New age constraints supporting the existence of protracted deformation in the Delamerian Orogen: ⁴⁰Ar/³⁹Ar geochronology on fabric forming minerals of the Kanmantoo Group metasediments

Abstract

Abstract. The final assembly of Gondwanaland following Rodinia breakup during the Neoproterozoic-Cambrian period was marked by tectonic mode switch leading to the transformation of a passive margin in the east to an active subduction margin. This period was marked by subduction of the Pacific plate leading to the initiation of the Delamerian Orogeny in South Australia at c. 515 Ma, culminating at c. 490 Ma. Most studies have attempted to constrain the timing of deformation through dating of magmatic intrusions as proxies for deformation, however, relatively few studies strived to date minerals in fabrics that developed during deformation. As opposed to existing age constraints on the timing of cessation of the Delamerian Orogeny at c. 490 Ma, the work presented here identifies new protracted periods of active regional tectonics in the eastern Mt. Lofty Ranges. This was first observed in the underlying Backstairs Passage Formation overlain by the Tapanappa Formation of the Cambrian Kanmantoo Group through ultra-high vacuum 40Ar/39Ar geochronology and 39Ar diffusion experiments conducted on mica separates. It is observed that younger ages up to c. 468 Ma are recorded in the stratigraphically older Backstairs Passage Formation, while the overlying Tapanappa Formation bears no 40Ar/39Ar apparent ages younger than c. 486 Ma. In addition, age data reveals a period of fluid activity at c. 497–493 Ma resulting in micaceous intrusions locally and regionally. The methodology adopted here involves compilation of the age information with microstructural analyses where the age interval of c. 493–487 Ma is interpreted as post peak-metamorphism garnet growth event. This period is likely associated with retrograde metamorphism and is co-eval with Cu-mineralization in the region at c. 492–485 Ma. A second generation of retrogressed biotite growth at the expense of garnet has been identified. Importantly, there are differences in metamorphic grade and timing of deformation between the stratigraphically older the Backstairs Passage Formation and the younger Tapanappa Formation. This can be explained through macroscopic and/or microscopic processes. Two interpretations are proposed here- first involving the lower activation energy of the Backstairs Passage Formation as a measure of argon retentivity, and therefore younger ages, and the second considering proximity of the two formations to the metamorphic core of the belt at Reedy Creek and the effect of dissipated heat affecting record of argon ages in each unit. These newly identified young ages indicate the protracted nature of the Delamerian Orogeny than previously documented.