Rubidium-strontium geochemistry of mafic and ultramafic inclusions, associated volcanic rocks, and basement rocks from the Ross Island area, Antarctica

Includes bibliographical references.Includes illustrations and maps.The alkalic volcanic rocks of the Ross Island area are part of a linear trend of Cenozoic volcanic centers, roughly paralleling the trend of the Transantarctic Mountains, which extends approximately 2000 km from the Balleny Islands to Mt. Weaver. The alkalic nature of these volcanic rocks suggest that the Ross Island area is part of a large structural tensional zone subparalleling the Transantarctic Mountain range. The volcanic rocks of the Ross Island area form a rock series, in which a primary basanitoid magma was extruded and part differentiated producing a basanitoidï –trachybasaltï –phonolite rock sequence. These rocks are cogenetic as based on their strontium isotopic compositions and have suffered negligible amounts of contamination from crustal materials, except perhaps for the phonolitic rocks. A single sample of trachyte, exhibits anomalous chemical and strontium isotopic compositions, and may represent either a contaminated differentiate of the Ross Island volcanic sequence or a differentiate of an unrelated magma. Found in association with the volcanic rocks are megacrystic inclusions of clinopyroxene, kaersutitic amphibole, and anorthoclase, in addition to mafic granulite and ultramafic inclusions. The megacrysts, with the exception of a chemically dissimilar clinopyroxene megacryst, are isotopically identical to their enclosing hosts and, therefore, appear to be cognate phases. Chemical and strontium isotopic data and structural relations suggest that the clinopyroxene megacrysts are the products of crystal nucleation on associated, pre-existing ultramafic inclusions, which were later dislodged during the ascent of the magma to the earth's surface. Several of the mafic granulite and ultramafic inclusions analyzed in this study have isotopically re-equilibrated to their hosts, which are represented by basanitoids and trachybasalts. One Iherzolite inclusion is accidental as based on its strontium isotopic composition. Rehomogenization of zones in the upper mantle and lower crust during the production and emplacement of the Ferrar Dolerites, approximately 155 M. Y. ago, is reflected by a Rb-Sr isochron of similar age as defined by whole- rock samples of mafic granulite and ultramafic inclusions. Incidental to this study, was the determination of ages of crystalline basement rocks from the drill core of DVDP 6, Lake Vida, Victoria Valley. Petrographic character and Rb-Sr systematics suggest that the quartz monzonite of this study is the Vida Granite. A piagioclase-biotite- hornblende gneiss, also recovered from DVDP 6, yields an age which is inconsistent to those reported for similar rocks from surrounding areas and is the result of the loss of radiogenic strontium or partial rehomogenization during the emplacement of the Ferrar Dolerites.M.S. (Master of Science