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

Metamorphosed turbidites from the Omeo Metamorphic Complex show only minor changes in delta(18)O values with increasing metamorphic grade from 13.4 +/ - 1.7 parts per thousand in the chlorite and biotite zones to 12.3 +/- 1.0 parts per thousand in the sillimanite + K-feldspar zone. Rocks within 5 km of the S-type granite at Hume Dam have delta(18)O values of 6.8-8.1 parts p er thousand 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-r ock interaction in this terrain did not occur, which limits the region's po tential for hydrothermal mineralisation. Anatexis at high grades was probab ly via dehydration-melting reactions that consumed muscovite and biotite, w hich is consistent with there being little fluid present during metamorphis m. Small (kilometre scale or less) S-type granites in the sillimanite + K-f eldspar zone have delta(18)O values similar to those of the surrounding met asediments and probably formed by melting of those rocks. By contrast, larg er (tens of kilometres scale) Ca-rich, peraluminous, S-type granites have l ower delta(18)O values than the surrounding metasediments, and may represen t melts of underlying middle to lower crust.