TRENDS IN OXIDES, SULFIDES AND TITANITES ALONG A LATE ARCHEAN, DEEP-CRUSTAL PROFILE IN TAMIL NADU, SOUTH INDIA

We studied textures, assemblages and chemistries of oxides, sulfides and titanites in felsic and intermediate orthogneisses across an amphibolite (north) to granulite (south) transition zone in northern Tamil Nadu. Magnetite is ubiquitous. Titanite is abundant in the amphibolite-facies terrain where ilmenite occurs as inclusions within titanite or is rimmed by titantite. Ilmenite appears as independent grains as titanite disappears just north of the orthopyroxene isograd. Pyrite is widespread while pyrhhotite is confined to roughly half the samples in the southern highest grade zone. These differences appear to reflect lower temperatures and higher fH2O in the lower grade zone. Magnetite + quartz and magnetite + pyrite veins are common south of the opx isograd. Thermodynamically these veins could form during uplift by reactions involving a residual metamorphic fluid, but this process is unable to account for their abundance. Magnetite-quartz veins could form either by the introduction of a oxygen-rich fluid or by a coupled retrograde reaction in which subsolidus reduction of the hematite component of ilmenite in one part of the rock provides oxygen for the formation of magnetite + quartz from ferromagnesium silicates in another portion. This second mechanism could account for the widespread occurrence of magnetite bands in ilmenite. The magnetite + pyrite veins are best explained by the influx of a fluid with a high H2S/H2O ratio (H2S-dominated conditions) or relatively high SO2 concentrations (SO2 – dominated conditions). Calculations based on coexisting magnetite and orthopyroxene indicate oxygen fugacities above both QFM and graphite saturation for almost all samples. A small population of rocks in the southern high-grade zone record low fO2 and many of these rocks have relatively high amounts of biotite and Rb. Biotite-rich, Rb-undepleted samples from further north record high fO2 indicating that biotite breakdown, Rb depletion and oxidation may not have been due to the same process.