Carbon‐isotope constraints on fluid advection during contrasting examples of incipient charnockite formation

Publisher Website
Google Scholar
Abstract
Incipient charnockite formation within amphibolite facies gneisses is observed in South India and Sri Lanka both as isolated sheets, associated with brittle fracture, and as patches forming interconnected networks. For each mode of formation, closely spaced drilled samples across charnockite/gneiss boundaries have been obtained and δ13C and CO2abundances determined from fluid inclusions by stepped‐heating mass spectrometry.Isolated sheets of charnockite (c.50 mm wide) within biotite–garnet gneiss at Kalanjur (Kerala, South India) have developed on either side of a fracture zone. Phase equilibria indicate low‐pressure charnockite formation at pressures of 3.4 ± 1.0 kbar and temperatures of about 700°C (forXH2O= 0.2). Fluid inclusions from the charnockite are characterized by δ13C values of −8% and from the gneiss, 2 m from the charnockite, by values of −15%. The large CO2abundances and relatively heavy carbon‐isotope signature of the charnockite 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 moved only 30 mm. This suggests that fluid advection has driven the carbon‐isotope front through the rock more rapidly than the reaction front. The carbon‐front/reaction‐front separation at Kalanjur is significantly 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. CO2abundances rise to a peak near one limb of the charnockite, and isotopic values vary from δ13C ofc.−5.5% in the gneiss to −9.5% in the charnockite. The shift to lighter values in the charnockite can be ascribed to the formation of a CO2‐saturated partial melt in response 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‐pervasive CO2influxed along zones of brittle fracture, possibly associated with the intrusion of charnockitic dykes. At deeper levels,in situmelting occurred under conditions of ductile deformation, leading to the development of patchy charnockites.