U-Pb detrital zircon ages (LAM-ICPMS)

U-Pb ages of detrital zircons from greywacke, and Rb-Sr metamorphic ages of slate from the Puncoviscana Formation of northwest Argentina are reported, and used to constrain the depositional age, metamorphic history, and sedimentary provenance of these rocks. The detrital zircon ages define mainly Late Mesoproterozoic-Early Neoproterozoic (1150-850 Ma) and Late Neoproterozoic-Early Cambrian (650-520 Ma) populations, the relative proportions of which vary inversely with the age of the youngest zircons in the samples. The 1150-850 Ma population is present in all samples and dominates in those with relatively old grains (> 600 Ma) in the Late Neoproterozoic-Early Cambrian population. However, the Late Mesoproterozoic-Early Neoproterozoic population is substantially smaller in those samples in which the Late Neoproterozoic-Early Cambrian population dominates and contains relatively young grains (> 520 Ma). The youngest zircons, c. 520 Ma, are in the Rancagua (Cachi, Salta province) sample. They form a narrow, unimodal peak and may have originated from volcanic sources active during deposition, in which case these youngest zircons would constrain the depositional age of the sample to the late Early Cambrian. This is consistent with Rb-Sr ages of 550-500 Ma for samples of slate from the Puncoviscana Formation. The detrital zircon age populations suggest a sedimentary provenance in a continental hinterland with a stabilized, extensive Late Mesoproterozoic orogen (with minor Paleoproterozoic and Archean precursors) and a more variable Late Neoproterozoic orogen containing an evolving sequence of less extensive subcomponents. A direct relationship with the Brazilian Shield is thus suggested with sedimentary detritus originating within the active-margin orogens of the interior, but with ultimate deposition in the passive-margin environment of western Gondwanaland.

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Detrital zircon U-Pb ages for 30 Late Jurassic and Cretaceous sandstones from the Eastern Province of eastern New Zealand, combined with previously-published geochronological and palaeontological data, constrain the time of deposition in the Pahau and Waioeka terranes of the Cretaceous accretionary margin of Zealandia, and their adjacent cover strata. The zircon age patterns also constrain possible sediment source areas and mid-Cretaceous geodynamic models of the transition from basement accretionary wedge to passive-margin cover successions. Pahau Terrane deposition was mainly Barremian to Aptian but continued locally through to late Albian time, with major source areas in the adjacent Kaweka and Waipapa terranes and minor inputs from the inboard Median Batholith. Waioeka Terrane deposition was mainly Albian, with distinctive and exclusive sediment sources, principally from the Median Batholith but with minor inputs from the Western Province. Alternative tectonic models to deliver such exclusive Median Batholith and Western Province-derived sediment to the mid-Cretaceous Zealandia continental margin are: (1) the creation of a rift depression across Zealandia or (2) sinistral displacement of South Zealandia with respect to North Zealandia, to expose Western Province rocks directly at the Zealandia margin. Detrital zircon age patterns of Cretaceous cover successions of the Eastern Province of eastern New Zealand demonstrate purely local sources in the adjacent Kaweka and Waipapa terranes. Cretaceous zircon components show a decline in successions of late Early Cretaceous age and disappear by late Late Cretaceous time, suggesting the abandonment or loss of access to both the Median Batholith and Western Province as sediment sources.

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