CHARACTERISTICS OF AN OBLIQUE DETACHMENT SYSTEM IN WEST ANTARCTICA, AND CONTRIBUTIONS TO THE BREAKUP OF GONDWANA'S ACCRETIONARY MARGIN

Detachment systems and MCCs, localized upon gneiss domes, formed due to intracontinental extension in East Gondwana in Cretaceous time, prior to breakup between West Antarctica and Zealandia. The Fosdick Mountains gneiss dome contains a rich record of the events within Antarctica, revealing an association between crustal melting and coalescence of granites with the formation of an oblique detachment system. Integrated structural and SHRIMP U‐Pb zircon geochronology of anatectic granites in the Cretaceous Fosdick Mountains gneiss dome reveal a progression in strain from wrench at 117‐115 Ma to transtension at 109‐102 Ma. The extent of crustal melting and degree of hybridization of magmas varied over this short time interval, according to SHRIMP oxygen and LA‐ICPMS hafnium isotope analyses of zircon, with U Pb geochronology. A further discovery comes from comparison of granites within the complex to regional Byrd Coast Granite plutons (BCG, 105–102 Ma) outside, at higher structural levels. Granites within the complex typically have large positive Eu anomalies and geochemistry due to early accumulation of feldspar and quartz, whereas BCG plutons have large negative Eu anomalies and elevated K₂O contents. Furthermore, the Hf and O zircon isotope compositions for BCG are consistent with derivation from crystallizing granites within the Fosdick complex. These attributes suggest a source-sink relationship between the Fosdick gneiss dome and outlying granites, wherein a cumulate granite component remained in the dome, creating a solid framework through which K20- enriched melts could migrate, ultimately, out of the dome. Possible evidence of strain localization within a cumulate framework (quasi-solid-state) is the presence of sillimanite shear zones from mm- to decimeters thick, within and bordering Kfeldspar porphyritic granite sills and sheets. The sillimanite is thought to be a product of stress localization due to impingement of feldspars in cumulates, causing dissolution and mass transfer of alkalis and silica away with the percolating melt, leaving behind a sillimanite ‘residue.’ These findings suggest that granites in gneiss domes form principally as accumulations of feldspar and quartz rather than as primary or minimum-melt compositions.