The Rio de la Plata Craton: the search for its full extent

RAPELA, C.W.; FANNING, C.M.; PANKHURST, R.J.

Mendoza, Argentina

Congreso; GONDWANA 12; 2005

The Rio de la Plata Craton (RPC) is widely considered to be autochthonous Palaeoproterozoic continental crust in south-western Gondwana, acting as a kernel for supercontinent growth by lateral accretion during Phanerozoic times. Its outcrops are restricted to classical areas near the Atlantic margin of South America, such as the Tandilia belt and the Piedra Alta terrane, thereby hampering more exact tectonic and crustal growth models for Gondwana. To better define the westward extension, age and compositional characteristics of the RPC, we have carried out geochronological, geochemical and isotopic studies on: a) four magmatic and metamorphic samples recovered from the bottom of deep drill holes (973–2608 m below surface) sited in the Sierras Pampeanas, far from the Paleoproterozoic type areas, and b) two samples of the sedimentary Neoproterozoic and Early Palaeozoic sequences that cover the basement in the Tandilia belt. Drill core samples from central and south-eastern Córdoba province have previously produced K–Ar ages of ca. 440 to 1515 Ma (Russo et al., 1979). The Saira drill core is a Ho+Bi quartz diorite (SiO2 = 51.0%). We have obtained a SHRIMP U–Pb zircon crystallization age of 2162 ± 3 Ma (Concordia age), and determined a total rock εNdt of +2.6 and a multi-stage TDM of 2303 Ma. The Ordoñez drill core is Bi+Mus monzogranite (SiO2= 72.5%): it has a U–Pb zircon crystallization age of 2086 ± 4 Ma (Concordia age), with an εNdt of +1.7 and TDM of 2312 Ma. Assuming a Paleoproterozoic age of 2100 Ma for the two other drill core samples, their calculated εNdt and TDM values are entirely consistent with those of the Saira and Ordoñez samples (Santiago Temple: olivine gabbro, SiO2=46.4 %, εNdt= +1.7, TDM= 2327 Ma. Camilo Aldao: hornblende schist, SiO2= 52.9%, εNdt= +1.5, TDM= 2340 Ma) indicating that all belong to a juvenile Paleoproterozoic continental crustal block. In the Tandilia belt, SHRIMP U–Pb determination on 60 zircons from a quartzite of the Neoproterozoic Sierras Bayas Formation (lower quartzite member, Barker area) show a dominant provenance at about 2240 Ma (mostly 2000–2550 Ma), and a minor peak at about 1135 Ma probably derived from a distant? Mesoproterozoic source. A sample of the ?Early Palaeozoic Balcarce Formation from its type area shows a more complex zircon provenance age pattern (n= 60) with conspicuous peaks in the ranges ca. 475–675, 975–1170 and 1685–2060 Ma, and minor peaks at ca. 800–890 and ca. 2540 Ma. The youngest zircons at about 475–480 Ma (Early Ordovician) provide a maximum estimate for the age of the Balcarce Formation. Conclusions obtained from the above results drastically change current knowledge of the areal extent of the RPC and raise important constraints for models on the accretion of SW Gondwana:  i. The basement underlying Late Palaeozoic and younger sediments in central and southern Cordoba is juvenile Palaeoproterozoic crust that closely resembles the age range and composition of the Tandilia belt (2065–2235 Ma, with peaks at ~2.16 and ~2.08 Ga, Hartmann et al., 2002). The εNdt and TDM values of the latter are –0.7 to –2.8 and ca. 2600 Ma, respectively (Pankhurst et al. 2003), suggesting minor reworking of older material.  The results strongly suggest that the RPC extends continuously at least 640 km to the north-west of Tandilia. ii. The Pampean mobile belt in the Sierras de Córdoba, and in general in most metasediments of the Eastern Sierras Pampeanas, do not record zircon provenance patterns with conspicuous peaks in the interval ca. 2065–2235 Ma (Schwartz & Gromet, 2004; PAMPRE unpublished SHRIMP data) and display peak TDM model ages in the interval 1600–1700 Ma (Pankhurst et al., 1998). Together these data indicate a remarkably sharp crustal boundary between the RPC and the Sierras Pampeanas with little influence of the now adjacent RPC as a source for the Palaeozoic mobile belts.   Hartmann, L.A., Santos, J.O.S., Cingolani, C.A., and McNaughton, N.J., 2002. Two Palaeoproterozoic orogenies in the evolution of the Tandilia Belt, Buenos Aires, as evidenced by zircon U-Pb SHRIMP geochronology: International Geology Review, v. 44, p. 528-543. Pankhurst, R.J., Rapela, C.W., Saavedra, J., Baldo, E., Dahlquist, J., Pascua, I. and Fanning, C.M., 1998. The Famatinian magmatic arc in the central Sierras Pampeanas: an Early-to-Middle Ordovician continental arc on the Gondwana margin. In: Pankhurst, R.J & Rapela, C. W., eds., The Proto-Andean Margin of Gondwana, Geological Society of London Special Publication 142, p. 343-367. Pankhurst, R.J., Ramos, A., and Linares, E., 2003. Antiquity and evolution of the Río de la Plata craton in Tandilia, southern Buenos Aires province, Argentina: Journal of South American Earth Sciences, v. 16, p. 5-13. Russo, A., Ferello, R. and Chebli, G., 1979. Llanura Chaco-Pampeana. Segundo Simposio de Geología Regional Argentina, Córdoba, Tomo I, 139-183. Schwartz, J.J. and Gromet, L.P., 2004. Provenance of Late Proterozoic-early Cambrian basin, Sierras de Córdoba, Argentina: Precambrian Research, v. 129, 1-21.