Mantle and crust interaction scenario at the crust-mantle transition zone: Depicted from inter-layered pyroxenite-granulite xenolith in Hannuoba area, North China Craton

Abstract

Mantle-crust interaction occurring in various ways has been proposed for a long time, but the details of its mechanism at the crust-mantle transition zone and the lower crust are poorly known. This study reports a layered pyroxenite-granulite xenolith from the Cenozoic basalts of Hannouba in the North China Craton. Petrological and mineral chemical investigations reveal that the pyroxenite layers are cumulates from underplating basaltic magmas, while the granulite portions represent pre-existing lower crust. The magmas were crustally contaminated by interaction with the granulite. Orthopyroxene and plagioclase in the granulite involving reaction with exotic melts are transformed to K-feldspar, clinopyroxene, Fe- and Mg-oxide, melt and/or sulfide. The mineral chemical similarities and overlapping temperature estimates suggest that the pyroxenites and the granulites had been incrementally equilibrated with each other, both physically and chemically, via significant chemical and thermal exchange. The phase transformation and interaction could represent crust anatexis to generate more felsic magmatism and related ore deposits for the middle to upper crust and subsequently increase crustal maturity. This is also an efficient way of forming crust-mantle transition zone below the lower crust and crustal reworking in the Hannuoba area and may be applicable to other places worldwide.