The African provenance of southern South America terranes: a record from Rodinia break-up to Gondwanan assembly.

RAPELA, C.W.; FANNING, C.M.; CASQUET, C.; PANKHURST, R.J.; SPALLETTI, L.A.; POIRÉ, D.G. Y BALDO, E.G.

Mar del Plata

Simposio; GEOSUR 2010, sesion "Rodinia in South America"; 2010

A remarkable characteristic of southern South America, is that the 2.26-2.02 Ga Palaeoproterozoic sequences of the Río de la Plata craton that define the oldest southern core of the continent, have not been affected by the widespread Neoproterozoic deformation and magmatism associated with the assemblage of Gondwana. In Uruguay, the Sarandí del Yi megashear separates the Paleoproterozoic basement unaffected by Neoproterozoic events (Piedra Alta and Pando terranes), from the complex Archean to Mesoproterozoic Nico Pérez terrane, which was reworked during the Mesoproterozoic (Bossi & Cingolani 2009, Oyhantçabal et al., 2009 and references therein), as well as the collage of terranes accreted during the Brasiliano-Panafrican orogeny (e.g., Punta del Este terrane and Dom Feliciano belt). Further south in the Tandilia belt in Argentina, SHRIMP analyses of the 2.23-2.06 Ga Paleoproterozoic basement do not show a Neoproterozoic overprint (Hartmann et al., 2002b); the 2.19- 2.09 Ga samples recovered from deep drill cores in the western side of the craton also show no such evidence (Rapela et al., 2007). Another important piece of evidence comes from the zircon provenance patterns of Neoproterozoic sedimentary and metasedimentary sequences located along the western and southern sides of the Río de la Plata craton (i.e. the Pampean Belt and the North Patagonian Massif). These sequences are dominated by a bimodal pattern, with peaks at 1250-960 Ma and 680-570 Ma and a minor peak c. 1900 Ma, with scarce, if any, Palaeoproterozoic zircons in the Río de la Plata craton range of ages (e.g. Basei et al., 2005, 2008; Pankhurst et al., 2006; Rapela et al., 2007). To explain these bimodal patterns, “African” (Namaqua-Natal) and “Brazilian” sources have been postulated (e.g. Schwartz & Gromet, 2004; Basei et al., 2005; Rapela et al, 2007). However, it is difficult to make a coherent scenario incorporating the several geodynamic models recently proposed for the pre-Atlantic (Adamastor ocean), and those for the western side of the Río de la Plata craton. Another important issue that remains poorly explained is the relative position of the Kalahari, Congo and Río de la Plata cratons during the early Neoproterozoic. This paper presents new U-Pb SHRIMP results on drill core samples from close to the present Atlantic coast, at the tip of the Tandilia belt on the eastern margin of the Río de la Plata craton (Punta Mogotes), as well as from the Sierra Ancasti and Sierra Brava in the Pampean belt, on the western side of the craton. Zircon provenance patterns on these critically located samples, together with previous results, allow an interpretation of the Neoproterozoic riftingdrifting of the Río de la Plata craton, and infer a close connection with similar processes in southwestern Africa. This process covers the transition from Rodinia dispersal to Gondwana assembly. The analyses of 5 samples from the 504 m deep Punta Mogotes borehole (Marchese & Di Paola, 1975) show the expected sharp contrast in zircon age pattern between the Neoproterozoic low-grade metapelites of the Punta Mogotes Formation and that of the overlying quartzites of the Balcarce Formation. The patterns of the two samples of the Punta Mogotes Formation are complex but remarkably similar, suggesting a similar source for at least the upper section of this sequence. Conspicuous younger peaks at 760-790 Ma defined by concordant zircons are the most important characteristic of these patterns, with significant populations in the Mesoproterozoic (peaks at 1250 and 1270 Ma respectively), and Upper Paleoproterozoic (peaks at 1735 and 1835 Ma), and minor but concordant populations at 1420-1560 Ma, 2070-2200 Ma, together with Early Palaeoproterozoic and Archaean zircons. This pattern and those found in the