Models for orthopyroxene–plagioclase and other corona reactions in metanorites, Dahomeyide orogen, West Africa

Coronal reaction textures occur in metanorite and related intrusions in the Pan-African Dahomeyide orogen of West Africa; they apparently formed by retrograde subsolidus reactions during cooling of the intrusions from 900 to 700 °C at c. 9±1 kbar. The coronas that formed around orthopyroxene (Opx) consist of sequential layers of diopside (Dio), hornblende (Hbl), garnet (Grt) and plagioclase (Pl) with inclusions of kyanite (Ky) or sillimanite (Sil). Three well-organized mineral assemblage sequences have been identified and modelled using steady-state diffusion theory for closed and open systems. The mineral sequence Opx|Hbl+Qtz|Hbl|Pl+Ky, formed by diffusion-controlled reactions in a closed system as layer thicknesses are very sensitive to relative component mobilities defined by Onsager diffusion coefficient (L_ii). Models of this corona (type i) that satisfy modes require L_CaCa≥L_MgMg≥L_FeFe>L_AlAl～L_SiSi ; however, small open-system fluxes involving loss of Al and gain of Ca are required to obtain the best fit between model and observed mineral proportions. Under steady-state diffusion, the monomineralic hornblende layer grew by replacing plagioclase whereas the |Hbl+Qtz| grew by Opx replacement. The type ii corona, which consists of the sequence Opx|Dio|Grt+Dio|Pl+Sil, is also stable under steady-state diffusion in a closed system. Modelling results show that the diopside grew by replacing Opx whereas most of Grt+Dio grew by replacing plagioclase. Stable solutions to the closed-system diffusion model that approximate the mode are restricted to the L-ratio regions where Fe, Mg and Ca are more mobile than Si and Al but are unstable when L_AlAl>100 L_SiSi . However, type ii corona mineral proportions were only closely matched when open-system loss of Al and gain of small amounts of Fewere considered in the diffusion models and relative mobilities were L_FeFe≥L_CaCa≥L_MgMg>L_AlAl～L_SiSi . The modelling results indicate that Ca and Mg were the most mobile elements in the formation of type i corona whereas Fe and Ca were the most mobile components in the growth of type ii coronas. A third corona type, consisting of the mineral sequence Act|Hbl|Grt|Pl+Sil, is only stable in open systems and requires large external fluxes involving gain of Fe, Al, Mg and Na and loss of Ca to obtain a solution of the diffusion model that approximates the estimated mineral proportions. Extensive recrystallization of plagioclase to produce sillimanite or kyanite inclusions accompanying corona formation may explain the open-system behaviour indicated by the diffusion models.