Oxygen and carbon isotope anomalies in the ultrahigh pressure metamorphic rocks of the Dabie-Sulu terranes: Implications for geodynamics

Abstract

Unusual isotope compositions of oxygen and carbon have been found in the ultrahigh pressure (UHP) metamorphic rocks of the Dabie-Sulu terranes, East China. Eclogites and coexisting coesite-bearing gneiss show negative δ 18O values as low as -10 to -2‰% relative to SMOW, while coesite-bearing marble associated with the eclogites has high δ 13C values up to +6‰ relative to PDB. The negative δ 18O values in the UHP rocks must be acquired by water-rock interaction between the precursors of the rocks and meteoric water prior to eclogite facies metamorphism, because oxygen isotope equilibrium has been preserved between different mineral pairs from the rocks which precludes hydrothermal alteration during exhumation. The positive δ 13C values in the UHP marble are interpreted to represent the carbon isotope composition of premetamorphic limestone, because there is no known process to enrich carbonate in δ 13C during regional metamorphism. The carbon isotope anomaly in the protolith of the marble suggests a local change in the redox state of water during deposition of limestone. Therefore, the Dabie area was a continental marginal basin that became consuming due to convergence prior to collision between the Yangtze and Sino-Korean plates. Survival of the unusually negative δ 18O and positive δ 13C values in the UHP rocks suggests that there was no significant isotopic exchange with the mantle during the subduction of supracrustal rocks to mantle depths. Because eclogitic and gneissic minerals undergo rapid oxygen isotope exchange by diffusion and recrystallization at mantle pressures and temperatures, preservation of meteoric water signature in the rocks implies a short residence time and a fluid-absent environment for the UHP slab at mantle depths. The consistency of oxygen isotope geothermometries for different mineral pairs suggests a rapid cooling and ascent process for the UHP eclogites subsequent to their formation at mantle depths and a lack of fluid during exhumation.