Fore-arc basement terranes of Zealandia: origin and tectonic evolution

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This thesis aims to unravel the origin and tectonic evolution of fore-arc basement terranes in Zealandia. The study is focussed on Permian to Lower Cretaceous volcanic and volcaniclastic rocks from several terranes in New Zealand and New Caledonia. The results provide new constraints on the timing, provenance, palaeogeography, and tectonic setting of the sedimentary basins, as well as establishing the cyclic nature of arc magmatism in Zealandia. Ultimately, the thesis provides insights into geodynamic processes associated with basin formation and arc magmatism along the eastern Gondwanan margin.The origin of basement terranes in New Caledonia (Teremba, Koh-Central, and Boghen terranes) is constrained by g2000 new concordant U/Pb ages and trace-element data from detrital zircons. Results show abundant pre-Mesozoic zircon ages, but a lack of early Permian to Middle Triassic ages. The results indicate that detritus was derived from a local Paleozoic continental fragment that was rifted from the Gondwanan margin prior to the initiation of Permo-Triassic magmatism in eastern Australia. This implies that the dispersal of the eastern Gondwanan margins might have started already in the early Permian, much earlier than previously thought.The Brook Street Terrane in South Island, New Zealand, was investigated by geological mapping in the Takitimu Mountains and UnPb zircon petrochronology. Results suggest that this terrane was an oceanic arc during the early Permian, possibly during a phase of trench retreat and back-arc extension. New constraints on the timing of hypabyssal intrusive rocks (White Hill Intrusive Suite; 288.6 p 6.0 Ma) coincide with the inferred biostratigraphic age of the Takitimu Group (290.1n272.3 Ma), and is substantially older than the age of the Longwood Suite magmatism (261n252 Ma) in the nearby Median Batholith. The White Hill Intrusive Suite, therefore, is spatially and temporally linked to the allochthonous Brook Street Terrane, which was amalgamated to the Gondwanan margin between 288 Ma and 261 Ma. Detrital zircon age spectra from the upper Permian to Middle Jurassic volcaniclastic successions match magmatic pulses in the adjacent Tuhua Intrusives, indicating that following terrane amalgamation, the Brook Street Terrane became a fore-arc basin.Detrital zircon data from the Murihiku Terrane, Dun Mountain-Maitai Terrane, and Kaka Point Structural Belt, show age spectra that match magmatic pulses in the adjacent Tuhua Intrusives (Median Batholith). Based on the cross-terrane and localized provenance links, we suggest that the Brook Street, Murihiku and Dun Mountain-Maitai terranes represent the proximal and distal parts of the same fore-arc basin along the Gondwanan margin. In the Murihiku Terrane, a change in the provenance of the detrital zircon, at 235n230 Ma, is indicated by a prominent change in the trace-element compositions (Th/U, Zr/Hf, U/Yb, Eu/Eu*, and zircon crystallization temperature). This change was coeval with the transition from the Longwood to Darran Suite magmatism in the Median Batholith. During this short period, at 235n230 Ma, magmatism, deformation, and uplift of basement rocks affected New Caledonia and eastern Australia (Hunter-Bowen Orogeny), thus suggesting that the Gondwanide Orogen was subjected to a large-scale secular change associated with a plate reorganization event.The cyclic nature of arc magmatism in Zealandia was investigated by combining detrital zircon geochronology with trace-element and new Hf isotope data. Zircon grains dated 360n160 Ma from New Zealand have a juvenile Hf isotope signature with temporal variations that suggest a geodynamic link to eastern Australia. During the Late Permian to Middle Triassic Gondwanide Orogeny, foreland basins were developed along the Gondwanan margin, and the oceanic Teremba Terrane arc (New Caledonia) was accreted. This arc accretion is reflected in a mix of juvenile and evolved Hf isotope signatures in 245n185 Ma zircon grains from New Caledonia.Based on the findings of this thesis, it is concluded that the evidence for proximal Zealandia sources for fore-arc basement terranes in Zealandia do not support (1) models that have invoked more distant or exotic sources, and (2) periods of margin-parallel tectonic transport to their present New Zealand position. Detrital zircons contain a reliable record of geodynamic processes occurring within the Zealandia continent, and are generally compatible with geodynamic processes record in east Australia. The demonstrated links between detrital zircon geochemistry and various geodynamic processes highlight its potential as an important tool in understanding the evolution of accretionary orogenic belts.n