THE FERRAR MAGMATIC SYSTEM OF THE MCMURDO DRY VALLEYS, ANTARCTICA: A BASALTIC MAGMATIC MUSH COLUMN

The Transantarctic Mountains stretch 4000 km across Antarctica from North Victoria Land on the Ross Sea to Coats Land on the Weddell Sea. Early explorers seeking the South Pole had to cross this range to gain access to the inner polar plateau. Geologists with R. F. Scott’s Discovery Expedition of 1901-1904 (H. Ferrar), E. H. Shackleton’s Nimrod Expedition of 1907-1909 (R. E. Priestley & D. Mawson), and R. F. Scott’s last expedition, the Terra Nova Expedition of 1910-1912 (G. Taylor & F. Debenham), readily recognized the abundance of dolerite sills between Victoria Land and the Beardmore Glacier, a distance of about 800 km. All succeeding expeditions have been similarly impressed by the thick dark bands of sills within massive basal expanses of pink granite and extensive upper sections of tan sandstones. The first modern comprehensive reports on the petrology of the dolerites were by Warren Hamilton and Bernard Gunn. The Ferrar magmatic system contains information vital to understanding the very fundamental processes of planetary magmatism. The heart of the system is an expansive (~10,000 km2) stack of five massive sills (~300-750 m thick), centrally interconnected and exceptionally well exposed over a vertical extent of about 4 km, culminating in the direct connection to regional flood basalts, the Kirkpatrick Basalt. These rocks record the episodic or serial establishment, loading, and eruption of a magmatic system with compositionally and temporally contiguous aliquots of compositionally diverse magma. Magma composition varies systematically upwards through the system from ultramafic (20% MgO) layered orthopyroxenite at the base to uniform tholeiite (5-7% MgO) in the uppermost sills and lavas. The overall bulk chemical compositions describe a strongly differentiated and diverse system exceedingly similar to that defined by the lavas of Hawaii (i.e., Kilauea, Mauna Kea, and Mauna Loa). And as at Hawaii, MgO content is directly related to the presence of magnesian phenocrysts, orthopyroxene in the Ferrar and olivine at Hawaii. In striking contrast to Hawaii, however, the spatial context of every sample defining the differentiation sequence within the Ferrar magmatic system is accurately known. This spatial control allows the processes giving rise to the compositional variations to be ascertained in some detail.