Mobility of components in metasomatic transformation and partial melting of gneisses: an example from Sri Lanka

Reaction textures, fluid inclusions, and metasomatic zoning coupled with th ermodynamic calculations have allowed us to estimate the conditions under w hich a biotite-hornblende gneiss from the Kurunegala district, Sri Lanka [h ornblende (N-Mg = 38-42) + biotite (N-Mg = 42-44) + plagioclase + quartz K-feldspar + ilmenite + magnetite] was transformed into patches of charnock ite along shear zones and foliation planes. Primary fluid inclusion data su ggest that two immiscible fluids, an alkalic supercritical brine and almost pure CO2, coexisted during the charnockitisation event and subsequent post -peak metamorphic evolution of the charnockite. These metasomatic fluids mi grated through the amphibolite gneiss along shear zones and into the wallro ck under peak metamorphic conditions of 700-750 degreesC, 5-6 kbar, and a(H 2O)(fl) 0 =0.52-0.59. This resulted in the formation of charnockite patches containing the assemblage orthopyroxene (N-Mg = 45-48) + K-feldspar (Or(70 -80)) + quartz + plagioclase (An(28)) in addition to K-feldspar microveins along quartz and plagioclase grain boundaries. Remnants of the CO2-rich flu id were trapped as separate fluid inclusions. The charnockite patches show the following metasomatic zonation patterns: a transition zone with the assemblage biotite (N-Mg = 49-51) + hornblende(N -Mg = 47-50) + plagioclase + quartz + K-feldspar + ilmenite + magnetite; a KPQ (K-feldspar-plagioclase-quartz) zone with the assemblage K-feldspar plagioclase + orthopyroxene (N-Mg = 45-48) + quartz + ilmenite + magnetite ; a charnockite core with the assemblage K-feldspar + plagioclase + orthopyro xene (N-Mg 39-41)+ biotite (N-Mg = 48-52) + quartz + ilmenite + magnetite. Systematic changes in the bulk chemistry and mineralogy across the four zon es suggest that along with metasomatic transformation, this process may hav e been complicated by partial melting in the charnockite core. This melting would have been coeval with metasomatic processes on the periphery of the charnockite patch. There is also good evidence in the charnockitic core tha t a second mineral assemblage, consisting of orthopyroxene (N-Mg= 36-42) biotite (N-Mg 50-51) + K-feldspar (Or(70-80))+ quartz + plagioclase (An(28- 26)), could have crystallised from a partial melt during cooling from 720 t o 660 degreesC at decreasing a(H2O)(fl) from 0.67 to 0.5. Post-magmatic evo lution of charaockite at T< 700 <degrees>C resulted in fluids being release d during the crystallisation of the charnockitic core. These gave rise to t he formation of late stage rim myrmekites along K-feldspar grain boundaries as well as late stage biotite, cummingtonite, and carbonates.