Geochronology of granite terranes in the Ford Ranges, Marie Byrd Land, West Antarctica

Abstract Rb‐Sr whole‐rock isochron and and K‐Ar mineral ages are presented for Ford Granodioritemineral ages are presented for Ford Granodiorite (Devonian) and Byrd Coast Granite (Jurassic‐Cretaceous) in the Ford Ranges, Marie Byrd Land, West Antarctica. K‐Ar mineral ages of the Ford Granodiorite are 324–375 Ma, cocentrating a 355ȁ375 Ma and those < 360 Ma are mainly in the central Denfeld Mountains. Rb‐Sr whole‐rock isochron ages of plutons in Denfeld and Sarnoff Mountains are 355–375 Ma (Late Devonian) for earliest adamellite granodiorites and 340–360 Ma (Early Carboniferous) for younger biotite granodiorite and muscovite granite. Devonian plutonism in West Antarctica is restricted to western Marie Byrd Land but has correlatives in northern Victoria Land (East Antarctica), Tasmania and Victoria (Australia), and South Island (New Zealand). Rb‐Sr whole‐rock isochron ages of Byrd Coast Granite (Mesozoic) indicate two plutonic phases: (l),leucogranites 140–150 Ma (Late Jurassic ‐ Early Cretaceous) in the Clark Mountains and at Mt Corey, with associated minor intrusives at Breeding and Stanford Nunataks; (2), widespread granodiorites, 110–115 Ma (mid Cretaceous), in the Allegheny and Sarnoff Mountains. Correlatives in the Fosdick Mountains are 95–100 Ma old. The older Jurassic plutonics are confined to the eastern margin of the Ford Ranges but mid‐Cretaceous granites are part of a widespread belt along the Pacific coast of West Antarctica. Strontium initial ratios of Ford Granodiorite and Byrd Coast Granite are mostly 0.704–0.706, characteristic of I‐type calcalkaline granite‐granodiorite plutonism.