Geochronology and geochemistry of Ferrar rocks from North Victoria Land, Antarctica.

Abstract

KL-Ar determinations on two igneous units from North Victoria Land, Antarctica, representative of the Ferrar Dolerite and Kirkpatrick Basalt, yield dates ranging from 144 to 180 Ma. The oldest date is in agreement with a Rb-Sr isochron age of 182 Ma provided by ost of the analysed samples. It is considered to represent the age of the magnmatic activity which produced the two suites; younger dates are attributed to argon loss subsequent to cooling. The chemical compositions of the rocks range from basaltic andesite to dacite. Major-element mass-balance, trace-element modelling, isotopic and geothermometric data, all imply a closed-system differentiation history from basaltic andesite to dacite. The basaltic andesites are characterized by high initial 87 Sr/ 86 Sr ratios (clustering around 0.7114) and low initial 143 Nd/ 144 Nd ratios (0.5121), high concentrations of incompatible elements, and relatively low abundances of Na 2 O, Nb, Ti and P. The normalized trace element contents of the least evolved basaltic andesite studied are intermediate between those of low-TiO 2 and high-TiO 2 lavas from the Mesa range to the south. Major and race element modelling shows that the studied basaltic andesites can be derived by crystal fractionation processes from the least differentiated low-TiO 2 rocks an,d, in turn, may generate high-TiO 2 rocks by crystal fractionation