Abstract
We present a data set of >1500 in situ O-Hf-U-Pb zircon isotope analyses that document the existence of a concealed Rodinian lithospheric keel beneath continental Zealandia. The new data reveal the presence of a distinct isotopic domain of Paleozoic–Mesozoic plutonic rocks that contain zircon characterized by anomalously low δ18O values (median = +4.1‰) and radiogenic εHf(t) (median = +6.1). The scale (>10,000 km2) and time span (>>250 m.y.) over which plutonic rocks with this anomalously low-δ18O signature were emplaced appear unique in a global context, especially for magmas generated and emplaced along a continental margin. Calculated crustal-residence ages (depleted mantle model, TDM) for this low-δ18O isotope domain range from 1300 to 500 Ma and are interpreted to represent melting of a Precambrian lithospheric keel that was formed and subsequently hydrothermally altered during Rodinian assembly and rifting. Recognition of a concealed Precambrian lithosphere beneath Zealandia and the uniqueness of the pervasive low-δ18O isotope domain link Zealandia to South China, providing a novel test of specific hypotheses of continental block arrangements within Rodinia.
Highlights
The Neoproterozoic amalgamation and subsequent breakup of the Rodinia supercontinent were significant events in Earth’s history
Central to the Rodinia debate is the location of cratonic blocks to the east of the Australia–East Antarctica margin, with arguments that the margin was adjacent to the western Canadian Laurentian margin (Dalziel, 1991; Moores, 1991) or the western United States Laurentian margin (Karlstrom et al, 1999)
It is hypothesized that the South China block was positioned between Australia–East Antarctica and Laurentia (Li et al, 1999)
Summary
The Neoproterozoic amalgamation and subsequent breakup of the Rodinia supercontinent were significant events in Earth’s history. Recognized as a distinct continent, Zealandia—of which 94% is currently underwater—formed following Late Cretaceous breakup of the Gondwana supercontinent (Mortimer et al, 2017). The basement rocks of continental Zealandia were created by multiple episodes of terrane accretion and arc-related magmatism along the paleo-Pacific Gondwana margin from the Cambrian to Early Cretaceous (Mortimer, 2004). The oldest basement rocks of Zealandia are divided into two provinces: the early Paleozoic Western province, comprising metasedimentary rocks and Paleozoic–Mesozoic intrusions, and the late Paleozoic–Mesozoic Eastern province, a series of plutonic-metasedimentary terranes accreted to the Gondwanan margin (Fig. 1) (Mortimer, 2004). Previous isotopic studies focused on this boundary demonstrate that Mesozoic Zealandia consisted of separate crustal blocks that are isotopically distinct (Schwartz et al, 2021). Our results reveal for the first time that continental Zealandia is underlain by a broad Precambrian lithospheric keel, which allows us to place Zealandia into the greater Rodinia supercontinent puzzle
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