Abstract
M.Sc.
A fluid inclusion study was performed on the following rock types:
● Metapelitic granulites that occur as xenoliths in the Bulai Pluton.
● Metapelitic granulites that occur around the Bulai Pluton.
● Granite from the Bulai Pluton.
These rocks outcrop on the farm Boston near Messina in the so called Three Sisters
area. The main aims of this study are the following:
● Which fluids are associated with granulite metamorphism?
● Do the fluid inclusions record more than one metamorphic event, if so, what
are the P-T conditions of this/these event(s)?
● How do the fluids compare to fluids in the Southern Marginal Zone in terms of
composition and density?
The metapelites are typically characterised by a peak metamorphic mineral
assemblages of (1) quartz, K-feldspar, plagioclase, garnet, biotite, cordierite and
sillimanite and (2) quartz, K-feldspar, plagioclase, garnet, orthopyroxene and biotite.
The first assemblage have been used by other workers (Van Reenen et al., in prep.) to
derive a P-T path for the metapelitic xenoliths and host rock, which is characterised by
decompression-cooling. This assemblage also shows typical high temperature
metasomatic formation of feldspar around quartz that is in contact with quartz. The
peak metamorphic conditions were estimated to be ~850°C and ~7.5 kbar.
Fluid inclusions were studied in garnet, quartz inclusions in garnet, and matrix quartz.
The following principle fluid types were identified:
● High salinity aqueous fluids.
● CO2-rich (±CH4) carbonic fluids.
ii
Petrographic evidence indicates that both fluids were present at peak metamorphic
conditions under conditions of fluid-fluid immiscibility. The high-salinity aqueous
fluid is most likely responsible for the high temperature metasomatic textures.
The density of the fluids trapped at peak metamorphic conditions have been reset to
lower values at pressures below that of the peak metamorphic conditions. This is
probably the result of the emplacement of the Bulai Pluton at shallow crustal levels (2-
3 kbar).
Later fluids are dominated by low-density carbonic fluids with significant amounts of
CH4. The CH4 is the result of retrograde hydration reactions at relatively low oxygen
fugacities.