ROLE OF SEDIMENT RECYCLING AT THE EARLY–MIDDLE PALEOZOIC SUBDUCTION ZONE IN THE CHINESE ALTAY INFERRED FROM SR–ND–HF ISOTOPES AND TRACE ELEMENTS OF THE SUBDUCTION-RELATED GRANITOIDS

Felsic igneous rocks are common constituents of volcanic arcs, and contain valuable information about subduction-related magmatism. In this study we investigate nine granitoids with S-type volcanic arc affinity from the Chinese Altay, emplaced from 507 to 391 Ma in an active subduction zone during the early–middle Paleozoic. These granitoids are characterized by moderate to high SiO₂ contents (61.01–75.30 wt.%), moderate total alkalis (Na₂O+K₂O, 3.43–7.64 wt.%), and high Al₂O₃ contents (13.29–17.18 wt.%). Negative εNd(t) values (−6.1 to −1.0), the wide range of εHf(t) values (−7.0 to +9.0), and enrichment of LILEs such as Pb, Th and U, all suggest that the granitoids were probably derived from the partial melting of subducting oceanic sediments and the associated mantle wedge. This inference is further supported by the Nd-isotope data. The high initial ⁸⁷Sr/⁸⁶Sr ratios (0.703963–0.719428), low Ba/Th ratios (7.00–118.93), and uniformly negative εNd(t) values (−6.1 to −1.0) indicate that slab-derived aqueous fluids were vital in generating the initial magma of these granitoids, and assimilation played only a minor role. Our data demonstrate that residual zircon retains a substantial amount of Hf during the partial melting of oceanic sediments, therefore, Hf may not be an effective tracer for the input of recycled sediments. We conclude that sediment recycling played an important role in the generation of arc magmatism and the growth of the Central Asian Orogenic Belt (CAOB).