Coronas around Olivine in the Miaowan Olivine Norite, Yangtze Craton, South China

Abstract

Coronitic microstructures have been used to interpret the late-stage solidification history of igneous rocks and to constrain the corresponding chemical and/or physical changes. Coronas with three shells were also recognized in the Miaowan olivine norite, Yangtze Craton, South China. In our study, orthopyroxene intergrowth with vermicular magnetite in the inner shell is in optical continuity with magnetite-free orthopyroxene in the middle shell. In the outer shell of brown amphibole remaining magnetite-free orthopyroxene inclusions sporadically occur. Meanwhile Mg# values of orthopyroxene (76–80) in the inner and middle shells are basically consistent with olivine (78–81). In this paper, we propose a multi-stage genetic model for the formation of coronas in the Miaowan olivine norite. In the first stage, the magnetite-free orthopyroxene shell formed through reaction between primocrystal olivine with the residual Si-rich melt at 990–1 053 °C and 6.2–6.5 kbar. In the second stage, the orthopyroxene-magnetite symplectite shell formed when primocrystal olivine reacted with the late-stage residual Fe-rich melt promoted by high oxygen fugacity condition at 927–1 035 °C and 6.0–6.5 kbar. In the third stage, the brown amphibole shell formed as the presence of residual hydrous melt and replaced the middle shell at 821–900 °C and 5.5–6.0 kbar.