Significance of orthopyroxene and major element constraints on the petrogenesis of Ferrar tholeiites from southern Prince Albert Mountains, Victoria Land, Antarctica

The least evolved Jurassic Ferrar tholeiites from southern Prince Albert Mo untains (Antarctica) are characterized by the occurrence of orthopyroxene ( opx), a mineralogical feature virtually absent in the tholeiites from the l arge igneous provinces of Karoo (South Africa) and Parand (Brazil). Petrogr aphy suggests that opx is the early phase in the sequence of crystallizatio n and mineral chemistry indicates that it is in equilibrium with the host r ock. In general, MELTS modeling predicts that opx is the liquidus phase in the Ferrar tholeiites with MgO higher than similar to7 wt% at P = 1.5-5 kba r, H2O = 0-1 wt% and f(O2) = QFM - I log unit conditions. MELTS results als o show that the early crystallization of opx is primarily controlled by hig h SiO2 and high SiO2/CaO, chemical characteristics typical of the Ferrar th oleiites, but not shown by the Karoo and Parand analogs with similar MgO co ntent. Major element geochemistry of the least evolved Ferrar tholeiite has been modeled through fractional crystallization and fractional crystalliza tion coupled with crustal assimilation processes, starting from natural per idotite-derived experimental melts. Mass balance and MELTS modeling support the argument that theoretical magma compositions suitable to be primary to the least evolved Ferrar tholeiites are compatible with hydrous (H2O = 0.3 -0.5 wt%) and anhydrous melts obtained at 10-15 kbar by high melting degree s (> 25%) of fertile and depleted spinel lherzolites, respectively, and lat er contaminated by the high-grade metamorphic rocks from the Victoria Land crystalline basement. The genesis of primary Ferrar tholeiites does not nec essarily reflect the generally assumed depleted source mantle being also co mpatible with a fertile one.