Refining granite generation by interrogation of zircon and monazite U-Th-Pb and Hf/Nd-O isotopes

In-situ geochemical and isotopic analyses of zircon and monazite can provide compelling and detailed insights into the source and evolution of their hosting granite. Here we investigated the U-Th-Pb and Hf/Nd-O isotopic signatures of zircon and monazite in two petrological zones (i.e., fine- to medium-grained zone and medium-grained zone) of the Laojunshan granite pluton in the Qinling Orogen, central China, to better constrain their source nature and evolution processes. Co-existing magmatic zircon and monazite in both zones gave identical crystallization ages of ca. 113 Ma. In the fine- to medium-grained zone, both magmatic and xenocrystic zircons were identified, which respectively have εHf(t) values of −6.5 to −5.0 and − 17.4 to +5.1, and δ18OVSMOW values of +5.25‰ to +5.78‰ and + 5.17‰ to +9.15‰. The age distribution pattern and HfO isotopic characteristics of the xenocrystic zircons are similar to those of detrital zircons from the wall rock, i.e., meta-sedimentary rocks in the Kuanping unit. Monazite grains in this zone have relatively low SiO2, ThO2 but high CaO contents, dominated by cheralite-type substitution. These monazites have εNd(t) values of −5.8 to −4.6 and δ18O values of +3.79‰ to +5.66‰, except one grain (εNd(t) = −10.4 to −9.6, δ18O = +9.22‰ to +9.81‰), which is likely a xenocrystic crystal assimilated from wall rocks. The identical UPb age but contrasting NdO isotopic compositions between the xenocrystic monazite and the magmatic monazite grains indicate that the former might have experienced solid-state recrystallization process and completely lost radiogenic Pb. Both zircon and monazite geochemical and isotopic signatures consistently indicate that crustal assimilation occurred in the fine- to medium-grained granite zone. In contrast, in the medium-grained zone, only magmatic zircons and monazites were identified. The magmatic zircons have εHf(t) values of −3.2 to +2.7, and δ18O values of +4.65‰ to +6.36‰. The magmatic monazites are characterized by huttonite-type substitution with high SiO2, ThO2 but low CaO contents. Their εNd(t) values vary from −5.7 to +1.6 (mostly < −4.4) and δ18O values from +4.65‰ to +6.36‰. Given that the isotopic features of zircons and monazites in this zone are consistent with their whole-rock isotope features and no inherited/xenocrystic grains were observed, the zircons and monazites should record the source isotope compositions. The presence of highly radiogenic Nd isotopes in monazite coupled with its normal mantle-like O isotopes suggest minor involvement of juvenile mantle-derived components in the magma source. Accordingly, we advocate combined study of zircon and monazite, which can better track granite source nature and evolution processes than individual mineral.