Eclogite-facies metamorphic relics in the blueschist-greenschist unit of the southern Tongbai region, eastern China

We report newly-discovered eclogite-facies metamorphic relics within the blueschist- greenschist unit in the south of the Tongbai tectonic block, which is structurally an extension of the Dabie-Hong-an ultrahigh-pressure (UHP) orogenic belt. The studied samples are garnet-hornblende rich metabasites, which occur as discontinuous boudins or pods enclosed within granitic gneiss (Type I) or impure marble (Type II). Type I eclogitic rock contains mainly garnet, quartz, hornblende, albite, phengite, epidote, and Al-bearing titanite. Fine-grained intergrowths of low-Na clinopyroxene and albite imply the existence of precursor omphacite. The inferred eclogite-facies assemblage is garnet + omphacite + rutile + quartz + phengite. Retrograde stages are from epidote amphibolite- to greenschist-facies metamorphism. The linear compositional profile of the porphyroblastic garnet indicates two different stages of metamorphic recrystallization. Type II carbonate-bearing eclogitic rock contains mostly garnet, amphiboles (glaucophane, hornblende, barroisite), quartz, and albite, with accessory omphacite, epidote, rutile, ilmenite, and titanite. The eclogite-facies assemblage includes garnet + omphacite + rutile + quartz + carbonate. Retrograde stages are from blueschist-facies, epidote amphibolite-facies to greenschist-facies metamorphism. Garnet porphyroblasts in the eclogitic rocks also contain different polyphase inclusions, which imply the existence of formerly trapped melts. Although no evidence of UHP metamorphism has yet been identified in these eclogitic rocks, some petrographic features, such as well-annealed polycrystalline quartz inclusions in porphyroblastic garnet and radial fractures surrounding some quartz inclusions, have led to our speculations about a precursor UHP stage. In summary, the Tongbai eclogitic rocks in the blueschist-greenschist unit record part of a subduction orogenic event, which was not previously recognized.