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, ε_Nd = + 5.1-+ 6.7, ε_Hf = + 10.6-+ 12.3, ²⁰⁶Pb/²⁰⁴Pb = 17.5-18.0) show some highly unusual correlations: ε_Nd correlates positively with ⁸⁷Sr/⁸⁶Sr, but negatively with ε_Hf. ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and ²⁰⁶Pb/²⁰⁴Pb ratios all correlate negatively with Δε_Hf (= deviation from the global ε_Hf-ε_Nd correlation). The correlations form two distinct mixing arrays with one common, high-ε_Nd 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 ε_Nd and ²⁰⁶Pb/²⁰⁴Pb, but high Δε_Hf. The ε_Nd-ε_Hf 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 Δε_Hf-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.