Disentangling magmatic-hydrothermal evolution of highly fractionated granite through the integration of zircon and monazite-(Ce) U-Th-Pb and Hf/Nd-O isotopes

Abstract

It is a great challenge to constrain the magmatic and hydrothermal evolution of highly fractionated granites due to complex evolution processes. In this contribution, we conducted an integrated study of in situ U-Th-Pb and Hf/Nd-O isotope analyses of zircon and monazite-(Ce) from the Wuduoshan granitic pluton in the Qinling orogen, central China, to decipher its complex magmatic-hydrothermal processes. There are two types of zircon. Zircon I occurs as clearly defined cores with higher cathodoluminescence (CL) intensity and lower U (190–1009 ppm) and Th (111–883 ppm) contents, whereas zircon II occurs as rims surrounding zircon I or as individual grains with higher U (1389–7698 ppm) and Th (450–3708 ppm) contents and lower CL intensity. Zircon I yielded coherent SIMS 206Pb/238U ages with a weighted mean of 401.1 ± 4.2 Ma (MSWD = 0.32, 2σ), which represents the emplacement age of the host granite. Whereas the U-Pb data of zircon II spread along the concordia curve with 206Pb/238U ages ranging from 392.4 ± 6.9 to 468.2 ± 6.8 Ma, which might have no geological meaning due to high-U matrix effect and/or Pb loss. Zircon I and II have εHf(t) values of −0.9 to +1.7 and −2.6 to +4.4, and δ18O values of +8.25‰ to +8.93‰ and +7.05‰ to +8.63‰, respectively. The differences in zircon morphologies and trace element contents suggest that zircon I crystallized from early-stage volatile-undersaturated magma, while zircon II may have crystallized from late-stage magmatic fluids. The larger variable and slightly lower δ18O values of zircon II imply the incorporation of external fluids during their formation or later alteration. Crystals of monazite-(Ce) from the Wuduoshan granitic pluton display faint oscillatory or sector zoning in BSE images and are characterized by huttonite-type substitution with high SiO2 and ThO2 contents but low CaO contents. The SHRIMP monazite-(Ce) U-Pb data gave clustered 206Pb/238U ages of 375 ± 5.6 to 396 ± 6.0 Ma with a weighted mean of 384.4 ± 2.5 Ma (MSWD = 0.9, 2σ). This age is up to 16 Ma younger than the age of zircon I, suggesting that monazite-(Ce) records a post-magmatic hydrothermal event later superimposed on the granite. The decoupled monazite-(Ce) and whole-rock Nd isotope compositions provide further support for the post-magmatic hydrothermal process. The samples of monazite-(Ce) have high δ18O values of +8.07‰ to +9.35‰ and negative εNd(t) values of −4.2 to −3.3, implying that the hydrothermal fluids were likely derived from evolved supracrustal materials. Thus, an integration of U-Th-Pb ages and Hf/Nd-O isotopes for zircon and monazite can be used to disentangle the complex magmatic-hydrothermal evolution processes of highly fractionated granite.