metasedimentary rocks of the Pampean Belt, characterized by prominent bi-modal peaks at 560-625 Ma and 1025-1110 Ma, with minor peaks at 730-760 Ma and c. 1900 Ma, have been used to constraint a plate reconstruction for various time periods involving the Río de la Plata, Congo and Kalahari cratons. The conspicuous peaks at 760-780 Ma of the Punta Mogotes Formation are unique among the Neoproterozoic successions, and these dominantly concordant detrital zircons define a minimum age for the siliciclastic succession. There are no Brasiliano- Panafrican ages (560-680 Ma) in the Punta Mogotes Formation, although this is a widespread event in southwestern South America, suggesting that the sequence is older than 680 Ma as well as younger than or coeval with the 760-780 Ma detrital peaks. Major detrital peaks at 635-660 Ma are otherwise observed in all samples of the overlying Balcarce Formatiom. Orthogneisses with U-Pb SHRIMP ages of 762 ± 8 and 776 ± 12 Ma have been described from the Punta del Este Terrane in eastern Uruguay, and inferred to be a portion of the Coastal Terrane of the Kaoko Belt (Hartmann et al., 2002a; Oyhantçabal et al., 2009). In southwestern Africa, this period is characterized by the inception of a large alkaline igneous province associated with rifting that was eventually superseded by drifting and finally by inversion of the basins (Jacobs et al., 2008 and references therein). A comparison with the Neoproterozoic detrital patterns of southwestern Africa and southeastern South America suggests that the most suitable source for the Punta Mogotes Formation was the basement of the Kaoko belt, on the southwestern edge of the Congo craton. The Piedras de Afilar Formation, a thick siliciclastic and carbonate sequence located on the edge of the Río de la Plata craton in Uruguay, shows a similar detrital pattern, which however lacks the 760-790 Ma peak (Gaucher et al., 2008 ), indicating that both sequences were derived from similar sources in the Congo craton. These similarities in detrital patterns strongly suggest that the Río de la Plata craton was a conjugate rift margin of the Congo craton at the time of the 760-830 Neoproterozoic rifting. The NW-SE branch of the aulacogenic triple point located at the western end of the Damara orogen (Goscombe et al., 2005 and references therein), is here considered as initiating separation of the Río de la Plata and the Congo cratons, resulting in development of the northern Adamastor ocean at the time of Rodinia breakup. It is also considered that discrete continental terranes might have rifted away from the Congo craton. The Archaean to Mesoproterozoic Nico Pérez terrane in Uruguay (Bossi & Cingolani, 2009) may have been produced during this episode. A second NE-SW branch of the triple point runs along the western edge of the Kalahari craton and faced an open ocean to the west. Discrete continental “African” terranes may have also rifted away during the opening of the southern Adamastor ocean. The continental terranes affected by rifting at the latitude of the Gariep Belt were mostly composed of the Mesoproterozoic complexes of the Natal-Namaqua orogen (c. 1000- 1100 Ma), and secondly by the 1700-2000 Ma Eburnean age basement, such as the Richtersveld terrane (Frimmel et al., 2001). Maximum expansion of the northern Adamastor and Khomas oceans took place at c. 700 Ma, while ocean opening continued in the southern Adamastor ocean. East-directed subduction started in the northern Adamastor ocean (Gray et al., 2006) at c. 680 Ma, with the possible formation of intraoceanic arcs. Closing of the northern branch took place at c. 640 Ma, involving transpression, e.g. the Kaoko Belt(Goscombe et al., 2005; Gray et al., 2006) and Punta del Este terrane (Oyhantçabal et al., 2009). West-directed subduction started in the southern Adamastor ocean with development of 620-580 “Brasiliano” magmatic arcs preserved in the “African” terranes , now located to the west and southwest of the Río de la Plata craton (present coordinates). Southward displacement of the Río de la Plata craton with the attached