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

Abstract

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 ɛHf(t) values (−0.7 to +6.2), and a limited range of whole rock ɛNd(t) values (−6.1 to −2.2) and zircon δ18O values (+8.49‰ to +12.91‰). Zircon xenocrysts are common in these plutons, and show markedly negative ɛHf(t) values (from −30.7 to −18) and highly positive δ18O values (from +9.61‰ to +13.03‰). 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.