STRUCTURAL CONTROLS ON MELT TRANSPORT PATHS AND DURATION OF DEFORMATION-INDUCED MELT MIGRATION, FOSDICK MOUNTAINS, ANTARCTICA

The influence of regional strain upon localization, migration and coalescence of melts produced during migmatization is being investigated in the Fosdick Mountains gneiss dome, West Antarctica. Recently deglaciated cliff faces and arrets provide 3-D control on geometry of fabrics, folds and shear zones on a m- to km-scale; expose the transition from voluminous diatexite in the dome's core to moderate-strain metatexite and overlying high-strain zones on the southern flank; and offer access to extensive leucosome networks considered to represent melt migration paths containing residual material.

The macroscopic structure of the dome, pervasive mesoscopic folds oriented ENE-WSW, and ENE-trending mineral lineation in metatexite indicate a maximum finite strain orientation of 070-250 during dome development, coincident with the stretching direction determined from brittle structures of Cretaceous age throughout the surrounding region. Contemporaneous ESE-striking dextral and less widespread NE-striking sinistral shears with steep fabrics indicate partitioning of regional strain in a transcurrent setting; dome emplacement occurred upon one of these inferred faults. Newly discovered during 2005-06 field study is a wide mylonitic zone recording dextral oblique sense of shear, potentially responsible for exhumation of the gneiss dome. Late structures including refolded folds, leucosome-filled shear bands and interboudin necks record a change in maximum kinematic axis orientation to NNE. U-Pb SHRIMP zircon, monazite and titanite geochronology is underway to assess the duration of migmatization and melt migration through leucosome networks in the dome, by obtaining ages of the U-Pb minerals situated in early vs. late structural sites for leucosome.

Preliminary SHRIMP ages for the narrow growth rims upon 375 Ma magmatic zircons (6 grains) in a migmatized orthogneiss exhibiting early fabrics are 119±2 Ma, interpreted as the time of high temperature metamorphism and melt migration during ENE-WSW transcurrent deformation. The timing coincides with waning plate convergence along the WANT-New Zealand sector of the Cretaceous active margin of Gondwana. The consistency in orientation of contemporaneous brittle to ductile structures suggests a continuum of deformation throughout the upper and lower crust.