Nico Pérez terrane led to the highly oblique collision against the southwestern Congo craton, developing sinistral transpression in the Kaoko and Dom Feliciano belts in the 640-600 Ma time interval. Protracted oblique subduction led to closure of the Adamastor ocean at ca. 545 Ma involving collision between the Río de la Plata and the Kalahari cratons (Frimmel & Frank, 1998). On the west and southwest, the Río de la Plata craton was involved at ca. 530-520 Ma into right-lateral collision with a large continental terrane, developing the transpressional Pampean Belt. References Basei, M.A.S., Frimmel, H.E., Nutman, A.P., Preciozzi, F., Jacob, J., 2005. A connection between the Neoproterozoic Dom Feliciano (Brazil/Uruguay) and Gariep (Namibia/South Africa) orogenic belts –evidence from a reconnaissance provenance study. Precambrian Research 139, 195-221. Bossi, J., Cingolani, C., 2009. Extension and general evolution of the Río de la Plata craton. In: Gaucher, G., Sial, A.N., Halverson, G.P., Frimmel, H.E. (eds.) Neoproterozoic-Cambrian Tectonics, Global Change and Evolution: A Focus on Southwesten Gondwana. Developments in Precambrian Geology, 16, Elsevier, pp.73-85. Basei, M.A.S., Frimmel, H.E., Nutman, A.P., Preciozzi F., 2008. West Gondwana amalgamation based on detrital zircon ages from Neoproterozoic Ribeira and Dom Feliciano belts of South America and comparison with coeval sequences from SW Africa. In: Pankhurst, R.J., Trouw, R.A., Brito Neves, B.B. and de Wit, M.J. (eds.) West Gondwana: Pre-Cenozoic Correlations Across the South Atlantic Region. Geological Society, London, Special Publications, 294, 239-256. Frimmel, H.E., Frank, W., 1998. Neoproterozoic tectono-thermal evolution of the Gariep Belt and its basement, Namibia/South Africa. Precambrian Research 90, 1- 28. Frimmel, H.E., Zartman, R.E., Späth, A., 2001. The Richstersveld Igneous Complex, South Africa: U-Pb zircon and geochemical evidence for the beginning of the Neoproterozoic continental break-up. The Journal of Geology 109, 493-508. Gaucher, C., Finney, S.C., Poiré, D.G., Valencia, V.A., Grove, M., Blanco, G. Paamoukaghlian, K., Gómez Peral, L., 2008. Detrial zircon ages of Neoproterozoic sedimentary successions in Uruguay and Argentina: insightsinto the geological evolution of the Río de la Plata Craton. Precambrian Research 167, 150-170. Goscombe, B., Gray, D., Armstrong, R., Foster, D.A., Vogl, J., 2005. Event geochronology of the Pan-African Kaoko Belt, Namibia. Precambrian Research 140, 1-41. Gray, D.R., Foster, D.A., Goscombe, B., Passchier, C.W., Trouw, R.A.J., 2006. 40Ar/39Ar thermochronology of the Pan-African Damara orogen, Namibia, with implications for tectonothermal and geodynamic evolution. Precambrian Research 150, 49-72. Hartmann, L.A., Santos, J.O.S., Bossi, J., Campal, N., Schipilov, A., McNaughton, N.J., 2002a. Zircon and titanite U–Pb SHRIMP geochronology of Neoproterozoic felsic magmatism on the eastern border of the Río de la Plata craton, Uruguay. Journal of South American Earth Sciences 15, 229-236. Hartmann, L.A., Santos, J.O.S., Cingolani, C.A., McNaughton, N.J., 2002b. Two Palaeoproterozoic orogenies in the evolution of the Tandilia Belt, Buenos Aires, as evidenced by zircon U–Pb SHRIMP geochronology. International Geology Review 44, 528-543. Marchese, H.G., Di Paola, E.C., 1975. Reinterpretación estratigráfica de la Perforación Punta Mogotes Nº 1, Provincia de Buenos Aires, República Argentina. Revista de la Asociación Geológica Argentina 30, 44-52. Oyhantçabal, P., Siegesmund, S., Wemmer, K., Presnyacov, S., Layer, P., 2009. Geochronological constraintson the evolution of the southern Dom Feliciano Belt (Uruguay). Journal of the Geological Society, London, 166, 1075-1084. Pankhurst, R.J., Rapela, C.W., Fanning, C.M., Márquez, M., 2006. Gondwanide continental collision and the origin of Patagonia. Earth Science Reviews 76, 235- 257. 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. Schwartz, J.J., Gromet, L.P., 2004. Provenance of Late Proterozoic-early Cambrian basin, Sierras de Córdoba, Argentina. Precambrian Research 129, 1-21.