Implications of detrital isotopic data from Cambro-Ordovician metasediments of the Famatina belt (Central Andes) in the Late Neoproterozoic-Early Paleozoic evolution of Western Gondwana

COLLO, GILDA; ASITNI, RICARDO A.

Gottingen, Alemania

Congreso; XXI International Lateinamerika-Kolloquium; 2009

The Famatina belt, located in the Argentine foreland, has been interpreted as part of an accretionary margin built along the western Gondwana during the early Paleozoic. Its low-grade metasedimentary rocks comprise a siliciclastic-volcaniclastic Upper Cambrian-Middle Ordovician succession, that experienced a complex post-depositional history related to the Ordovician Ocloyic Orogeny. Detrital zircon data (U-Pb SHRIMP) of sandstones from the Achavil and Negro Peinado formations allow interpreting Gondwanan sources, which assisted to analyze the early stages of the Proto-Andes in this region (Collo et al. 2009). The sample from the Achavil Formation has two main populations at 521 ± 6 Ma (n=8) and 630 ± 9 Ma (n=26), and some subordinated ages (ca. 781, 1038, 1118, 1322, 1908 and 2096 Ma). The sample of the Negro Peinado Formation has four main populations at 522 ± 4 Ma (n=12); 638 ±16 Ma (n=8); 997 ±20 Ma (n=14) and 1220 ± 25 Ma (n=17), and subordinated ages (ca. 732, 1496 and 1690 Ma). CL images suggest their derivation from largely igneous sources with the youngest ages approximating the time of sediment accumulation. The Negro Peinado Formation is not older than Middle Cambrian (youngest grain at ca. 505 Ma.), while a slightly younger depositional age can be established for the Achavil Formation (youngest grain at ca. 490 Ma). A younger age for the last unit is also consistent with the regional trend from pre- to post-Pampean units in the southern area of the Central Andean segment (Schwartz and Gromet, 2004; Steenken et al., 2006; Rapela et al., 2007), which essentially show a progressive decrease of Mesoproterozoic grains and increasing of the Neoproterozoic ages. Disregarding the Cambrian (Pampean) input, similarities between Negro Peinado Formation and the older Ancasti Formation (located further east of the study region) and between Achavil Formation and the younger La Cébila Formation are sticking. This trend, in which typical Brasiliano and Grenville populations persist through the post-Pampean units, was also suggested by Adams et al. (2008) to the north based in detrital zircon populations analyzed in the Puncoviscana Formation and the overlying Mesón Group. eNd(500) and TDM values from samples of Negro Peinado and Achavil formations range between -6.3 and -9.8 and between 1.81 and 1.58 Ga (n=12), respectively and suggest that they were derived from a composite source area, separated from the mantle during the Paleoproterozoic with little contribution from juvenile younger material. The Ordovician volcaniclastic succession (Volcancito, Bordo Atravesado, Suri and Molles formations) shows TDM between 1.87 and 1.27 Ga and eNd(490) between-3.1 and -12.2 (n=18), consistent with progressive contribution from juvenile material related to the Ocloyic magmatic arc (Collo et al. 2008). The mixture of early Paleozoic, Neo- and Mesoproterozoic populations in the Cambrian units of Famatina, associated with Paleoproterozoic TDM ages, is typical of lower Paleozoic Gondwana rocks. We suggest that Pampean (~520 Ma), Braziliano (~635 Ma) and Grenville-Sunsás (~1000 and ~1200 Ma) sources were relatively close to the basin during their sedimentation. It seems probable that a Grenville terrane, with an isotopic Gondwanic affinity (a southern extension of the Arequipa-Antofalla massif ?; Lucassen et al. 2000, Casquet et al. 2006), accreted against the Río de La Plata craton at ca. 560-630 Ma. The development of a Brazilian arc upon Mesoproterozoic basement as a product of a west-dipping Neoproterozoic subduction in the central proto-Andean region, allows better explanation for the recorded Meso and Neoproterozoic zircon populations. The closest source for Cambrian populations seems to be the Pampean belt to the east of Famatina, implying rapid uplift and partial exhumation of the Pampean orogen and allowing to interpret earlier deposition within a west prograding peripheral foreland basin. The east-dipping subduction along the western margin of Gondwana, associated to the Ocloyic Orogeny and to the Terra Australis orogenic supercycle, is evidenced by the Ordovician Famatina arc rocks which show a higher contribution of juvenile material. References Adams, C.J., Miller, H., Toselli, A.J. and Griffin, W.L., 2008. The Puncoviscana Formation of Northwest Argentina: U-Pb geochronology of detrital circons and Rb-Sr metamorphic ages and their bearing on its stratigraphic age, sediment provenance and tectonic setting. N. Jb. Geol. Paläont. Abh., 247 (3): 341-352. Casquet, C., Pankhurst, R.J., Fanning, M., Baldo, E., Galindo, C., Rapela, C.W., González-Casado, J.M. and Dahlquist, J.A., 2006. 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