Summary of results. Zircon isotopic and trace element data from the Rover field, Warramunga Province

Abstract

Zircon Lu–Hf and O isotopic and trace element data were collected to complement existing geochronological data from the Rover field, southeast of Tennant Creek, central Northern Territory. The analysed samples represent the main volcanic and volcaniclastic units and hosts of the gold–copper–bismuth and base metal mineralisation of the Rover field. Zircon U–Pb dates from these samples span ca 1850–1700 Ma. This period encompass the ca 1850–1840 Ma Tennant Event (associated with the Tennant Creek Supersuite and gold–copper–bismuth mineralisation), the 1820–1800 Ma extensional Murchison Event, and the younger Davenport Event associated with the ca 1710 Ma Devils Suite magmatism.

Farias et al (2022b) proposed a model in which the Rover field and Tennant Creek mineral field evolved from the compressional tectonic setting of the Tennant Event to an extensional tectonic setting, which continued until the onset of the Davenport Event. This extensional phase is partly evidenced in the Rover field by the ca 1.76 Ga E-MORB magmatism (Bluebush basalt; Farias et al 2022b), which is likely associated with the latest stages of the Murchison Event, and the ca 1.73 Ga base metal mineralisation in the western Rover field (Farias et al 2022a).

Zircon Hf, O and trace element data presented in this study uphold the supposition that ca 1850–1840 Ma zircon in the Rover field formed from evolved fractionated and oxidised magmas (strongly to moderately positive zircon Ce anomalies), most likely associated with the nearby Tennant Creek Supersuite and/or the bimodal magmatism that produced the extrusive rocks of the ca 1850–1840 Ma Yungkulungu Formation.

The range of zircon REE compositions suggest differences in the parental magmas across the Rover field. Zircon from the central–eastern parts of the field show lower light- to mid- rare earth elements (LREE to MREE) content compared with zircon of the western Rover field. We suggest that competition for REE in the magma between zircon and other mineral phases (eg titanite, monazite or apatite) most likely influenced the zircon REE composition (Garber et al 2017). More mafic to intermediate magmas compositions in the central zone (with titanite and hornblende in equilibrium with zircon) restricted LREE and MREE access to zircon.

The majority of zircon in the 1850–1840 Ma age range yielded negative eHf values, with one sample (porphyritic rhyolite; sample BW19DLH0002) returning values lower than -25. These strongly negative eHf values could be explained by reworking of an evolved continental crust and/or sediment input into magmas that migrated through a thickened continental crust. The d18O values obtained from 1850–1840 Ma zircon grains are mostly in the range expected for zircon derived from a significantly reworked supracrustal source (eg Kemp et al 2006); they are above the expected range for mantle-derived melts defined by Valley et al (2003; 5.3 ± 0.6‰). The isotopic data from zircon younger than ca 1820 Ma yielded eHf values close or slightly above the CHUR, suggesting some level of mantle input. Some of the younger grains yielded d18O values expected in deep crustal material that has not been supracrustally reworked. This indicates that the Rover field received juvenile mantlederived material, or had exposed deep crustal blocks after ca 1820 Ma, most likely associated with the extensional phase of the Murchison Event.