The late crystallization stages of low-Ti, low-Fe tholeiitic magmas: Insights from evolved Antarctic and Tasmanian rocks

Abstract

Examples of the latest stages of crystallization in evolved tholeiitic rocks can be found in the low-Ti, low-P Ferrar Magmatic Province in Antarctica (e.g., Thern Promontory and Archambault Ridge) and Tasmania (e.g., the Red Hill intrusion and Mount Wellington sill). Evolved rocks of this magmatic province include: a) rocks of basaltic andesite composition containing plagioclase, augite, pigeonite, and interstitial Fe–Ti oxides; b) rocks of andesitic composition with plagioclase as a liquidus phase, a single Fe-rich subcalcic clinopyroxene and Fe–Ti oxides; and c) rocks of dacitic composition displaying granophyric texture, containing sodic plagioclase, alkali feldspar, ferrohedenbergite, fayalite, Fe–Ti oxides, quartz and accessory phases. The latest crystallization stages are thus characterized by the striking chemical convergence of subcalcic Fe-augite and Fe-pigeonite followed by more evolved rocks that preserve quartz-alkali feldspar cotectic (granophyric) relationships, lack subcalcic clinopyroxene and contain coexisting olivine and calcium-rich clinopyroxene. These features are consistent with crystallization at (or below) the quartz–fayalite–magnetite oxygen buffer at temperatures close to 800°C. The trace element geochemistry of coexisting pyroxenes, feldspar and Fe–Ti oxides are entirely consistent with magmatic evolution caused by fractional crystallization of the observed phases. Similarities and differences with other silicic volcanic rocks elsewhere are also described. There is no evidence for liquid immiscibility.