Authigenic monazite-xenotime-rutile-zircon growth in white-mica schist – complex fluid-rock interaction at 530 Ma, Espinhaço Range, Brazil

Petrographic observations, mineral-geochemical data and results of in situ U-Pb dating and Nd isotope analyses provide evidence not only for authigenic growth of zircon, xenotime, rutile and monazite in white-mica schist at ca. 530 Ma, but also for the preservation of detrital grains of zircon, rutile and monazite that crystallized between ~3100 and 1150 Ma. The authigenic character is indicated by zircon outgrowths closely intergrown with xenotime and rutile crystals, and by a U-Pb lower intercept age of 530 ± 5 Ma, estimated for xenotime and monazite rims. The zircon outgrowths occur where detrital zircon faces are dissolved in contact with Fe-Mg-rich phengitic muscovite, suggesting the involvement of an aqueous fluid enriched in K-Mg-Fe-Al-Ti-P near the thermal peak at 510 °C. In contrast, authigenic monazite rims overgrowing rounded cores were formed during the retrograde evolution, as indicated by their local occurrence in assemblage with kaolinite and extreme geochemical compositions. The monazite rims show very low U contents (4–14 ppm), extremely high Th/U ratios (up to 1670), and nearly identical ¹⁴³Nd/¹⁴⁴Nd_t (0.51184 ± 0.00006), values that markedly differ from the detrital cores (¹⁴³Nd/¹⁴⁴Nd_t=0.51000±0.00050; U=162-16418 ppm; Th/U=2.6-153). The high Th/U ratios point to the involvement of an oxidizing fluid, in line with late goethite formation. Monazite microstructures, results of geothermobarometry, and Nd isotope characteristics point to a chain of processes, from partial dissolution of detrital monazite, through REE transport and Nd isotope homogenization in an aqueous fluid, to monazite-rim growth at T = 280 °C and P <3 kbar.