusions At the Pacific Gondwana margin, after the prominent Pampean orogeny in Middle Cambrian, a shallow water through developed in NW Argentina. Material came partly from the underlying Puncoviscana Formation and/or its metamorphic equivalents, but continuous provenance from the Brazilian Shield cannot be excluded. In any case, frequent grains restricted to 2200 to 2000 Ma are considered to have derived from the Río de la Plata craton of southeast Brazil and Uruguay, and its extension to the area east of the Sierra de Córdoba (Rapela et al., 2007).

References Aceñolaza, G.F., 2003 — The Cambrian System in Northwestern Argentina: stratigraphical and palaeontological framework. Geologica Acta, 1: 23-39. Aceñolaza, G.F., 2005 — The Cambrian System in Northwestern Argentina: stratigraphical and palaeontological framework. Reply. Geologica Acta, 3 (1): 73-77. Adams, Ch., Miller, H., Toselli, A.J., 2008 — The Puncoviscana Formation of northwest Argentina: U-Pb geochronology of detrital zircons and Rb-Sr metamorphic ages and their bearing on its stratigraphic age, sediment provenance and tectonic setting. Neues Jahrbuch für Geologie und Paläontologie, in press. Bachmann, G., Grauert, B., Kramm, U., Lork, A., Miller, H., 1991 — El magmatismo del Cámbrico Medio/Cámbrico Superior en el basamento del Noroeste Argentino: Intrusivos de Santa Rosa de Tastil y Cañaní. Actas X. Congreso Geológico Argentino, Tucumán, 4: 125-127. Loewy, S.L., Connelly, J.N., Dalziel, I.W.D., 2004 — An orphaned basement block: The Arequipa-Antofalla Basement of the central Andean margin of South America. Geological Society of America Bulletin, 116: 171-187; doi: 10.1130/B25226.1. Pankhurst, R.J., Rapela, C.W., 1998 — The proto-Andean margin of Gondwana: an introduction. In: Pankhurst, R.J., Rapela, C.W. (eds): The Proto-Andean Margin of Gondwana. Geological Society of London, Special Publications 142: 1-9. Rapela, C.W., Pankhurst, R.J., Casquet, C., Fanning, C.M., Baldo, E.G., González-Casado, J.M., Galindo, C., Dahlquist, J., 2007 — The Río de la Plata craton and the assembly of SW Gondwana. Earth-Science Reviews, 83: 49-82. Saavedra, J., Toselli, A., Rossi, J., Pellitero, E., Durand, F., 1998 — The early Paleozoic magmatic record of the Famatina System: a review. In: Pankhurst, R.J., Rapela, C.W. (eds): The Proto-Andean Margin of Gondwana. Geological Society of London, Special Publications, 142: 283-295. Sánchez, M.C., Salfity, J.A., 1999 — La cuenca cámbrica del Grupo Mesón en el Noroeste Argentino: desarrollo estratigráfico y paleogeográfico. Acta Geológica Hispánica, 34: 123-139. Sims, J.P., Ireland, T.R., Camacho, A., Lyons, P., Pieters, P.E., Skirrow, R.G., Stuart-Smith, P.G., Miró, R., 1998 — U-Pb, Th-Pb, and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin. In: Pankhurst, R.J., Rapela, C.W. (eds): The Proto-Andean Margin of Gondwana. Geological Society of London, Special Publications, 142: 259-281.

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