Disequilibrium partial melting of metasediments in subduction zones: Evidence from O-Nd-Hf isotopes and trace elements in S-type granites of the Chinese Altai

Abstract

Late Cambrian to Devonian granitic magmatism in the Chinese Altai provides a critical view of geodynamic processes active during crustal growth in the Central Asian orogenic belt. In this study, we report results of zircon U-Pb and Hf-O isotopic compositions, whole-rock geochemical signatures, and Nd isotopic data for late Cambrian–Early Devonian granites in the Chinese Altai. These granites were emplaced between 497 and 397 Ma and have high SiO2 (66.02–72.07 wt%) and K2O (3.18–5.19 wt%) contents, and low Fe2O3t (1.94–5.63 wt%), MgO (0.21–2.23 wt%), and CaO (0.62–1.25 wt%) contents, with A/CNK ratios of 1.16–1.53 (where A/CNK = molar ratio of Al2O3/[CaO + Na2O + K2O]). Moreover, these granites are geochemically similar to S-type granites. They are characterized by negative εNd(t) values (–3.84 to –1.54), high δ18O values (+9.34‰ to +13.82‰), and low CaO/Na2O and (Na2O + K2O)/(MgO + FeO + TiO2) ratios, implying a mafic-metapelitic source. The εHf(t) values of the granites (−11.17 to +13.27) are decoupled from the εNd(t) and δ18O isotope values. This is suggested to be a result of disequilibrium during melting of the source wherein residual zircons were preserved and retained large amounts of Hf, producing Hf-depleted melts and Hf-enriched xenocrystic zircons. Variable zircon dissolution rates during melting and melt loss are ascribed to explain the observed variance in Hf concentrations. Based on the results and published data, a ridge subduction model was established to explain the 497–397 Ma high-temperature magmatism in the Chinese Altai.