Sources of Anfengshan basalts: Subducted lower crust in the Sulu UHP belt, China

Deep subduction of lower continental crust to mantle depths has been recognized in outcrops and drill cores of eclogites and other ultra-high pressure (UHP) rocks of the Sulu intra-continental orogenic belt. In a search of evidence for such subducted crustal sources of melts, we study the elemental and isotope geochemistry of basalts from Anfengshan, a Miocene volcano located in the Sulu UHP belt, as well as Nd–Hf isotopes of eclogites from the Sulu belt itself. The Anfengshan basalts are basanites and nephelinites with low SiO2, high incompatible element contents, positive Nb, Ta, Sr, and negative K, Pb, Zr, Hf, and Ti anomalies. Radiogenic isotopes (87Sr/86Sr = 0.70337–0.70359, εNd = + 5.1–+ 6.7, εHf = + 10.6–+ 12.3, 206Pb/204Pb = 17.5–18.0) show some highly unusual correlations: εNd correlates positively with 87Sr/86Sr, but negatively with εHf. 87Sr/86Sr, 143Nd/144Nd, and 206Pb/204Pb ratios all correlate negatively with ΔεHf (= deviation from the global εHf–εNd correlation). The correlations form two distinct mixing arrays with one common, high-εNd end-member. Superchondritic Zr/Hf ratios (~ 55), and negative Zr, Hf, Ti anomalies indicate that the common mantle source component has been metasomatized by carbonatitic liquids. We suggest that the other two source components are eclogites derived from subducted lower crust: both of these differ from ordinary mantle components by their low 87Sr/86Sr and low εNd and 206Pb/204Pb, but high ΔεHf.The εNd–εHf values of the eclogites form two groups, both of which lie close to the Hf–Nd mantle array, and are therefore not direct analogues of possible source eclogites for the basalts. We explain the shift toward high ΔεHf-basalt sources as follows: an early Cretaceous igneous event extracted partial melts from the eclogites residing in the mantle, thereby increasing their (residual) Lu/Hf ratios, while changing Sm/Nd only slightly. During the Anfengshan melting event, these garnet-rich sources produced melts with low Zr/Hf and increased Nb/Nb*, Hf/Hf*, Sm/Yb ratios relative to the peridotitic source end-member. We therefore suggest that the eclogites represent the residues of mafic lower continental crust subducted during the Triassic continent–continent collision. This interpretation is supported by recent seismic tomography, which revealed a high-velocity anomaly in the uppermost mantle beneath the Sulu belt.