Sources, evolution and ages of A-type granites from the post-orogenic Itu batholith, SE Brazil: Inferences from zircon U–Pb dating, Lu–Hf isotope ratios and trace-element geochemistry

Abstract

A combination of U–Pb dating by SHRIMP and Lu–Hf isotope and trace-element analyses by LA-ICPMS in zircon crystals from representative samples of the Itu Granitic Batholith, allowed to determine timing, sources and intensive parameters of crystallization (T, fO2) of the plutons that generated this large (>300 km2) occurrence of A-type granites that locally marks the end of the Neoproterozoic orogeny in SE Brazil. The main episode of batholith construction is dated at 578 ± 4 Ma, with possible minor manifestations as young as ∼560 Ma. Negative zircon εHf(t) as low as −19 to −17 in the Indaiatuba pluton requires a significant crustal input, but a component from enriched mantle (which has εHf(t) as low as −10, reflecting contamination from previous subduction) is evident in all plutons by the presence of small mafic bodies and frequent mafic microgranular enclaves showing evidence of mixing with the host granites. Distinct sources are implied by large contrasts in redox state, estimated qualitatively from negative Eu anomalies and quantitatively from Ce anomalies in zircon, with samples from the Itupeva and Salto plutons showing fO2 ∼FMQ+2 and a sample from the Cabreúva pluton closer to FMQ. Significant intra-sample variations of εHf(t) and trace-elements (e.g., Th, U) in zircon are also indicative that multiple sources have contributed to individual magma batches and were probably preserved by fast crystallization related to magma mixing processes.