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

Fluid inclusions in gabbroic xenoliths from Cenozoic alkaline centres of the Mt.Melbourne Volcanic Field at the margin of the Ross Sea Rift Sytem have been studied in order to derive fluid compositions and cond itions of entrapment. Most of the inclusions are secondary, and only f ew are considered primary with respect to igneous crystallization. Tex tures indicate that the former inclusions formed from complete necking -down of fluid-filled, healed cracks. Most inclusions studied, primary or secondary, contain almost pure CO2, as has been tested by heating/ freezing experiments. Other fluid components (N2, hydrocarbons) have b een shown by Raman spectroscopy to be present in some inclusions. Hist ograms of homogenization temperatures for large (up to 50 mm) secondar y inclusions in plagioclase and pyroxene have maxima between ca. 16-de grees and 26-degrees, 12-degrees and 18-degrees. Inclusions in olivine were measured in one rock only and gave lower values from 0-degrees t o 10-degrees. We interpret these results as fluid entrapment and crack healing at different P-T-conditions along the steep geotherm previous ly documented for the lower crust in the Ross Rift region (Berg et al. , 1989), which followed metamorphic equilibration of these rocks at co nditions of 900-1000-degrees-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-degrees-C, estim ated 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 es timate is a minimum value due to additional fluid components, this dec ompression and cooling is interpreted to be related to uplift and cool ing of the lower crust in the Mt.Melbourne area during the formation o f the Ross Sea Rift and uplift of the Transantarctic Mountains.