Origin and emplacement of a middle Cretaceous gneiss dome, Fosdick Mountains, West Antarctica

The Fosdick Mountains, West Antarctica, form an 80 × 15 km migmatite dome comprising massive paragneisses that exhibit polyphase fabrics, nappe-scale folds that involve granodiorite to leucogranite intrusions, and diatexite. High strain zones developed on the NE flank of the dome. Multiple generations of leucogranite sheets, dikes and diatexite intrude the dome, and evidence for partial melt in structural sites is widespread. Macroscopic folds and the maximum anisotropy of magnetic susceptibility (AMS) direction are oriented NE-SW, generally parallel with the N65W regional finite strain axis determined from brittle faults and a mafic dike array outside the dome. The direction is oblique to the inherited fault that bounds the dome, to regional trends in the surrounding Ford Ranges, and to the nearby continental margin. Paragneiss assemblages yield thermobarometry results that indicate ≥18 km depth for growth of texturally early garnet and ∼10 km depth for growth of texturally late cordierite at the expense of biotite. Nodular and dendritic forms of cordierite that develop at shallow crustal depths completely overprint dynamic fabrics. The cordierite-K feldspar-sillimanite-garnet-biotite gneisses are determined by U-Pb SHRIMP (sensitive high-resolution ion microprobe) zircon analysis to contain inherited zircon populations of 1100-1000 Ma and 500 Ma age. The U-Pb distribution is characteristic of sediments shed from the Ross Orogen of the Paleozoic Gondwana margin, represented by Swanson Formation in the Ford Ranges. A granodiorite gneiss yields 375 Ma prismatic zircon grains characteristic of Ford Granodiorite in the region. Zircon rim ages in both rock types suggest a protracted growth history during polyphase high-temperature metamorphism. The peak of metamorphism was attained at 106-99 Ma, based on prior U-Pb monazite ages and regional relationships, followed by rapid cooling through the range of ⁴⁰Ar/³⁹Ar mineral systems between 101 and 94 Ma. The timing coincides with a change from convergent to divergent tectonics along the West Gondwana margin prior to breakup. Considered together, the partial melt evidence, decompression record and rapid thermal evolution of the partially molten rocks suggests diapiric processes in effect during emplacement the Fosdick Mountains dome along the Balchen Glacier fault. The consistent NE-SW orientation of folds, AMS strain axes, stretching direction from mafi c and felsic dikes, kinematic axes from minor faults, and sparse mineral lineation attest to structural controls on dome emplacement. These are interpreted as evidence of dextral transcurrent strain across the region at ca. 100 Ma.