Authigenic growth of monazite, xenotime, rutile and zircon by fluid-rock interaction in the Southern Serra do Espinhaço, Brazil

Abstract

The minerals zircon, monazite, xenotime and rutile commonly occur in metasedimentary rocks as detrital but also authigenic grains, which can result from different fluid-driven processes. In this study such processes are investigated on the basis of detailed petrographic observations, mineral-geochemical data, and results of in situ U-Pb dating and Nd isotope analyzes in a kaolinitized micaschist. The data provide evidence for the preservation of detrital grains of zircon, rutile and monazite crystallized between ~ 3100 and 1150 Ma, and for authigenic zircon, xenotime, rutile and monazite formed at ca. 530 Ma. The authigenic character is indicated by zircon outgrowths closely intergrown with xenotime and rutile crystals. The outgrowths occur where detrital zircon faces are intensely 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 and > 0.8 GPa. In contrast, authigenic monazite rims overgrowing rounded cores were formed during the retrograde evolution, as indicated by their occurrence in assemblage with kaolinite, and results of geothermobarometry (T = 280 °C, P < 0.3 GPa). The monazite rims show very low U contents (4–14 µg/g), extremely high Th/U (up to 1670), and nearly identical ¹⁴³Nd/¹⁴⁴Nd_t (0.51184 ± 0.00006) values that markedly differ from those of the detrital cores (¹⁴³Nd/¹⁴⁴Nd_t = 0.51000 ± 0.00050; U = 162–16418 µg/g; Th/U = 2.6–153). The high Th/U points to the involvement of an oxidizing fluid, in line with late goethite formation. Monazite microstructures, and Nd isotope characteristics require a chain of processes, from partial dissolution of detrital monazite, through REE transport and Nd isotope homogenization in an aqueous fluid, to new monazite growth.