| Summary: | Includes bibliographical references (pages 177-185) Basalts of the Erebus Volcanic Province, which have been erupted across the Transantarctic Mountains - Ross Embayment boundary, have brought up abundant xenoliths of both mantle and crustal origin. A wide range of lithologic types is represented in the xenolith suite, including spinel lherzolites, garnet granulites, and three-pyroxene granulites from the Transantarctic Mountains, and spinel granulites from the Ross Embayment. Garnet itself is actually rare in the garnet granulite suite. Most of the garnet has decomposed, by heating and/or decompression, to symplectites of plagioclase + orthopyroxene + olivine ± spinel. Additionally, the three-pyroxene granulites contain the first known metamorphic occurrence of "uninverted" pigeonite coexisting with augite and orthopyroxene. Thermobarometry of these xenoliths, which are from a continental rift environment, enabled the construction of a petrologic geotherm. The barometry on the Transantarctic Mountains crustal xenoliths indicates that crystallization took place at depths of »15-40 km, while the Transantarctic Mountains mantle xenoliths equilibrated between »43 and 53 km. These pressure estimates are in good agreement with the geophysically-defined depth to the base of the crust under the Transantarctic Mountains (»40-43 km) . The thermometry yields very high temperatures (900-1100°C) throughout the length of the geotherm. The very high temperatures and the shape of the geotherm indicate that advection, not conduction, was the dominant heat-transfer mechanism that operated below the Trans - antarctic Mountains and Ross Embayment lower and middle crust. The advective nature of the geotherm is likely controlled by heat loss from the Cenozoic alkaline basaltic magmas that have been underplat ing and invading the crust. Today, the geophysically-defined depth to the base of the Ross Embayment crust is estimated to be »20-23 km. However, the presence of garnet pseudomorphs in Ross Embayment xenoliths clearly implies that the Ross Embayment crust was once much thicker, at least 33 km, and perhaps as much as 43 km. This illustrates that the Ross Embayment crust has locally undergone a 35% or more reduction in thickness during the process of extension and rifting. One granulite sample from the Transantarctic Mountains contained a pyroxene megacryst exhibiting spectacular exsolution lamel lae of augite, OPX, and pigeonite. The exsolution textures are interpreted to result from the combined effects of igneous crystallization and cooling, followed by regional metamorphism. M.S. (Master of Science)
|
|---|