- Webb 1100
Title: Geochemistry of Lamprophyres, Dry Valleys, Antarctica: Insight into the end of a subduction regime
Abstract: Lamprophyres are mafic dikes characterized by an abundance of volatile-bearing silicate minerals such as amphibole and biotite-phlogopite, enrichment in both Mg and in incompatible elements such as Rb and K, and intermediate to low SiO2 content. They are often among the final plutonic rocks intruded during subduction regimes, and they are found cross-cutting subduction-generated plutons in regions such as the Sierra Nevada, the Variscan Orogeny of western Europe, and the Ross Orogeny of Antarctica. The Dry Valleys of Antarctica lie along the margin of the Ross Orogeny (c. 615-480) and provide a valuable record of a subduction regime and its end because of exceptional exposure, a wealth of previous data on the age, chemistry, and extent of older rocks in the region, and the unusually dense concentration of lamprophyre dike swarms in several areas.
To better understand why lamprophyres often appear during and after the termination of subduction, whole-rock major- and trace-element geochemistry was performed on lamprophyre dikes from the Dry Valleys. This research addresses several questions related to the generation of lamprophyre dikes, primarily 1) whether these rocks originate as primary melts 2) whether lamprophyres derive from melting of enriched mantle domains and 3) whether, and if so how, the generation of porphyritic felsic dikes often found alongside lamprophyres is linked to the formation of lamproic melts. The results suggest that the lamprophyres found in the Dry Valleys include samples of primary composition, although most of the samples have undergone mixing with an as-yet unidentified source during ascent. The incompatible element concentrations of even the most primitive melts are too elevated for lamprophyres to derive from depleted mantle, supporting the hypothesis that they derive from enriched mantle domains. Furthermore, geochemical data support the hypothesis that porphyry dikes are differentiates of lamprophyres, and cross-cutting relationships suggest that these dikes differentiated from a lamproic magma after the main intrusion of lamprophyre dikes. The geochemical data obtained from the Dry Valleys are consistent with those obtained by previous workers elsewhere along the Ross Orogeny margin, suggesting widespread intrusion of enriched mantle melts during the termination of subduction.