Source of the Cooma Granodiorite, New South Wales — a possible role of fluid‐rock interactions

Abstract

The bulk rock chemistry and O and Sr isotopic composition of the S‐type Cooma Granodiorite, NSW, is compared with the rocks of the surrounding metasedimentary envelope. Major and trace element abundances confirm previous suggestions that the high‐grade metapelitic and metapsammitic gneisses immediately surrounding the granodiorite are the geochemical equivalents of the more distant, low‐grade pelitic and psammatic metasediments. The Cooma Granodiorite can be modelled as a subequal mixture of pelite and psammite and probably represents a partial melt from which a solid residue has not significantly separated.

Based on slight but consistently lower δ¹⁸O values in the granodiorite (average δ^l8O = + 12.2) compared with the probable mixed pelitic‐psammitic source (average δ¹⁸O = + 13.5), it is suggested that a water‐rich fluid flux was an important factor in the formation of the Cooma Granodiorite. This fluid, possibly derived either from an underlying more‐mafic magma or, alternatively, from within the pelitic‐psammitic sequence itself, may selectively have lowered the δ¹⁸O value of the source of the granodiorite.

Rb‐Sr isotope data suggest younger whole‐rock ages (∼ 375 Ma) than previously published mineral isochron ages (406 Ma) dating the formation of the Cooma Granodiorite and its metamorphic envelope. The Cooma sediments were probably characterized by an initial negative slope in the ⁸⁷Rb/⁸⁶Sr vs ⁸⁷Sr/⁸⁶Sr diagram, and seem not to have been fully equilibrated on a regional scale during metamorphism and melting.

Key words:

• Cooma Granodiorite
• fluid‐rock interaction
• major and trace elements
• O and Sr isotopes
• partial melting of sedimentary source
• Rb‐Sr dating