8 - Compositional Characteristics of the Ferrar McDV Magmatic System

Summary

This chapter shows the whole rock chemical compositions for all the sills in the Dry Valleys region. Harker diagrams for the major and selected trace elements clearly reflect the strong role of orthopyroxene (and subordinate clinopyroxene) as the major mineral phase controlling rock composition. These are low titanium, non-alkaline dolerites, which are common to this entire region and continuing to Tasmania and even as far as some of the Karoo system. On an AFM diagram the system almost mimics that of Skaergaard. This Opx control extends throughout the system. And upwards with the loss of Opx the upper sills overlap in composition with the Kirkpatrick Basalts. Various other diagrams, meant to discriminate between pyroxene and feldspar control, show the effects of crystal sorting locally and regionally, with the chilled margins being tholeiitic and the sill centers tending to ultramafic. A pattern persisting outwards from Bull Pass for at least 50 km north and south. Relative to the Basement Sill this appears as a large plume of crystal-laden magma, extending outward in all directions, exhibiting locally the strong effects of crystal sorting during emplacement.