Multistage metamorphism of garnet orthopyroxenites from the Maowu mafic–ultramafic complex, Dabieshan UHP terrane, eastern China

Abstract

Garnet orthopyroxenites of the Maowu mafic–ultramafic body occur in coesite-bearing paragneisses in the Dabieshan ultrahigh-pressure (UHP) metamorphic complex. We have distinguished six stages of metamorphism based on integrated mineralogical–petrological investigations: M1 – high-T/low-P metamorphism (˜1.4 GPa, ˜850°C); M2 – low-T/low-P metamorphism (˜1.4 GPa, ˜750°C); M3 – low-T/high-P metamorphism (2.1–2.5 GPa, 740–760°C); M4 – UHP metamorphism (5.3–6.3 GPa, ˜800°C); M5 – early retrogression (<3 GPa, >750°C); M6 – late retrogression (<2.3 GPa, <670°C). Detailed textures and mineral compositions indicate that the protolith of the Maowu garnet orthopyroxenites was a mantle refractory harzburgite or dunite that had been metasomatized by crust-derived silica-rich fluid or melt. M1–M2 defines an isobaric cooling P–T path, probably resulting from corner-flow in the mantle wedge above the subduction slab. M2–M4 defines an isothermal compressional P–T path, suggesting that the complex subsequently recycled into the deep upper mantle (up to ˜200 km depth) and underwent HP–UHP metamorphism. M4–M6 defines a retrograde P–T path, implying that the rocks ascended to crustal levels attending exhumation of the UHP terrane, and was continuously metasomatized by fluid derived from the continent country rocks.