VOLCANIC HISTORY OF MOUNT SIDLEY, A MAJOR ALKALINE VOLCANO IN MARIE-BYRD-LAND, ANTARCTICA

Mount Sidley is a complex, polygenetic stratovolcano composed primaril y of phonolitic and trachytic lavas and subordinate pyroclastic lithol ogies at the southern extremity of the Executive Committee Range, a li near chain of volcanoes in central Marie Byrd Land, Antarctica. Detail ed field investigation coupled with 14 high precision Ar-40/Ar-39 age determinations reveal a 1.5 million year life span between 5.7 and 4.2 Ma in which three major phonolitic central vent edifices (Byrd, Weiss and Sidley volcanoes) and their calderas were developed (5.7-4.8 Ma). This was followed (4.6-4.5 Ma) by the eruption of trachytic magmas fr om multiple vent localities further south, and then by small volume be nmoreite-mugearite lavas and tephras around 4.4-4.3 Ma at the southern end of Mount Sidley. The final phase of activity was the eruption of basanite cones at approximately 4.2 Ma. The southward migration of vol canic activity was accompanied by distinct changes in magma compositio n and is best explained by the sequential release of magmas stored wit hin an intricate system of conduits and chambers in the crust by tecto nically driven (magma assisted?) fracture propagation. The style of vo lcanic migration at Mount Sidley is emulated on a larger scale by othe r volcanoes in the Executive Committee Range, in which progressive sou thward displacement of volcanic activity corresponds with significant petrological variations between major centers.