CARBON-ISOTOPE CONSTRAINTS ON FLUID ADVECTION DURING CONTRASTING EXAMPLES OF INCIPIENT CHARNOCKITE FORMATION

Incipient charnockite formation within amphibolite facies gneisses is observed in South India and Sri Lanka both as isolated sheets, associa ted with brittle fracture, and as patches forming interconnected netwo rks. For each mode of formation, closely spaced drilled samples across charnockite/gneiss boundaries have been obtained and delta(13)C and C O2 abundances determined from fluid inclusions by stepped-heating mass spectrometry. Isolated sheets of charnockite (c.50 mm wide) within bi otite-garnet gneiss at Kalanjur (Kerala, South India) have developed o n either side of a fracture zone. Phase equilibria indicate low-pressu re charnockite formation at pressures of 3.4 +/- 1.0 kbar and temperat ures of about 700 degrees C (for X(H2O)=0.2). Fluid inclusions from th e charnockite are characterized by delta(13)C values of -8% and from t he gneiss, 2m from the charnockite, by values of -15%. The large CO2 a bundances and relatively heavy carbon-isotope signature of the charnoc kite can be traced into the gneiss over a distance of at least 280 mm from the centre of the charnockite, whereas the reaction front has mov ed only 30 mm. This suggests that fluid advection has driven the carbo n-isotope front through the rock more rapidly than the reaction front. The carbon-front/reaction-front separation at Kalanjur is significant ly larger than the value determined from a graphite-bearing incipient charnockite nearby, consistent with the predictions of one-dimensional advection models. Incipient charnockites from Kurunegala (Sri Lanka) have developed as a patchy network within hornblende-biotite gneiss. C O2 abundances rise to a peak near one limb of the charnockite, and iso topic values vary from delta(13)C of c. -5.5% in the gneiss to -9.5% i n the charnockite. The shift to lighter values in the charnockite can be ascribed to the formation of a CO2-saturated partial melt in respon se to influx of an isotopically light carbonic fluid. Thus, incipient charnockites from the high-grade terranes of South India and Sri Lanka reflect a range of mechanisms. At shallower structural levels non-per vasive CO2 influxed along zones of brittle fracture, possibly associat ed with the intrusion of charnockitic dykes. At deeper levels, in situ melting occurred under conditions of ductile deformation, leading to the development of patchy charnockites.