Devonian magmatism in the southern New England Orogen: evidence from dolerite intrusions within the Gordonbrook Serpentinite (New South Wales, Australia)

The geochemistry and geochronology of dolerite intrusions within the Gordonbrook Serpentinite were investigated to constrain the age of the ultramafic massif and assess its potential correlation with the Great Serpentinite Belt in New South Wales. Whole‑rock geochemical data define a continuous trend from tholeiitic to calc‑alkaline compositions, indicating derivation from a subduction‑modified mantle source emplaced in an arc‑proximal extensional setting. A Late Devonian age of the dolerite intrusion is established by U–Pb LA‑ICP‑MS analyses of primary apatite (lower‑intercept age of 379 ± 34 Ma) and an apatite Lu–Hf age (378 ± 24 Ma). Our data further suggest that the apatite U–Pb system was variably reset, with a younger age of 231 ± 24 Ma reflecting thermal overprinting by hydrothermal fluids associated with granitic intrusions of the Dumbudgery Creek Complex. Inherited zircon grains within the dolerites yield significantly older U–Pb dates (630–415 Ma; lower intercept 451 ± 26 Ma). Their high Hf contents, low Y/Hf and Zr/Hf ratios, and strongly fractionated rare-earth element patterns indicate crystallisation from evolved felsic melts. These xenocrystic zircons were most likely assimilated from sedimentary material containing Ordovician–Silurian detrital zircon populations. Their age range overlaps with detrital zircon signatures from sediments of the Gamilaroi and Silverwood provinces. Regionally, the Gordonbrook Serpentinite shares key characteristics with ultramafic units of the Great Serpentinite Belt, including Devonian mafic intrusions with arc‑proximal backarc geochemical signatures and inherited zircon populations older than 380 Ma. These similarities support the interpretation that the Gordonbrook massif represents a displaced segment of a once‑continuous ultramafic belt subsequently fragmented and reoriented during oroclinal bending.