The roots of the Dabieshan ultrahigh-pressure metamorphic terrane: constraints from geochemistry and Nd-Sr isotope systematics

The Dabieshan area between the Sino-Korean and Yangtze cratons in east-cent ral China has become the focus of much recent attention because of the disc overy of abundant coesite and rare micro-diamond inclusions in both eclogit es and their enclosing country rocks. The Dabieshan metamorphic complex, pr eviously regarded as Archean continental basement of the Yangtze craton, is mainly made up of Precambrian felsic orthogneiss, amphibolite, and migmati tic gneiss with minor eclogite, granulite, ultramafic rock and marble. Our geochemical analyses and Nd-Sr isotope data show that most Dabieshan orthog neisses are distinctly different from the nearby Kongling gneisses of the Y angtze basement, which are Archean high-Al TTG rocks with an average Nd mod el age of 3.3 +/- 0.2 Ga with volcanic are granitic affinity. The protolith s of the Dabieshan orthogneisses are diverse, and three types of rocks are distinguished: (1) the majority of the felsic gneisses in the eclogite unit s display geochemical signatures of post-Archean granites and may have resu lted from Neoproterozoic magmatism in a rift environment; (2) some of the f elsic gneisses in the eclogite units have an affinity with Kongling gneiss, and were presumably derived from the Yangtze basement by tectonic extrusio n during Mesozoic exhumation of the ultrahigh-pressure (UHP) metamorphic ro cks; and (3) the felsic gneisses of the dome region show geochemical signat ures of Archean granitoids and Nd model ages between 3.1 and 1.0 Ga? attrib uted to mixing between Neoproterozoic mantle-derived material and the Arche an Kongling gneisses. Numerical modeling shows that mixing between mantle-d erived melts and Kongling gneiss can account for the Nd-Sr isotopic Variati on of Mesozoic mafic monzodiorites in the UHP eclogite unit, implying that the Kongling complex was extended beneath the Dabieshan terrane possibly du ring early Mesozoic continental collision. We suggest that the dome region was originally part of the Yangtze craton, and was separated from it by Neo proterozoic rifting. The orogen was later significantly modified, especiall y by Jurassic-Cretaceous migmatization and magmatism. (C) 2000 Elsevier Sci ence B.V. All rights reserved.