Extreme negative Eu anomalies in peraluminous granites of South China: implications for extensive fluid extraction

Abstract

Mesozoic peraluminous granites in South China (aluminum saturation index > 1.0) often exhibit strong Eu depletions. These extreme Eu anomalies (Eu/Eu* down to <0.01) are observed primarily in the early Yanshanian period, coeval to extensive W-Sn mineralization in the area (162–141 Ma), but the origin of such extremely low Eu/Eu* remains elusive. Here, we analyzed representative Mesozoic granite samples from South China for whole rock major and trace elements, zircon U-Pb ages, trace elements, and oxygen isotopes. Our results suggest that amount of sediment incorporation is insufficient in generating the extremely low Eu/Eu*. If plagioclase retention played a major role in achieving such low Eu/Eu*, oxygen fugacities of <FMQ–5 would be required, even under conditions of extreme feldspar retention and fractionation; such reducing conditions do not align with zircon redox indicators (FMQ–2 to FMQ + 1). On the other hand, modeling results show that fluid extraction can substantially decrease Eu/Eu* in granitic melts, but the efficiency strongly depends on fluid salinity. Achieving Eu/Eu* < 0.01 in residual melts requires a fluid-to-melt mass ratio of >0.2 for brine systems and >0.8 for low-salinity fluids. Correlations between the Eu/Eu* and Ba/Th and Ba/Nb ratios further support this fluid-driven mechanism. We thus propose that extensive halogen-rich fluids exsolution and extraction were critical to generating extremely low Eu/Eu* in the peraluminous granites and likely contributed to W-Sn mineralization in South China. As much of South China existed as an isolated inland sea at low latitudes in the Indosinian period, intense evaporation combined with high productivity may have formed massive halogen- and organic-rich sedimentary strata. These strata were buried to deep crustal level during the Early Jurassic in response to the low-angle subduction of the Paleo-Pacific Plate. We further propose that reworking of these sedimentary strata provided halogens and other volatiles essential for the magmatism and W-Sn mineralization during the early Yanshanian period.