Mode of occurrence, bulk chemical compositions, and mineral textures of ultramafic rocks in the Lutzow-Holm Complex, East Antarctica

摘要

Available data on the mode of occurrence, bulk chemical compositions, and mineral textures of ultramafic rocks (ultrabasic rocks plus "normatively" ultramafic rocks (normative olivine+pyroxenes+ilmenite>70%)) in the Late Proterozoic high-grade Lutzow-Holm Complex are summarized. The bulk chemical compositions suggest that most of them are derived from various parts of layered gabbro. Some are troctolitic, being composed mainly of normative olivine and plagioclase without pyroxenes. The mode of occurrence in the field suggests that they were tectonically fractured and emplaced in the metasedimentary gneisses. Their occurrence mainly in the western part of the Lutzow-Holm complex suggests that original gabbroid masses were present in the presently missing part (ocean floor?) between the adjacent Lutzow-Holm and Yamato-Belgica Complexes. The mineral textures, especially those of garnet and spinel, suggest changing metamorphic conditions within the spinel-amphibole lherzolite facies during prograde recrystallization of the rocks; from earlier relatively high-pressure and low-temperature conditions to later lower-pressure and higher-temperature conditions.