Missing pieces in the deformation and thermal history along the Cambro-Ordovician Delamerian Orogen, South Australia

Abstract

The Neoproterozoic-Cambrian period was one of the most active periods of crust formation, largely shaped by the Grenvillian (1.0Ga) and Pan-African (0.55Ga) orogenies. During this time, Gondwana was undergoing final assembly following Rodinia breakup. This plate reorganisation event led to the transformation to an active margin from a passive margin at the east with simultaneous subduction of the Pacific plate. This active period of subduction following Gondwana assembly led to the Delamerian Orogen at c. 515Ma culminating at c. 490Ma.

The Delamerian Orogeny initiated following the deposition of Kanmantoo Group sediments during the Early Cambrian times (c. 524Ma) in an extensional setting, and comprises three phases of deformation recognised as D1, D2 and D3 of varying styles and intensity. Presently, the timing of these orogenic events is estimated through U-Pb ages on magmatic intrusives as proxies for deformation/metamorphic/mineral growth events. This study focusses on re-evaluating and re-establishing the timing of Delamerian deformation and hydrothermal alteration, and constrain thermal evolution across the orogen through 40Ar/39Ar geochronology on selected samples from pre-, syn- and post-tectonic units with particular emphasis on analysing fabric forming minerals. This in turn has provided a) new constraints on timing of younger events previously not identified in the Delamerian Orogen, and b) a coherent dataset from different geochronometers (U-Pb, Ar/Ar etc.). 40Ar/39Ar geochronology was conducted on selected samples from Kanmantoo Group metasediments in the eastern Mt. Lofty Ranges due to their subsequent folding post-deposition during Delamerian Orogeny. Samples have also been selected from Summerfield Intrusive Suite (SIS) (late syn-Delamerian) and Padthaway Ridge suite (post-Delamerian A-type bimodal suite) and Anabama Hill Granite Complex (Cu-Mo porphyry prospect) to bracket the entire temporal span of the Delamerian Orogeny.

The picture emerging through the 40Ar/39Ar results indicates existence of significantly similar younger age components (up to Late Devonian) across all units studied here, irrespective of their varied emplacement times and mechanisms. Though all units except the Padthaway Ridge suite and Anabama Hill Granite Complex were deformed during the Delamerian Orogenesis, the samples preserve a spectrum of ages ranging from Late Cambrian to Early Devonian. In the Mt. Lofty Ranges, ages younger than c. 430Ma do not exist likely due to its exhumation and closure to Ar-diffusion in minerals. Units from SIS, Anabama Hill Granite Complex and Padthaway Ridge suite preserve significantly younger ages with major peaks at c. 480Ma, c. 465-462Ma, c. 425-423Ma and c. 385-380Ma. These age intervals correlate well with onset of Lachlan Fold Belt (LFB) in the east, and could likely record its effects in older Delamerian units. This study identifies two possible reasons for the existence of younger ages in the Delamerian Orogen, a) previously unidentified protracted periods of tectonic activity than previously anticipated i.e., Delamerian Orogeny was longer-lived than presently known until c. 468Ma, and b) effects of commencement of younger LFB orogenesis that affected the cratonised Delamerian Orogen in eastern Australia during Early Ordovician through reactivation of older structures in the region. The findings in this study recognise previously unidentified younger age peak intervals that correspond to deformation, mineralisation, alteration and/or mineral growth events at key locations, and contribute to our current understanding of Delamerian Orogen. Show more