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 Mountains (Antarctica) are characterized by the occurrence of orthopyroxene (opx), a mineralogical feature virtually absent in the tholeiites from the large Mesozoic igneous provinces of Karoo (S. Africa) and Paranà (Brazil). Petrography suggests that opx is the early phase in the sequence of crystallization and mineral chemistry indicates that it is in compositional equilibrium with the host rock. In general, MELTS modeling does apparent that opx is the first liquidus phase in the Ferrar tholeiites with MgO> 7 wt%, at P= 1.5-5 kbar, H2O= 0-1 wt% and fO2 = QFM –1 log unit conditions. MELTS results also evidence that the opx early crystallization is primarily controlled by high SiO2 and high SiO2/CaO, chemical characteristics typical of the Ferrar tholeiites, but not of the Karoo and Paranà analogues with similar MgO content. Mass balance and MELTS modellings support that the magma compositions suitable to be primary to the least evolved Ferrar tholeiites are compatible with water-undersaturated (H2O = 0.3-0.5 wt%) experimental melts obtained at 10 kbar (30-38% melting degree) and later contaminated by the high-grade metamorphic rocks of the Victoria Land crystalline basement. The involvement of crustal contamination during fractional crystallization is inescapable if we consider that SiO2 content of mantle-derived melts decreases with pressure increase. A pressure higher than 10 kbar is expected for the primary Ferrar tholeiites associated with 30-40 km thick continental crust. Crustal contamination was responsible for the high SiO2 content of the Ferrar tholeiites and the opx early crystallization. All data indicate that major element chemistry of the Ferrar tholeiites requires high degrees of melting of H2O-undersaturated fertile peridotite mantle and subsequent evolution through AFC, consistently with our previous studies on trace elements and Sr-Nd-Pb isotopes.