Coronitic and Symplectitic Textures in Meta-Troctolite Reveal the Transition from Magmatism to Granulite-Facies Metamorphism in the Early Paleozoic Tongbai Orogen, Central China

Newly discovered meta-troctolite enclaves in Early Paleozoic Huanggang diorite within dominantly meta-volcaniclastic greenschist to amphibolite facies Erlangping unit in central China extends the known extent of granulite-facies metamorphism in the Tongbai Orogen. Evidence for this granulite facies event is preserved in coronitic and symplectitic textures developed in the meta-troctolites enclaves. The primary assemblage comprises olivine and plagioclase with minor orthopyroxene, clinopyroxene, spinel and amphibole. The late magmatic stage comprising an orthopyroxene (Opx(1)) corona around olivine and clinopyroxene corona around spinel formed by reaction of cumulus and intercumulus phases with trapped melt at around 1040-1060 degrees C near the solidus. During troctolite cooling, metamorphic solid-state replacement of primary olivine and plagioclase produced orthopyroxene (Opx(1))/clinopyroxene (Cpx(1)) + vermicular spinel (Sp(1)) symplectite, followed by an inclusion-free amphibole (Amp(1)) layer surrounded by amphibole + spinel symplectite (Amp(2) + Sp(2)). Microstructural, pseudosection and mineral isopleth analysis coupled with prominent compositional zoning in the Opx(1) corona and chrome-rich spinel (Cr-Sp) suggests that the symplectites formed by diffusion-controlled solid-state replacement at the interface of reactant minerals, probably during cooling to granulite-facies conditions of similar to 820-880 degrees C and 6.7-7.0 kbar. U-Pb dates of rare metamorphic zircon, as indicated by growth zonation and identification of metamorphic minerals enclosed in grains by Raman probe, are ca. 450 Ma. The reaction textures record the transition from late magmatic to granulite-facies metamorphism with the heat supplied by magma upwelling at the base of Erlangping arc triggered by extension of Erlangping back-arc.