Oxygen isotope geochemistry of the Omeo Metamorphic complex, Victor: implications for metamorphic fluid flow, mineralisation and anatexis

Metamorphosed turbidites from the Omeo Metamorphic Complex show only minor changes in δ18O values with increasing metamorphic grade from 13.4 ± 1.7 per mill in the chlorite and biotite zones to 12.3 ± 1.0 per mill in the sillimanite + K-feldspar zone. Rocks within 5 km of the S-type granite at Hume Dam have δ18O values of 6.8-8.1 per mill that probably reflect interaction with heated meteoric-igneous fluids. Interaction with igneous fluids has also occurred close to other I- and S-type granites in this region. However, pervasive metamorphic fluid-rock interaction in this terrain did not occur, which limits the region's potential for hydrothermal mineralisation. Anatexis at high grades was probably via dehydration-melting reactions that consumed muscovite and biotite, which is consistent with there being little fluid present during metamorphism. Small (kilometre scale or less) S-type granites in the sillimanite + K-feldspar zone have δ18O values similar to those of the surrounding metasediments and probably formed by melting of those rocks. By contrast, larger (tens of kilometres scale) Ca-rich. peraluminous, S-type granites have lower δ18O values than the surrounding metasediments, and may represent melts of underlying middle to lower crust . Copyright (1998) Blackwell Science Asia