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 350-450m 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 degrees 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 (<2m diameter), which we interpret as fossil segregation channels. Textures and mineral compositions in these felsic channels are very similar both to UZ and MZ gabbronorites, and to the groundmass separating accumulated orthopyroxene primocrysts in the LZ and MZ. We infer that plagioclase-charged, hydrous pore melt from the pyroxenite may have segregated, pooled and ascended through these conduits to feed growth of the UZ gabbronorite. Detailed mapping shows that the pipes are separated by about 15 m on average. Calculations suggest that this number density of conduits could have drained the LZ cumulates of their interstitial melt plagioclase in about 8 days. Sequences (each 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.