Analysis and ages of Dykes from Shackleton Range

In addition to some mafic dykes which are already known and which were resampled, a number of mafic dykes were discovered in the Shackleton Range during the 1987/88 GEISHA expedition and sampled for the first time. Field data for 29 dykes, as well as analytical results of petrographic, geochemical, and isotope-geochemistry studies on the basalts of 26 of these dykes, are presented and discussed. The mafic dykes can be subdivided into five groups on the basis of their petrography and degree of alteration. According to the geochemical analyses, the dyke rocks are continental tholeiites. Geochemical characterization on the basis of trace-elements, especially rare earths, permits subdivision according to magma type. This subdivision shows reasonable agreement with the subdivision according to petrographic criteria. On the basis of radiometric dating and field relationships, the following ages can be assigned to the five groups of dykes: Group I, Early Jurassic; Groups II and III, early Palaeozoic (Middle Devonian to Late Ordovician?); Group IV, probably Late Proterozoic and Group V, probably Middle Proterozoic. These groups of mafic dykes, which can be regarded as indicating extension phases, are discussed with respect to the geotectonic history of the Shackleton Range, and comparisons with neighbouring regions are made