Geochemistry and Rb-Sr geochronology of the alkaline-peralkaline Narraburra Complex, central southern New South Wales; tectonic significance of Late Devonian granitic magmatism in the Lachlan Fold Belt

Three suites of alkaline granite can be recognised in the Narraburra Complex at the triple junction of the Tumut, Giralambone-Goonumbla and Wagga Zones, central southern New South Wales. On the basis of K2O/Na2O ratios, biotite and hornblende-biotite potassic I-type granites have been assigned to the Gilmore Hill (K2O/Na2O 1.00) and Barmedman Suites (K2O/Na2O > 1.2). These are metaluminous to weakly peraluminous suites that crystallised from high-temperature,reduced magmas with the least fractionated members of each suite having high Ba and low Rb abundances compared to other Lachlan Fold Belt granites. Fractionated members of these suites have high abundances of high-field-strength elements, similar to those observed in A-type granites. Arfvedsonite and aegirine-arfvedsonite granites have been assigned to the peralkaline Narraburra Suite. Granites from this suite have chemistry consistent with them being the intrusive equivalents of comendites and they are also similar in some respects to A-type granites: they have, for example, particularly high abundances of Zr. The A-type signature is, however, at least in part the result of strong fractionation. Total-rock Rb-Sr isotopic analyses from both I-type suites plot on the same isochron, giving an age of 365 ± 4 Ma (Srl = 0.70388 ± 53). A total-rock isochron for the peralkaline Narraburra Suite gives a less well-defined age of 358 ± 9 Ma (Srl = 0.7013 ± 80). The Late Devonian Rb-Sr ages may be emplacement ages or a result of resetting during fluid-rock interaction. Although granites of the Narraburra Complex have geochemical affinities with alkaline granites formed late in orogenic cycles, they post-date arc magmatism by at least 75 million years and they formed in a within-plate setting. Magmatism was related to localised reactivation of major faults (Gilmore Fault and the Parkes Thrust) in the region, and to partial melting involving both enriched mantle and Ordovician shoshonitic crustal components. Emplacement of the Narraburra Complex was contemporaneous with magmatism in the Central Victorian Magmatic Province and A-type magmatism in eastern New South Wales. Collectively, all these magmatic events were related to extension post-dating amalgamation of the western and central/eastern subprovinces of the Lachlan Fold Belt