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 SiO 2 (66.02-72.07 wt%) and K 2 O (3.18-5.19 wt%) contents, and low Fe 2 O 3 t (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 Al 2 O 3 /[CaO + Na 2 O + K 2 O]). Moreover, these granites are geochemically similar to S-type granites. They are characterized by negative ε Nd (t) values (-3.84 to -1.54), high δ 18 O values (+9.34‰ to +13.82‰), and low CaO/Na 2 O and (Na 2 O + K 2 O)/(MgO + FeO + TiO 2 ) ratios, implying a mafic-metapelitic source. The ε Hf (t) values of the granites (-11.17 to +13.27) are decoupled from the ε Nd (t) and δ 18 O 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.