Isotopic dating of basin inversion-The Palaeozoic Cobar Basin, Lachlan Orogen, Australia

R. A. Glen*, R. D. Dallmeyer, L. P. Black

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

42 Citations (Scopus)


The age of inversion of the Early Devonian Cobar Basin is poorly defined by field geological data which have to be extrapolated from the coeval shelf west of the basin. In order to better constrain the age of deformation, a whole-rock isotopic dating program was undertaken and yields the following results. Rb-Sr isotopic dates predate the depositional age, giving a regional isochron of 442 ± 4 Ma. Three K-Ar dates from Zone 1 are mutually indistinguishable (402 ± 8, 402 ± 8, 405 ± 8 Ma), suggesting a mid to late Early Devonian age of cleavage formation. Four whole-rock 40Ar/39Ar spectra from the high-strain Zone 1 give intermediate-temperature plateaux ranging from ca. 389 Ma to 404 Ma. In the lower strain Zone 2, total-gas ages from three samples range from ca. 401 Ma to ca. 423 Ma. An approximate intermediate-temperature plateau for one sample indicates an age ca. 405 Ma: the other sample yields a good plateau of ca. 437 Ma. These data are interpreted as reflecting a low-grade cleavage-forming inversion event which took place at ca. 395-400 Ma. Isotopic data from Zone 1 suggest (near) complete rejuvenation of detrital phases during the 395-400 Ma event. In contrast, the older Zone 2 Rb-Sr ages and some of the 40Ar/39Ar ages reflect, at least in part, basement provenance ages preserved within detrital micas and feldspars which did not undergo complete rejuvenation because of lower deformational strain and/or temperature of metamorphism. The intense 395-400 Ma late Early Devonian inversion event in zones 1 and 2 of the former Cobar Basin largely bypassed the southwestern part of the basin and the adjacent shelf to the west. These only underwent minor block faulting and uplift at this time, but were more strongly deformed, along with the overlying fluviatile Mulga Downs Group, in the Carboniferous.

Original languageEnglish
Pages (from-to)249-268
Number of pages20
Issue number1-4
Publication statusPublished - 30 Nov 1992
Externally publishedYes


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