Discovery of a microcontinent (Gulden Draak Knoll) offshore Western Australia: implications for East Gondwana reconstructions

Analysis of dredged samples from the Gulden Draak Knoll demonstrates that it is a submarine rifted continental fragment that lies at the boundary between the western Perth Abyssal Plain and Wharton Basin, Indian Ocean. The Knoll comprises a granulite facies basement, including pelitic paragneiss and mafic orthogneiss, with a Cambrian granite inferred to intrude the other rocks. Boulders and cobbles of felsic gneiss with Mesoproterozoic and Cambrian protolith ages were also sampled likely reflecting a complex basement to variable sedimentary and volcanic rocks. The U-Pb isotopic system in the Archean and Mesoproterozoic zircon is significantly disturbed, reflecting Cambrian orogenesis that affected all samples. The protolith to garnet-sillimanite-biotite paragneiss has a maximum deposition age of 1163±24Ma and includes older detrital zircon grains with populations at c. 2.65Ga and between 1.4 and 1.1Ga. A younger population in this sample is interpreted as a mix of newly grown metamorphic zircon and isotopically reset zircon, implying that the granulite facies metamorphism occurred at c. 511±5Ma. Protracted Cambrian orogenesis is indicated by a metamorphic age in the mafic orthogneiss of 530±6Ma and isotopic disturbance shortly following emplacement of granite (c. 540Ma with zircon ages disturbed to 509±7Ma) and the protolith to the felsic orthogneiss (c. 528Ma with zircon ages disturbed to 510±3Ma). Xenocrystic zircon grains in the Cambrian rocks include Archean (c. 2839±9Ma) and Mesoproterozoic (1230-1370Ma) populations also isotopically disturbed during Cambrian orogenesis. Igneous Cambrian zircon grains have less radiogenic Hf-isotope compositions (Hf_i=0.281821-0.281367) than Mesoproterozoic xenocrysts (Hf_i=0.282267-0.281993), indicating limited involvement of the Mesoproterozoic crust in granite production. A more likely source includes Archean crust represented by xenocrysts with Hf_i=0.281399-0.280863. The Gulden Draak Knoll is reconstructed in Gondwana ('Leeuwin' full-fit model) along strike of a major structure termed the Indo-Australo-Antarctic Suture (IAAS), recently mapped from geophysical interpretations in Wilkes Land, Antarctica. New isotopic data suggest that basement rocks from the Gulden Draak Knoll have affinity to crust exposed either side of the IAAS. Determining if this structure is a suture zone sensu stricto remains to be tested.