THE LASHLY MOUNTAINS OF SOUTHERN VICTORIALAND, ANTARCTICA: INVESTIGATING A POSSIBLE ANCIENT VOLCANO

The Lashly Mountains of Southern Victorialand, Antarctica are composed primarily of Ferrar dolerite, but are poorly constrained in terms of magmatic environment – namely, are they intrusive or extrusive? The reason for this confusion is that the Lashly Mountains sit at an interesting junction between McMurdo Dry Valley’s intrusives and the Transantarctic Mountain’s extrusives. Several previous studies have interpreted the exposed magmatic systems in the Lashly Mountains and other nearby systems as being intrusive. However, it isn’t entirely clear how these studies determined that the magmas were intrusive, because in many cases contact with overlying host rock is not present. We feel it is necessary to reinvestigate the magmas of the Lashly Mountains with the intent of providing an evidence informed decision. The unnamed nunatak to the north of Mount Crean reveals, in an exposed cliff face, a complex assemblage of varying units. At the base of the outcrop is a thick single layer of vertically jointed dolerite. This layer is then capped, along a horizontal interface, by rhythmically layered dolerite, which has been previously reported as containing variable quantities of glass. Towards the center of the outcrop is a thick bulbous mass of dolerite. The layered unit in part underlays this bulbous mass, and is also draped over it. Samples collected from the talus pile range from coarse grained dolerite to highly altered amygdaloidal basalt to poorly consolidated tuff. We interpret this evidence holistically as indicative of an extrusive origin. Extrusives have not been previously identified in this region. This interpretation thus provides a roof, allowing for accurate estimates of paleodepth for the exposed sills of the nearby McMurdo Dry Valleys.