LA-ICPMS ZIRCON U-PB GEOCHRONOLY AND CHEMISTRY: IMPLICATIONS TO THE EVOLUTION OF THE REWORKED PALEOPROTEROZOIC QUIRINO COMPLEX WITHIN THE NEOPROTEROZOIC RIBEIRA BELT, SE-BRAZIL

The Quirino complex represents the reworked Paleoproterozoic basement of the Paraíba do Sul terrane, the uppermost thrust slice of the central segment of the Neoproterozoic-Cambrian Ribeira belt that resulted from the youngest lithospheric convergence of the Pan African-Brazilian collage.

A granitic hbl-bt-orthogneiss outcrop with Archean, Paleoproterozoic and Neoproterozoic zircons was selected. U-Pb geochronology reveals protolith crystallized at 2308 ± 9Ma (upper intercept) and metamorphosed at 606 ± 22 Ma (lower intercept) with fabric related to the development of the 530-520 Ma Paraíba do Sul mylonitic zone. New LA-ICPMS zircon chemistry was performed at the same spot that previous U-Pb LA-ICPMS analysis.

One Archean (3388 ± 16 Ma, minimum age inheritance) grain is transparent, 200µm prismatic size, Th/U = 0.1. The Paleoproterozoic grains (35) are concentrated in the diamagnetic fraction (32), are transparent, 200-400µm size, prismatic, subeuhedral to euhedral. Chemistry in ppm: Mg 0.6-217, Fe 12-1476, Ca 108-3496, Ti 5 -22, Ce 46-49, Th 68-270, U 120-278. High (0.1-0.5) negative Eu anomaly and Th/U 0.1-1.5.

Most (8 of 11) of Neoproterozoic zircons belong to F.F 5° paramagnetic fraction, being yellowish to brownish, prismatic or rounded, 200-400 µm size. Enriched in Mg (683-878 ppm), Fe (1569-3322 ppm), Ca (8140-59763 ppm), Ti (10-167 ppm),Ce (68-430 ppm), Th (678-9626 ppm) and/or U (487-3209 ppm), Ce and Th/U (0.6-3.0) positive correlation. The Ce_N content are 40-256X chondrite, weak positive Ce (1-2) and negative Eu (0.6-0.9) anomalies. The Neoproterozoic grains in the diamagnetic fraction were related to rim overgrowths or new transparent zircons, with Ti 2-5 ppm, Th/U 0.1-1.0, Eu/Eu* 0.1- 0.6, Ce/Ce* 1-3.

We concluded that the Neoproterozoic grains grew after or simultaneously with the others grains of the matrix, in a solid state, including hornblende, allanite, apatite, and/or ilmenite, in a metamorphic process under 620-670 °C (Ti-in-zircon thermometer, diamagnetic fraction). In the other hand the igneous Paleoproterozoic grains show none or little inclusions (hornblende), Zr saturation temperature 780 °C, and 650-816 °C crystallization temperature (Ti-in-zircon thermometer). It is clear that the Th/U ratio alone is not useful to the distinction between igneous and metamorphic zircons.