Cumulate Origin and Polybaric Crystallization of Fe-Ti Oxide Ores in the Suwalki Anorthosite, Northeastern Poland

摘要

Fe-Ti oxide ores in the Proterozoic Suwalki massif-type anorthosite, northeastern Poland, have been recognized through geophysical exploration and extensive drilling down to 2,600 m depth. The Fe-Ti oxide-rich rocks from Suwalki consist of vanadium-poor Ti-magnetite in lenses varying from the centimeter- to the kilometer-scale. Fe-Ti-rich rocks are commonly layered and have gradational contacts with the host anorthosite; they do not represent well-defined intrusions such as the major Tellnes and Lac Tio deposits. Based on petrography, modal proportions, whole-rock analyses, and liquidus phase compositions of Fe-Ti oxide ores, the sequence of crystallization is as follows: plagioclase + Ti magnetite + ilmenite, followed by orthopyroxene, then clinopyroxene, and finally apatite. The comparatively low V content (0.20-0.67 wt percent V) in Ti magnetite results from relatively oxidized crystallization conditions. The diapiric emplacement of the anorthositic pluton influenced the crystallization of the Fe-Ti ores and is responsible for the crystal sorting controlled by the density contrast of liquidus phases. Polybaric crystallization of Fe-Ti oxide ores is evidenced by variable Al_2O_3 (ca. 1-5 wt percent) content of the associated orthopyroxene. Polybarism is responsible for the appearance of Fe-Ti oxides before orthopyroxene, due to the decreasing stability field of orthopyroxene with pressure.