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

摘要

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 igneous provinces of Karoo (South Africa) and Paraná (Brazil). Petrography suggests that opx is the early phase in the sequence of crystallization and mineral chemistry indicates that it is in equilibrium with the host rock. In general, MELTS modeling predicts that opx is the liquidus phase in the Ferrar tholeiites with MgO higher than ~7 wt% at P=1.5–5 kbar, H2O=0–1 wt% and % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8YjY-vipgYlH8Gipec8Eeeu0xXdbba9frFj % 0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9 % vr0-vr0-vqpWqaaeaabiGaaiaacaqabeaabaqaamaaaOqaaiabdAga % MTWaaSbaaeaacqqGpbWtaeqaaWWaaSbaaeaadaWgaaqaaiabikdaYa % qabaaabeaakiabg2da9iabbgfarjabbAeagjabb2eanjabgkHiTiab % igdaXiaaysW7cyGGSbaBcqGGVbWBcqGGNbWzaaa!381A! $f_{rm O} _{_2 } = {rm QFM} - 1;log $ unit conditions. MELTS results also show that the early crystallization of opx is primarily controlled by high SiO2 and high SiO2/CaO, chemical characteristics typical of the Ferrar tholeiites, but not shown by the Karoo and Paraná analogs with similar MgO content. Major element geochemistry of the least evolved Ferrar tholeiite has been modeled through fractional crystallization and fractional crystallization coupled with crustal assimilation processes, starting from natural peridotite-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 degrees (>25%) of fertile and depleted spinel lherzolites, respectively, and later contaminated by the high-grade metamorphic rocks from the Victoria Land crystalline basement. The genesis of primary Ferrar tholeiites does not necessarily reflect the generally assumed depleted source mantle being also compatible with a fertile one.