| Abstrakt: |
The Devonian Narraburra Granite in southern New South Wales is located on the western margin of the Bogan Gate Synclinorium in the Temora Rift and is associated with other post- or late orogenic sodic granites and tholeiitic and transitional basic intrusive rocks. This region represents the rifted boundary between the Wagga-Omeo and Kosciusko Terranes. The granite is peralkaline (Al2O3/Na2O + K2O = 0.95-0.96), contains aegirine and arfvedsonite, and is only the third occurrence of peralkaline granite recorded in the Lachlan Fold Belt of southeastern Australia.Chemically the Narraburra Granite is distinct with high SiO2, total alkalis, Zr, Nb, Ga, Y and rare earth element abundances, relatively low Al2O3, MgO and CaO, and a pronounced negative Eu anomaly. The geochemical features are similar to those exhibited by A-type granites of the Bega Batholith, but the negative Eu anomalies are more pronounced (Eu/Eu* = 0.08-0.16) and the overall shape of the rare earth patterns is different. The abundances of Ba, Sr, V, Cr and Sc are low and there is considerable lateral variation in the rare earth elements, which are enriched towards the western margin of the intrusion.The chemistry and chemical variations within the granite are best explained by crystal fractionation combined with post-magmatic alteration by a sodium- and fluorine-rich fluid phase. The major element abundances and high Fe/Mg, common to all A-types, cannot be explained by an origin from remelting of a residual source unless these granites have been fractionated.The discovery of peralkaline granites in the rifted margin of the Wagga-Omeo and Kosciusko Terranes is regarded as further evidence that a well-developed continental crust was in place in southeastern Australia by Devonian times. The dominant tectonic style from the Late Silurian onward was that of strike-slip tectonics with a dextral transtensional regimen existing in the Temora region during the Early Devonian. The deformation, sedimentation and style of magmatism reflect the extensional tectonics. |
