Metamorphic P-T conditions and thermal structure of Chinese Continental Scientific Drilling main hole eclogites: Fe-Mg partitioning thermometer vs. Zr-in-rutile thermometer

Core rocks recovered from the main hole (5158 m deep) of the Chinese Continental Scientific Drilling (CCSD-MH) project, southern Sulu UHP terrane, east-central China, consist of eclogites, various gneisses and minor metaperidotite cumulates; this lithological section underwent subduction-zone UHP metamorphism. Coesite-bearing eclogites are mainly present between the depths of 100-2000 m, but below 2000 m, mafic eclogites are rare. Selected elements (Zr, Nb, Cr, Fe, Si, Mg, Al and Ti) in rutile from 39 eclogite cores from 100 to 2774 m, and major elements of minerals from representative eclogites were analysed by electron microprobe. Zirconium and Nb concentrations of rutile cluster ∼100-400 and 200-700 ppm respectively. However, Zr and Nb contents in rutile from strongly retrograded eclogites show larger variations than those of fresh or less retrograded eclogites, implying that somehow fluid infiltration affected rutile chemistry during retrograde metamorphism. Zr contents in rutile inclusions in garnet and omphacite are slightly lower than those of the matrix rutile, suggesting that the rutile inclusions formed before or close to the peak temperature. The P-T conditions of the CCSD-MH eclogites were estimated by both Fe-Mg exchange and Zr-in-rutile thermometers, as well as by the Grt-Cpx-Phn-Ky geothermobarometer. The maximum temperature range of 700-811 °C calculated at 40 kbar using the Zr-in-rutile thermometer is comparable with temperature estimates by the Fe-Mg exchange thermometer. The temperature estimates of eclogites in a ∼3000 m thick section define a continuous gradient, and do not show a distinct temperature gap, suggesting that the rocks from 100 to 3000 m depth might belong to a single, large-scale UHP slab. These data combined with P-T calculations for CCSD-MH peridotites yield a low geotherm (∼5 °C km^-1) for the Triassic subduction zone between the Sino-Korean and Yangtze cratons; it lies ∼30-35 mW m^-2 conductive model geotherm.