Nd-O-Hf isotopic decoupling in S-type granites: Implications for ridge subduction

The generation of S-type granites in the continental crust generally requires partial melting of metasedimentary rocks at relatively high temperatures, and is typically associated with syn- or post-collisional tectonics. These melts are generally expected to inherit the geochemical characteristics of their metasedimentary protoliths, and their Nd, Hf, and O isotopic compositions have been widely used to trace the origin of the magmas. Here we show that these isotopic systems are decoupled in S-type granites from the Chinese Altai, displaying a wide range of zircon epsilon(Hf)(t) values (-0.7 to +6.2), and a limited range of whole rock epsilon(Nd)(t) values (-6.1 to -2.2) and zircon delta O-18 values (+8.49 parts per thousand to +12.91 parts per thousand). Zircon xenocrysts are common in these plutons, and show markedly negative epsilon(Hf)(t) values (from -30.7 to -18) and highly positive delta O-18 values (from +9.61 parts per thousand to +13.03 parts per thousand. In addition, these granites exhibit high Lu/Hf (0.15 to 1.38) ratios, and substantial depletions in Zr and Hf. We develop a new model to explain the observed relationship between residual zircon content and Nd-Hf-O isotope decoupling. We propose that these S-type granites formed from partial melting of metasedimentary crustal rocks, associated with a ridge subduction-related heat source. Limited crustal heating, due to mantle upwelling through the slab window on a small scale, may have resulted in significant disequilibrium melting of the metasedimentary crust. Our findings provide new insights into melting processes in the deep continental crust. (C) 2019 Elsevier B.V. All rights reserved.