Amphibole Content and Composition of the Vanda Dike Swarm: Dry Valleys, Southern Victoria Land, Antarctica

The Ordovician Vanda Dike swarm is composed of hundreds of dikes in the Dry Valleys of Antarctica. We examined over 600 dikes across Bull Pass, Taylor, Victoria, and Wright Valleys. A previous age determination on one of the dikes yields 470 +- 7 Ma. The dikes are mostly emplaced in pre-tectonic and syn-tectonic granites that range in age from 589 to 490 Ma. A few dikes are found in coarse-grained post- tectonic granites that are 486 to 477 Ma. The dikes occur in concentrated clusters, and the different clusters are characterized by limited compositional ranges. For example, in Wright Valley near Lake Vanda, dikes are predominantly mafic, whereas felsic porphyry is common at the mouth of Bull Pass. Most of the dikes and all of the granites contain amphibole, which is being applied as a thermobarometer to understand the conditions of emplacement and uplift history of the terrain. The temperatures of felsic dikes obtained from apatite-zircon saturation thermometry range from 791 to 826° C for zircon and for apatite, 921 to 946° C. Most of the Vanda dikes have non-vertical dips (typically 40 to 80°), irregular margins, and served as ductile shear zones, all of which point to mid-crustal emplacement. Thus, the dike swarms were likely emplaced during regional uplift at the end of the Ross orogeny.