SHRIMP U/Pb Zircon Dating of Neoproterozoic Granitic Magmatism and Collision in the Pelotas Batholith, Southernmost Brazil

Abstract

The introduction of robust geochronological methods for age determinations of the southernmost segment of the Neoproterozoic terranes of Brazil, namely the Dom Feliciano Belt, provides important clues for unraveling the complex evolution of the Brasiliano/Pan-African orogeny in this southwest ern portion of the Gondwana supercontinent. Except for associated small schist belts and postorogenic foreland basins, the belt is represented in this region of southeastern South America by the Pelotas Batholith. Precise SHRIMP U/Pb zircon geochronological techniques based on the study of 95 individual spots on 74 zircon crystals (three samples) and on Nd-isotopic determinations (three samples) are used to assess the late Neoproterozoic history of the belt, especially the orthogneisses interleaved with the batholithic plutons. Three petrotectonic associations were selected for detailed isotopic investigations-the Pinheiro Machado syncollisional monzogranites, the widespread Piratini gneiss tonalitic xenoliths, and the Arroio dos Ratos (now Encantadas) gneiss. The results allow the establishment of the timing and ages of the metamorphic peak and early magmatism. We dem onstrate that evolution of Neoproterozoic magmatism within the Pelotas Batholith occurred through a long-lived crustal recycling process from the Paleoproterozoic Rio de la Plata craton. Three major events are recognized, two corresponding to crustal granite generation by partial melting of Paleo proterozoic protoliths at ~780 and ~610 Ma, and one related to the high-grade syncollisional meta morphic peak at ~630 Ma. The data also yield precise criteria to distinguish between thrust-related granitoids of the Dom Feliciano belt and older orthogneisses, both previously interpreted as a unique, pre-collisional, Brasiliano Cycle magmatic-arc association.

The Río de la Plata Craton: a review of units, boundaries, ages and isotopic signature

“…Magmatic ages at 850-750 Ma on zircon and high-temperature metamorphism dated at 650-600 Ma confirm the correlation between the Punta del Este Terrane and the Coastal Terrane of the Kaoko Belt (Oyhantçabal et al 2009;Gross et al 2009). Similar ages have also been reported in xenoliths of the Pelotas Batholith (da Silva et al 1999) and in the Neoproterozoic basement of the Brusque Group (843 ± 12 Ma, U-Pb SHRIMP on zircon; Basei et al 2008b). …”

Tectonic Setting and Sources of Magmatism Related to the Southern Brazilian Shear Belt

“…2.1Ga. Calc-alkaline magmatism with arc-type signature was produced during the period between 800 and 700 Ma, and is represented mainly by metamorphosed tonalite associations, recognized in the western and eastern regions of the Sul-rio-grandense Shield (Silva Filho & Soliani Jr. 1987, Babinski et al1997, Kraemer 1995, Fragoso-Cesar et al 1990, Silva et al 1999. Their ages, indicated by whole rock Rb-Sr and U-Pb isotopic data in zircons, are about 710 and 780 Ma, respectively.…”

Structural evolution and tectonic setting of the Porongos belt, southern Brazil

“…The granite-gneiss unit in the east (Dom Feliciano belt) is characterized by extensive Neoproterozoic crustal reworking of Trans-Amazonian (2.2-2.0 Ga) gneisses (Babinski et al 1996(Babinski et al , 1997Silva et al 1999Silva et al , 2000. It contains numerous and voluminous c. 630-600 Ma Neoproterozoic granitoids.…”

“…High-precision Pb-Pb and U-Pb zircon ages, comprising both conventional and sensitive, high-resolution ion-microprobe (SHRIMP) data, are now available from a number of granitoid rocks in Rio Grande do Sul (e.g. Babinski et al 1996Babinski et al , 1997Leite et al 1998Leite et al , 2000Silva et al 1999;Hartmann et al 1999Remus et al 2000a). They focus on plutonic rocks in the São Gabriel block and Dom Feliciano belt, whereas only limited reliable isotope data are available for the Porongos belt.…”