U–Pb and Hf isotopes in granitoids from the Eastern Bolivian basement: Insights into the Paleoproterozoic evolution of the western part of South America

Abstract

Granitoids from the Eastern Bolivian basement at Santo Corazón, Correreca, and Santa Terezita, and the Santana Gneiss from the Corumbá region (Mato Grosso de Sul state, Brazil) were studied by geochemistry and Sm–Nd and U–Pb isotope analysis. The granites have a medium to high calcium-alkaline potassic character and are typically peraluminous S-type granites. For The Santo Corazón quartz monzonite and the Correreca granite we obtained Orosirian upper intercept U–Pb zircon ages of 1874 ± 15 Ma and 1862 ± 7 Ma. For Santa Terezita granite two upper intercept U–Pb zircon ages of 1849 ± 11 and 1852 ± 6 Ma were obtained. The geochemical and Nd and U–Pb isotope data suggest that these complexes were part of a crust created between 2.23 and 1.96 Ga, which was reworked mainly in Orosirian times in a magmatic arc setting. This is further supported by Hf isotope data for zircon from the Correreca granite. The garnet-hornblende-biotite Santana Gneiss gave an upper intercept zircon U–Pb age of 1764 ± 23 Ma, which is interpreted as the crystallization age of the igneous protolith, a quartz-monzonite to granodiorite. The petrological, geochemical, and Nd as well as U–Pb isotope data show similarity between the Eastern Bolivian basement granites (EBB) and the Santana Gneiss with the Alumiador Intrusive Suite and the Caracol Gneiss, respectively. This suggests that this crustal fragment could have been an extension to the west of the Rio Apa Block, which today extends for 300 km from the southwest of Mato Grosso do Sul and northeast of Paraguay to Eastern Bolivia. The presence of continental crust that dates back to 1.7–1.8 Ga is inferred from isotope data for the cores of zircon from the Santana Gneiss and the Eastern Bolivian granites (EBB). In contrast, U–Pb ages obtained in rims of zircon crystals may indicate that two collisional events took place: a first one at 1.7 Ga, which we can relate to the collision of the Eastern and Western terranes of the Rio Apa Block with the Paraguá Block located to the west; and a second one at 1.3 Ga, related to the collision between the Paraguá Block, Eastern Bolivian basement granite terrane and the Rio Apa Block (EBB + RAB) with the Amazonian Craton during the San Ignacio Orogeny. This assemblage remained stable during the amalgamation into the Rodinia supercontinent.