Zircon Hf isotope behavior during the magmatic-hydrothermal processes: A case study from the Yashan pluton, South China

Zircon is a common accessory mineral in granites. Zircon Hf isotope analysis has been widely applied in the studies of crustal evolution and granite genesis. However, previous studies have demonstrated that zircon can be altered by reacting with fluids. At present, it is poorly known how hydrothermal fluids exsolved from evolved magmas may affect the Hf isotope system of zircon. To address this issue, we carried out a case study by evaluating the U-Pb and Lu-Hf isotope systems as well as trace element analyses on zircons from granites of the Yashan pluton, South China, which is well-known for the Nb-Ta ore deposit related to magmatic-hydrothermal activities. Previous geochronological studies indicate that the crystallization and mineralization ages are contemporaneous, at ca. 150–160 Ma. The hydrothermal fluids that are responsible for hydrothermal alteration and mineralization were exsolved from late evolved magmas. Based on micro-textures and trace element compositions, zircon in these rocks can be categorized into three types: magmatic, early hydrothermally altered, and lately hydrothermally altered. Magmatic zircons in these rocks are characterized by typical oscillatory zones in CL images, crystalline zircon bands in Raman spectrum, high major element concentrations (32.5 wt% SiO2 and 61.6 wt% ZrO2), and steeply increasing chondrite-normalized REE patterns from La to Lu with positive Ce anomalies. Early altered zircons display inward-penetrating dendritic dark zones and cracks in BSE images. Furthermore, these zircons have flat LREE patterns, redshift and wide Raman characteristic peaks, suggestive of fluid influence. Lately hydrothermally altered zircons are distinguished by four types of secondary textures: featureless domains, porous domains with abundant inclusions, irregular patchy reaction zones and domains containing both pores and patchy reaction zones. They have disturbed U-Pb dates, disappeared Raman characteristic peaks, relatively lower major element concentrations and higher trace element concentrations relative to the magmatic zircons. The lately altered zircons were formed by coupled dissolution-reprecipitation and diffusion-reactions. Magmatic zircons have largely varied εHf(t) values (at 150 Ma) of −10.5 to −1.2 (mean = −7.4) and the first and third quartiles (Q1 and Q3) of −8.4 and − 6.7. Such a variation could be related to incremental assembly of distinct magma batches with different isotopic compositions from the heterogeneous magma sources. Similarly, lately hydrothermally altered zircons have largely varied εHf(t) values of −9.4 to −3.7 and − 12.2 to 1.3 for different samples. We speculate that the large Hf isotope variations of this zircon type were inherited from heterogeneous magmas. It indicates that even if melts in the late periods of magma evolution are rich in fluorine and thus can efficiently transport high field strength elements like Hf, the resultant zircons do not have homogeneous Hf isotope compositions as previously suggested. Therefore, the circulation scales and durations as well as the contents of complexing agents may all determine whether Hf isotopes can be homogenized in the later stages of magma evolution.