Fine-grained mafic bodies as preserved portions of magma replenishing layered intrusions: the Nadezhda gabbronorite body, Lukkulaisvaara intrusion, Fennoscandian Shield, Russia

Abstract

The 100 m thick and 700 m long Nadezhda body in the Lukkulaisvaara layered intrusion exhibits concentric zonation with an inward progression from a 0.5 to 1.0 m thick marginal layer of medium- to coarse-grained norites and gabbronorites that abruptly give way to fine-grained oikocrystic gabbronorites composing the rest of the body. The concentric zonation is additionally emphasized by well-developed alignment of plagioclase laths and orthopyroxene oikocrysts parallel to the outer contacts of the body, pegmatitic gabbronorite segregations in the centre of the body and slight inward decrease in whole-rock Mg# and Cr and increase in incompatible elements. The body has distinctly higher whole-rock Mg# and lower concentrations of all incompatible components than its host rocks. It is enveloped by highly altered marginal anorthosites belonging to host norites and gabbronorites. We interpret the Nadezhda body as a portion of high Mg# (∼75%) and incompatible element-poor (∼20 ppm, Zr; ∼10 ppm, total REE; ∼0.20 wt%, TiO2) magma that replenished the evolving chamber and became trapped within the cumulate pile. Recrystallization of adjacent rocks by volatiles exsolved from the magma upon emplacement resulted in formation of marginal anorthosites. Upon cooling the magma started to crystallize medium- to coarse-grained norites along its margins, but subsequent decompression and loss of volatiles led to rapid crystallization of magma into fine-grained oikocrystic gabbronorites. Solidification of the remaining residual liquid gave rise to pegmatitic gabbronorite segregations.