New structural and geochronological data from the Eastern Palmer Land Shear Zone, in the vicinity of Beaumont Glacier, provide the first evidence of dated structural control on emplacement of the 119–95 Ma, >13000 km2 Cretaceous Lassiter Coast Intrusive Suite, during the accretionary mid-Cretaceous Palmer Land Event orogeny. A previously undated >100 m thick dyke-like quartz tonalite intrusion was emplaced at 116.5 ± 1.5 Ma (Ar–Ar biotite cooling age) along a NW–SE axis, coeval with NW–SE compressional deformation, and possibly controlled by sinistral transpression along the Beaumont Glacier shear zone. The quartz tonalite preserves two styles of deformation and was probably progressively deformed by normal-sinistral shearing during magmatic cooling with initial deformation of mafic enclaves in the dyke interior and final formation of proto-mylonite to mylonite on the dyke margin. The quartz tonalite cuts folding associated with Phase 1 of the Palmer Land Event, indicating that this folding is older than 116 Ma, extending the onset of Phase 1 back in time. Mylonite on the quartz tonalite sheet margin was subsequently thermally reset at c. 107 Ma (Ar–Ar biotite method), during a peak in Lassiter Coast Intrusive Suite magmatism.
Country rock lithological data, petrographic description of tonalite and Ar–Ar geochronology data are available at www.geolsoc.org.uk/SUP18552.