Abstract
In the three areas studied, the McMurdo volcanics have been subdivided into four formations: Melania (oldest, and predominantly basaltic); Aurora Trachyte; Nubian Basalt; and the Trachyte Hill Formation. Tentative correlations have been made among the three areas. In addition to these four divisions, maps are presented showing the lavas subdivided on the basis of their petrography (particularly phenocryst contents). Five broad petrographic types are recognised: Olivine-augite basalts; plagioclase basalts; hornblende basalts; pyroxene trachytes, and hornblende trachytes. Black Island and Brown Peninsula are believed to have been originally stratovolcanoes, similar to the present form of Mt Bird. In each area, this early conebuilding phase (Melania Formation) is predominantly olivine-augite basalt, although locally trachyte and trachybasalt are recognised. This phase was followed, after a period of prolonged quiescence, by the Aurora Trachyte Formation, comprising extensive viscous flows of pyroxene-bearing and later hornblende-bearing trachytes. The third phase is recognised as the small to moderate sized scoria cones (Nubian Formation) of both olivine-augite and hornblende basalts. The final phase was the eruption of relatively crystal-rich hornblende trachytes, best represented on Cape Bird. During field work, only two broad lava types were recognised—“basalts” and “trachytes”. This bimodal distribution was confirmed by the frequency distribution of (Na2O + K2O) values based on 93 partial alkali analyses. This data also indicates a tendency for differences in chemistry between the three (petrographic) basalt types and also between the pyroxene and hornblende trachytes. Three new complete chemical analyses of lavas from Cape Bird are presented, confirming the undersaturated nature of these lavas.

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