Geochemistry of the Cenozoic Volcanic Rocks of Ross Island and Vicinity, Antarctica

Abstract

The Cenozoic volcanic rocks of Ross Island and vicinity, Antarctica, are surface flows, tuffs, breccias, and small intrusives. The prominent rock is basanitoid. Alkali-basalt magma reached the surface over an appreciable length of time and was differentiated to produce a rock series : basanitoid → trachybasalt → phonolite. The alkalic, silica-undersaturated rocks are part of a larger petrologic province composed of volcanic centers in a belt nearly 2,000 km long roughly parallel to the Trans-antarctic Mountains. Two trends of magmatic differentiation are apparent. At depth the fractional crystallization of olivine, clinopyroxene, and opaque oxide minerals, all of which are abundant as phenocrysts in the basanitoid flows, produced trachybasalt magma. At crustal levels, fractional crystallization of clinopyroxene, apatite, opaque oxides, kaersutite, plagioclase, and anorthoclase developed the phonolitic rocks. Alkali enrichment is marked, and the end-member phonolites contain 11% Na₂O, 5.6% K₂O, and 57% SiO₂. The basanitoids contain relatively large amounts of Ba (~400 ppm), Sr (~1,000 ppm), and Rb (~30 ppm). The concentrations of these and other trace elements in the various rock types support the model of small degrees of partial melting of peridotitic mantle for the derivation of the basanitoid magma, and of magmatic differentiation for the subsequent development of the rock series.