Ultrahigh-ϵ Nd eclogites from an ultrahigh-pressure metamorphic terrane of China

Abstract

The SuLu ultrahigh-pressure (UHP) metamorphic terrane is part of the Qinling-Dabie suture zone formed by the collision between the Sino-Korean and Yangtze cratonic blocks. Geochemical and isotopic investigation of the UHP metamorphic rocks has revealed a spectacular history for some eclogitic rocks. Retrograded coesite-bearing eclogites from the Weihai area in the eastern end of the SuLu terrane have recorded an extraordinary Nd isotopic growth with a world-record high ϵ Nd-value of + 264 (range = + 170 to + 264). Such isotopic characteristics are consistent with the observed extreme LREE fractionation and very high Sm/Nd ratios. The SmNd isotopic data allow us to calculate the minimum age of protolith formation at ∼ 1.7 Ga, assuming their evolution from a depleted or a chondritic mantle. Geochemical data indicate that the eclogite protoliths were of basaltic compositions originally, but underwent metasomatism (water-rock interaction) at high pressures and temperatures leading to the present apparent andesitic composition and extreme LREE depletion. This process took place in the Proterozoic (∼ 1.7 Ga) and was not related to the Mesozoic continental collision and ultrahigh-pressure metamorphism at ∼ 210−220 Ma ago. Because of a protracted retrograde metamorphism, garnet SmNd systems were not closed until ∼ 160 Ma ago, ∼ 50 Ma after the peak eclogite metamorphism. This implies that the exhumation history is significantly different in different parts of the Dabie and SuLu UHP metamorphic terranes. From geochemical analyses and isotopic age determinations we argue that at least two ancient tectonic events in the middle and late Proterozoic can be identified prior to the last Mesozoic continental collision and accompanying UHP metamorphism in the SuLu and Dabie terrane.