Fluid-inclusion studies on lower crustal gabbroic xenoliths from the Mt.Melbourne Volcanic field (Antarctica): Evidence for the post-crystallization uplift history during Cenozoic Ross Sea Rifting

Abstract

Fluid inclusions in gabbroic xenoliths from Cenozoic alkaline centres of the Mt.Melbourne Volcanic Field at the margin of the Ross Sea Rift System have been studied in order to derive fluid compositions and conditions of entrapment. Most of the inclusions are secondary, and only few are considered primary with respect to igneous crystallization. Textures indicate that the former inclusions formed from complete necking-down of fluid-filled, healed cracks. Most inclusions studied, primary or secondary, contain almost pure CO 2 , as has been tested by heating/freezing experiments. Other fluid components (N 2 , hydrocarbons) have been shown by Raman spectroscopy to be present in some inclusions. Histograms of homogenization temperatures for large (up to 50 mm) secondary inclusions in plagioclase and pyroxene have maxima between ca. 16° and 26°, 12° and 18°. Inclusions in olivine were measured in one rock only and gave lower values from 0° to 10°. We interpret these results as fluid entrapment and crack healing at different P-T-conditions along the steep geotherm previously documented for the lower crust in the Ross Rift region (Berg et al., 1989), which followed metamorphic equilibration of these rocks at conditions of 900-1000°C and 3-5 kbar (Hornig et al., 1992). Our fluid inclusion data then indicate an uplift path for the lower crust from the early high-T conditions to a later stage of crack formation, fluid migration, and fluid entrapment around 300-400°C, estimated from the maximum temperature for brittle behaviour of plagioclase and the 1-2 kbar derived from fluid inclusion measurements. With the limitation of the method in mind, and considering that the pressure estimate is a minimum value due to additional fluid components, this decompression and cooling is interpreted to be related to uplift and cooling of the lower crust in the Mt.Melbourne area during the formation of the Ross Sea Rift and uplift of the Transantarctic Mountains