Inferred Eclogite-Facies Relics in the Migmatite Unit of the Dabie

Inferred Eclogite-Facies Relics in the Migmatite Unit of the
Dabie Block and the Blueschist Unit of the Tongbai Complex,
Eastern China
Chin-Ho Tsai (Hualien Teachers College); Hanwen Zhou (China University of
Geoscience (Wuhan)); Yoshiyuki Iizuka (Academia Sinica (Taipei))
Two new occurrences of metabasites with inferred eclogite-facies relics included in
deformed marbles have been recently recognized in the Dabie and Tongbai region. One is
garnet metabasite as sheared boudins enclosed within dolomitic marble, which crops out in
the migmatite unit of the Dabie block. The other is garnet metabasite as discontinuous pods
within deformed marble in the blueschist unit of the southern Tongbai. The Dabie marble
contains calcite and dolomite, with accessory phlogopite, quartz, plagioclase, tremolite,
and olivine. The mineral assemblages indicate amphibolite-/granulite-facies metamorphism.
The included metabasite contains garnet, quartz, hornblende, clinopyroxene, plagioclase,
titanite, and rare rutile. Mineral textures indicate that hornblende, sodic plagioclase,
clinopyroxene (diopsidic), and titanite are post-peak reaction products. On the basis of
reaction textures and mineral compositions, we infer the existence of precursor omphacite
(i.e. eclogite-facies assemblage) in the metabasite. Post-collisional migmatization might
have caused the amphibolite-/granulite-facies overprinting on the metabasite-marble
association. Our new discovery explains the shortage of UHP records in the migmatite unit.
The Tongbai metabasite contains garnet, green hornblende, and quartz, with accessory
rutile, and relict clinopyroxene. Some garnet grains contain unusually abundant tiny TiO2
precipitates with or without orientation. The host marble contains calcite and accessory
white mica, quartz, apatite, relict garnet, and rutile. Although no evidence of UHP
metamorphism has yet been identified in these Tongbai rocks, some petrographic features,
such as polycrystalline quartz inclusions in garnet (metabasite) and “palisade”-textured
quartz (marble), have led to our speculation about a precursor UHP stage. We interpret the
metabasite as former eclogite associated with marble, and propose a need to revise the
regional tectonic framework.