Potassium‐argon (K‐Ar) total‐rock and rubidium‐strontium (Rb‐Sr) whole‐rock isochron age determinations of slates in the Swanson Formation, the oldest rock unit of Marie Byrd Land, West Antarctica, are reported. The K‐Ar ages form two groups, 350–380 Ma and 410–450 Ma. The younger group reflects a pervasive thermal effect associated with intrusion of Late Devonian granites in the area, and the maximum age of the older group (about 450 Ma) dates an early phase of deformation, cleavage formation, and low‐grade regional metamorphism during the Late Ordovi‐cian. Rb‐Sr whole‐rock isochron ages of these slates also yield Late Ordovician ages (c. 444 Ma) which determine the time of metamorphism of the Swanson Formation. A model Rb‐Sr age of the least metamorphosed pelite indicates that Swanson Formation is not older than Precambrian. Re‐examination of published Rb‐Sr whole‐rock data of the Fosdick metamorphic rocks suggests they are migmatised Swanson Formation with Cretaceous granite as the injection phase. Good correlations can be made with similar quartzose turbidite sequences elsewhere, in eastern Marie Byrd Land, on Campbell Island, and the West Coast of the South Island of New Zealand, where evidence of Late Ordovician low‐grade metamorphism, 440–450 Ma, has also been found. However, these differ from similar sequences in northern Victoria Land and Tasmania, which show only evidence of a latest Cambrian and/or earliest Ordovician low‐grade metamorphism, 500–510 Ma ago.
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C.J. Adams (1986) studied this question.