Reassessment and new chronological data (Ar/Ar, Rb/Sr and Sm/Nd) from the Pelotas batholith, Rio Grande do Sul state: petrogenesis implications and reactivation age of the shear zones

SHRIMP and conventional U-Pb and Pb/Pb evaporation dating of zircon from the Pelotas batholith indicates one group of ages between 625 and 633 Ma for the Pinheiro Machado suite, and another group between 575 and 599 Ma for the Viamao, Encruzilhada do Sul and Dom Feliciano suites. Sr 87/Sr 86 isotopic ratios of 0.7060-0.7016, epsilonNd values of -5 and -10, and Nd TDM model ages between 2200 and 1600 Ma suggest that the suites were mainly derived by melting of Paleoproterozoic crust formed during the Tranzamazonic Cycle. Basic magmatism and heterogeneous magma mixing in the Pinheiro Machado, Viamao, Encruzilhada do Sul and Dom Feliciano suite and the Piquiri alkaline magmatism, with Pb/Pb zircon evaporation ages between 610 and 615 Ma, suggest that a mantle component also contributed to the formation of the Pelotas batholith. 39 Ar/40 Ar determinations in micas from mylonitic rocks of high- and low-dip shear zones showed ages between 540 and 530 Ma, which show that important tectonic events occurred after the emplacement of the batholith. This event developed in a transpressive regime, was responsible for the reactivation of older shear zones, and was probably contemporaneous with the development of the Camaqua basin. (author)