Contrasting zircon Hf–O isotopes and trace elements between ore-bearing and ore-barren adakitic rocks in central-eastern China: Implications for genetic relation to Cu–Au mineralization

Abstract

The petrogenesis of Early Cretaceous adakitic intrusions in the Lower Yangtze River belt (LYRB), central-eastern China, and their genetic association with Cu–Au mineralization have recently been debated. This study presented integrated in-situ zircon U–Pb–Hf–O isotopic and trace elemental data for the LYRB adakites, and a comparison with ore-barren adakites from the south Tan-Lu fault (STLF) adjacent to the LYRB. Magmatic zircons from these two series of intrusions have U–Pb ages of 145–132Ma and 136–132Ma respectively. The STLF zircons have δ18O ranging from 5.6 to 6.7‰ and εHf(t) from −28.8 to −16.4, plotted within the range of global lower crustal metabasaltic xenoliths, consistent with low-radiogenic Pb of the host adakitic rocks. In contrast, both Hf and O isotopic compositions of zircons from the LYRB are greatly variable with heavier δ18O (4.7 to 9.6‰) and higher εHf(t) values (−25.5 to +2.0) compared with the STLF series. The co-variations of Hf–O isotopes in the LYRB series reflect source heterogeneity as a result of mixing of basaltic oceanic crust with sediments (10–20%), consistent with high-radiogenic Pb and enriched Sr–Nd isotopic compositions of the host adakites. The high La, U and low Ti concentrations in the LYRB zircons also imply a volatile (perhaps, CO32−-rich, carbonatite-like) source. Combined with whole-rock geochemical data, the new results further suggest contrasting origins of the LYRB and STLF adakites from subducted oceanic crust and foundering lower continental crust, respectively.The LYRB zircons have much higher ratios of Ce4+/Ce3+ (avg.417) and Eu/Eu* (avg. 0.67) than the STLF zircons (avg. 84 and 0.44). This difference confirms that the ore-bearing adakitic magmas are more oxidized relative to the ore-barren ones. There is roughly a positive correlation between zircon Ce4+/Ce3+ and δ18O in the LYRB series, probably indicating that the elevated fO2 was related to components enriched in heavy oxygen isotopes. A possible candidate is sediments, carried by subducting slabs. The involvement of sediments may significantly promote oxidation of the resulting adakitic melts, a key factor for generation of Cu–Au mineralization, e.g., in the LYRB. This study also indicates that combined in-situ analysis of zircon REEs and δ18O could be a powerful tool to decipher the intrinsic links of fO2 with sediment components in subduction zones.