Evidence from the Fosdick Mountains, Antarctica for Rapid Cooling of Mid- to Lower Crustal Rocks during Cretaceous Intracontinental Extension of the East Gondwana Margin

The Fosdick Mountains migmatite dome, Marie Byrd Land, Antarctica, exposes a middle Paleozoic metaplutonic complex of continental arc affinity that underwent Cretaceous migmatization. Migmatization and cooling occurred during the development of a magmatic arc and subsequent back-arc extension and transtension along the East Gondwana margin of Antarctica – New Zealand. The South Fosdick Detachment zone (SFD) forms the southern boundary of the Fosdick Mountains and is in part responsible for exhumation of the mid- to lower crustal rocks. Segments of the SFD are exposed over 30 km and record dextral normal oblique motion with top-to-the SW, WSW transport along azimuth 240. Fold axes and mineral lineation within the dome and brittle kinematics outside the dome indicate regional stretching oriented 070-250. Throughout the detachment structure, brittle fractures and shears overprint high temperature fabrics and solid-state fabrics. Microstructural observations record the transition from melt presence to solid-state deformation. Former presence of melt microstructures are preserved as euhedral Kfs grains in contact with interstitial, optically continuous quartz and magmatic rims on sub-solidus cores with overgrowths of different compositions. Solid-state fabrics such as C-S fabrics and asymmetric porphyroclasts with tails overprint the former melt presence microstructures. Biotite and hornblende ⁴⁰Ar/³⁹Ar thermochronology from the SFD and the Fosdick migmatite dome record cooling ages at ca. 100 Ma. Cooling ages and structures indicate the Fosdick migmatite dome experienced rapid cooling related to a period of intracontinental extension and transtensional deformation in Marie Byrd Land along the East Gondwana margin. Additionally, the rapid cooling rates coupled with the microstructural evidence for former melt textures in solid-state deformed rocks suggest a rapid transition from melt to mylonite.