Complex ⁴⁰Ar/³⁹Ar age spectra from low metamorphic grade rocks: resolving the input of detrital and metamorphic components in a case study from the Delamerian Orogen

Abstract

Abstract. Low metamorphic grade rocks contain both detrital minerals and minerals newly grown or partly recrystallised during diagenesis and metamorphism. However, rocks such as these typically yield complex 40Ar/39Ar age spectra that can be difficult to interpret. In this study, we have analysed a suite of variably deformed rocks from a region of low metamorphic grade within the c. 514–490 Ma Delamerian Orogen, South Australia. The samples analysed range from siltstone and shale to phyllite and all contain either muscovite or phengite determined by hyperspectral mineralogical characterisation. Furnace step heating 40Ar/39Ar analysis produced complex apparent age spectra with multiple age components. Using the concept of asymptotes that define minimum and maximum ages for different components, we interpret the age spectra to preserve a range of detrital mineral ages, along with younger components related to either cooling or deformation- induced recrystallisation. Two samples contain Mesoproterozoic detrital age components, up to c. 1170 Ma, while the c. 515 Ma Heatherdale Shale which has both c. 566 Ma and c. 530 Ma detrital components. All samples contain younger lower (younger) asymptotes in the age spectra defined from multiple heating steps that range from c. 476 to c. 460 Ma. One interpretation of these younger ages is that they are caused by post-metamorphic cooling. However, the shape of the age spectra and the degree of deformation in the phyllites suggest the ages may record recrystallisation of detrital minerals and/or new mica growth during deformation. Potentially these c. 476 to c. 460 Ma ages suggest deformation in the upper portion of the orogen was facilitated by movement along regional faults and shear zones up to around 20 million years after the cessation of deformation in the high-metamorphic grade regions of the Delamerian Orogen.

Metallogenic Setting and Temporal Evolution of Porphyry Cu-Mo Mineralization and Alteration in the Delamerian Orogen, South Australia: Insights From Zircon U-Pb, Molybdenite Re-Os, and In Situ White Mica Rb-Sr Geochronology

“…15; Reid et al, 2022). These ages have been interpreted as a result of far-field geodynamic events along the eastern paleo-Pacific margin of Gondwana (Reid et al, 2022;Fig. 15).…”

“…Muscovite-and chlorite-rich phyllites from the Mount Lofty Ranges and Fleurieu Peninsular of the Delamerian Orogen yielded 40 Ar/ 39 Ar spectrum ages of 458 to 430 Ma (Fig. 15; Reid et al, 2022). These ages have been interpreted as a result of far-field geodynamic events along the eastern paleo-Pacific margin of Gondwana (Reid et al, 2022;Fig.…”

“…These dates are also similar to ca. 470-458 Ma ages for deformation obtained from thrust sheets in the western region of the orogen (Reid et al, 2022), suggesting that tectionic activity certainly did not cease with the emplacement of ca. 490-470 Ma granitoids.…”

“…(Foden et al, 2006), in comparison to porphyry Cu deposits in the Macquarie Arc of Lachlan Orogen. 40 Ar/ 39 Ar ages of metasediments from Reid et al (2022). Zircon U-Pb and molybdenite Re-Os ages for porphyry Cu-Au deposits in the Cadia Valley and Cowal Lake districts, NSW from Wilson et al (2007), Zukowski (2010), Rush (2013), Foster et al (2015, and Harris et al (2020) and references therein.…”