Protolith age of Santa Maria Chico granulites dated on zircons from an associated amphibolite-facies granodiorite in southernmost Brazil

Abstract

U-Pb dating of zircon was undertaken with the Beijing SHRIMP II (sensitive high resolution ion microprobe) on an amphibolite facies granodiorite and an almandine-albite granulite from the Santa Maria Chico Granulitic Complex, southern Brazilian Shield. This work was also done to unravel protolith ages which are often hidden in the array of partly reset data. The obtained metamorphic ages of the granodiorite gneiss and the granulite are 2035 ± 9 Ma and 2006 ± 3 Ma, respectively. These data are within the range of metamorphic ages determined in previous studies (2022 ± 18 Ma and 2031 ± 40 Ma). However, protolith ages for the granodiorite (2366 ± 8 Ma) and the granulite (2489 ± 6 Ma) were obtained which are outside the previously recognized range (> 2510-2555 Ma). The magmatic protolith age of the granodiorite refers to a previously little known magmatic event in the shield. Further investigations may demonstrate that amphibolite facies zircon crystals are useful as a window into geological events in associated granulites, because zircon ages are blurred in the studied granulites.

Paleo- and Neoproterozoic magmatic and tectonometamorphic evolution of the Isla Cristalina de Rivera (Nico Pérez Terrane, Uruguay)

“…Siderian to Neoarchean inheritance in zircon and Sm/ Nd model ages are also indicated in the Santa María Chico Granulitic Complex (Hartmann et al 1999(Hartmann et al , 2008 and suggest it represents the prolongation of the basement of the Nico Pérez Terrane in southern Brazil.…”

The Paleoproterozoic in the South-American continent: Diversity in the geologic time

“…On the other hand, there are (up to now) some isolated observations, such as those in Tickyj et al (2004) and Hartmann et al (2008) referring to the presence of pre-Rhyacian rocks, which are worth mentioning. In the northernmost of the craton, in the socalled "Taquarembó block", west of the exposure window of the Rio Grande do Sul basement, in the Santa Maria Chico Complex granulitic rocks (polycyclic, with partial retrograde metamorphism to the amphibolite facies) of granodioritic composition, presented UePb (SHRIMP) ages of 2.35 Ga, obtained from zircon cores.…”

Lithologies, structure and basement-cover relationships in the schist belt of the Dom Feliciano Belt in Uruguay

“…1C). The Santa Maria Chico complex is defined in Rio Grande do Sul and contains ortho-and paragneisses reaching granulite facies at ~ 2.4 Ga (Hartmann 2008). The Valentines-Rivera complex is constituted by mafic and felsic orthogneisses, yielding multistage magmatic ages of ~ 2.17 -2.11 Ga and reaching granulite facies at ~ 2.08 Ga (Oyhantçabal et al 2012).…”