USING LA-ICPMS ZIRCON FINGERPRINTING TO INVESTIGATE THE RELATIONSHIPS BETWEEN NEOPROTEROZOIC-CAMBRIAN MAGMATISM AND TECTONICS IN SOUTHERN BRAZIL

A series of ~14 Neoproterozoic-Cambrian basins extends for ~1500 km from northern Uruguay through southern Brazil and offers excellent exposures of sedimentary, volcanic, and shallow intrusive units that formed following the construction of the supercontinent Gondwana. Typically, the basins have been studied and interpreted independent of one another, leading to different explanations for their origins (e.g., forearc basin, rift basin, strike-slip basin; cf. Almeida et al., 2010). Almeida et al. (2010) recently proposed that the basins may be components of one extensive rift system. Testing of this hypothesis, however, has been hindered by difficulty in comparing igneous units whose ages were determined over several decades using variable sample sizes and diverse dating methods (Rb-Sr, U-Pb zircon SIMS, U-Pb zircon TIMS, etc.).

To clarify age and petrogenetic relationships between igneous units in these basins, we applied whole rock elemental analysis and LA-ICPMS zircon trace element/U-Pb age fingerprinting to 10 silicic (~69 – 77 wt% SiO₂) samples (five intrusive, five volcanic; ~50-100 zircons per sample) from the central and southern portions of the basin system in Brazil (the Itajaí and Camaquã basins, respectively). We integrated our analyses with data that Braun et al. (2010) collected using the same approach for five silicic plutons from the Graciosa province at the north end of the basin system. The benefits of LA-ICPMS in this instance are multifold: it is relatively fast and cost-effective; it permits simultaneous in-situ elemental and U-Pb isotopic analysis; and it produces data that are comparable between basins.

Zircons in Camaquã volcanics yield a robust ²⁰⁶Pb/²³⁸U age of ~540 Ma, whereas Camaquã intrusions display peaks at ~580 Ma (coeval with Graciosa magmatism) and ~540 Ma (coeval with alteration in the Graciosa plutons). Most zircons have low LREE concentrations consistent with magmatic crystallization, but some – particularly those in the ~540 Ma volcanics – are rich in LREE, suggesting alteration or presence of inclusions. Volcanics and intrusives from the Itajaí basin yield a single discrete ²⁰⁶Pb/²³⁸U age peak of ~520 Ma, reflecting a magmatic event not recorded in the other two basins.