Timing of events during the late Proterozoic Beardmore Orogeny, Antarctica: Geological evidence from the La Gorce Mountains

The Beardmore Orogeny previously has been designated for deformational and magmatic activity that occurred during the late Proterozoic in the central Transantarctic Mountains. It is recognized in folding of Beardmore Group turbidites, unconformably overlain by Cambrian limestones, and by silicic magmatism dated ∼650 Ma. Until this study, the relationship between the deformation and the magmatism had not been known. In the La Gorce Mountains, La Gorce Formation (Beardmore Group) was tightly folded during a regional metamorphic event producing biotite and limited axial-plane cleavage. The late Precambrian Wyatt Formation is a silicic porphyry with both volcanic and hypabyssal phases, possibly representing the eroded roots of a caldera complex. Wyatt Formation is conformably overlain by Ackerman Formation, a sequence of interbedded volcanics and shallow-marine sedimentary rocks. At a newly discovered locality, Wyatt Formation intrudes folded La Gorce Formation. This relationship demonstrates that the episode of folding and low-grade regional metamorphism was completed prior to the silicic magmatism of the Wyatt and Ackerman Formations. A contact between Ackerman and La Gorce Formations, which previously had been interpreted as conformable, has been shown by this study to be a fault. We advocate that the usage of the term “Beardmore Orogeny” be restricted to the deformational and metamorphic event and that the late Proterozoic magmatism be viewed as the initial stage in an episode of widespread silicic volcanism that continued during the Early and Middle Cambrian in the central Transantarctic Mountains.