Large-scale Mechanical Redistribution of Orthopyroxene and Plagioclase in the Basement Sill, Ferrar Dolerites, McMurdo Dry Valleys, Antarctica: Petrological, Mineral-chemical and Field Evidence for Channelized Movement of Crystals and Melt.

The Jurassic Ferrar dolerite sills of the McMurdo Dry Valleys, Antarctica represent the plumbing system for flood basalt eruptions associated with the breakup of Gondwana. Among the Ferrar sills, the 350–450 m thick cumulate-textured Basement Sill is differentiated into a Lower Marginal Zone (LMZ) gabbronorite, a thick Lower Zone (LZ) orthopyroxene–plagioclase orthocumulate pyroxenite, a strongly layered mela- to leuco-gabbronorite Middle Zone (MZ), a thick Upper Zone (UZ) gabbronorite with ferrogabbroic pods, and an Upper Marginal Zone (UMZ) gabbronorite. Textures and mineral compositions in the LZ pyroxenite and MZ–UZ gabbronorites are nearly identical, the main distinction being the greater relative proportion of plagioclase in the MZ–UZ gabbronorites, and of pigeonite in the UZ. Most orthopyroxene in the LZ, MZ and UZ occurs as sub-euhedral, normally zoned primocrysts, commonly with rounded plagioclase inclusions. Plagioclase is usually sub-euhedral and normally zoned, but can contain sodic cores interpreted to be xenocrystic. Orthopyroxene and feldspar compositions thoughout the sill are generally fairly uniform, and resemble the compositions of micro-phenocrysts in the chilled margins. We infer that the sill was filled by a c. 1250°C slurry of orthopyroxene plagioclase phenocrysts or primocrysts that subsequently unmixed in response to buoyancy forces. The LZ websterite contains numerous anorthosite to gabbronorite schlieren, veins and pipes (c. 5–10 m thick) of layered leuco-gabbronorite in the MZ could represent intra-cumulate sills that formed from plagioclase-rich slurries ascending in segregation channels. Fe–Ti-rich pyroxenitic veins and pods (some pegmatitic) and an unusual coarse-grained plagioclase facies occur at the contacts between massive leuco-gabbronorite layers in the MZ. Discordant ferro-pegmatite pods and dykes occur throughout the UZ. We interpret these Fe-rich pegmatoidal rocks as evolved residual melts expelled from the compacting gabbronoritic cumulates of the MZ and UZ. [ABSTRACT FROM AUTHOR]