The Vanda Dike Swarm, Dry Valleys, Antarctica II: Geochemistry and Geochronology

Abstract:

One of the most spectacular features of the Dry Valleys of Antarctica is the Vanda Dikes, an exceptionally well exposed swarm of >600 Cambro-Ordovician dikes emplaced near the end of the Ross Orogeny. Whereas their compositions range from mafic to felsic, they are primarily bimodal, with few intermediate compositions between 56 and 66 wt.% SiO₂; mafic dikes vary from calc-alkaline to shoshonitic. The suite of dikes exhibits ⁸⁷Sr/⁸⁶Sr from 0.704-0.711 and ¹⁴³Nd/¹⁴⁴Nd from 0.51217-0.51242; the wide range of Nd isotopic ratios may reflect variable degrees of assimilation of the subcontinental lithospheric mantle inherited during Proterozoic crustal generation and the Ross Orogeny. The mafic dikes have a subduction-related trace element signature and we propose they originated as melts of the subcontinental lithospheric mantle. The siliceous suite is characterized by elevated ⁸⁷Sr/⁸⁶Sr_i and an average Nd model age (τDM) of 1444 Ma, suggesting that the silica-rich dikes were generated by melting of or contamination by a Neoproterozoic crustal source, followed by variable fractional crystallization and further continental crust contamination. Analysis of zircons by CA-TIMS U-Pb geochronology reveals that the dikes were emplaced in a narrow time window, from 491 to 495 ± 0.3 Ma, which falls between emplacement of a synorogenic pluton (DV1) and extension-related plutons (DV2). Thus, we conclude that the dikes were intruded during the transition of the Ross Orogeny from collision to extension. During orogenic uplift and collapse, decompression melting of the subcontinental lithospheric mantle was initiated, resulting in the emplacement of the Vanda Dike Swarm. Because there is a strong correlation between dike age and latitude, the dikes can be used to map the progress of the collision-extension transition during the Ross Orogeny across the Dry Valleys.