The Tandilia System as a southern extension of the Río de la Plata craton: an overview

CINGOLANI, CARLOS ALBERTO

INTERNATIONAL JOURNAL OF EARTH SCIENCES

SPRINGER

Lugar: Berlin; Año: 2010 vol. 100 p. 412 - 432

1437-3254

THE TANDILIA SYSTEM AS A SOUTHERN EXTENSION OF THE RÍO DE LA PLATA CRATON: AN OVERVIEW Carlos Cingolani Centro de Investigaciones Geológicas (CONICET-UNLP) and División Científica de Geología Museo de La Plata. Calle 1 n. 644, 1900-La Plata, Argentina carloscingolani@yahoo.com; ccingola@cig.museo.unlp.edu.ar Key words: Río de la Plata craton, Buenos Aires province, Argentina, Igneous-metamorphic complex, Paleoproterozoic evolution, Neoproterozoic-Early Paleozoic cover, Tectonic model. ABSTRACT The southernmost outcrops of the Río de la Plata cratonic region are exposed in the Tandilia System in the central part of the Buenos Aires province in eastern Argentina.The geological evolution of Tandilia comprises mainly a juvenile igneous-metamorphic Paleoproterozoic basement named Buenos Aires Complex, which is covered by thin Neoproterozoic to Early Paleozoic sedimentary units which display subhorizontal bedding. The Paleoproterozoic basement consists mainly of granitic-tonalitic gneisses, migmatites, amphibolites, some ultramafic rocks and granitoid plutons. Subordinate rock-types include schists, marbles, and dykes of acid and mafic composition. Tandilia was recognized as an important shear belt district with mylonite rocks derived mainly from granitoids. The available geochemical data indicate that the basement igneous-metamorphic rocks are mainly of calc-alkaline signature. After U-Pb zircon crystals data the tectonic scenario seems related to juvenile accretion event (2.25-2.12 Ga) along an active continental margin, followed by continental collision (2.1-2.08 Ga). It was postulated that the collisional tectonic setting caused thrusting and transcurrent faulting favouring the anatexis of the crustal rocks. The tholeiitic dykes constrain the time of crustal extension associated with the last stages of the belt evolution. A lack of recrystallization or new zircon growth in the Neoproterozoic suggests that the Tandilia Paleoproterozoic basement was preserved from younger orogenies such as those of the Brasiliano cycle (0.64-0.59 Ga) After a long paleoweathering process the Sierras Bayas Group (c. 185 m thick) represents a record of the first Neoproterozoic sedimentary unit (siliciclastic, dolostones, shales, limestones), superposed by Cerro Negro Formation (c.150-400 m thick, siliciclastics) assigned to Upper Neoproterozoic age, and the final sedimentary transgression at the Early Paleozoic (Balcarce Formation (c. 90-450 m thick) deposited over all the mentioned Precambrian units reflecting an independent marine basin evolution. Depositional sequences and unconformities were recognized during the transgression-regression events. The Neoproterozoic marine deposits were correlated to the Nama Group in southwest Africa and more recently a direct connection with the lower section of the Arroyo del Soldado Group (Nico Pérez terrane, Uruguay) was described. Based on the geological background an interpreted chronological tectonic evolution model for the Tandilia System is offered.