Revisiting tectonic models for the evolution of the Ellsworth Mountains in Antarctica: A key component for understanding the African-Antarctic section of the paleo-Pacific margin

Abstract

Our current understanding of the Ellsworth Mountains stratigraphy suggests the oldest sedimentary sequence (Heritage Group) was deposited in a Cambrian rift setting. This Early Palaeozoic age is then used as a key piercing point to help define Cambrian paleogeography for the southern paleo-Pacific margin of Gondwana, which places the Ellsworth Mountains between southern Africa and East Antarctica as part of West Gondwana. Interpretations of this continental rift during the Cambrian had led to several tectonic models because require the reconciliation of seemingly contradictory Cambrian tectonic scenarios, including simultaneous subduction along the paleo-Pacific margin of Gondwana and rifting along this margin within the Ellsworth sector. However, U-Pb zircon dating of a micro-diorite from the Heritage Group reveals a crystallization age of 682 ± 10 Ma, which is not in the expected Cambrian range. This finding challenges chronostratigraphic and tectonic interpretations that now need to be revisited. We present a revision of the proposed tectonic models for the evolution of the Ellsworth Mountains during the late Neoproterozoic until the Cambrian in light of new findings. We have analysed the oldest sedimentary and volcanic rocks from the Heritage Group, utilizing U-Pb, Lu-Hf, and O isotopes in detrital and igneous zircons, along with whole-rock and mineral chemistry in igneous rocks. Positive εHft and mantle-like δ18O values for the igneous Cryogenian zircons suggest that the rifting, affecting Mesoproterozoic crust, occurred during the Cryogenian rather than in the Cambrian. Despite being scarce in the igneous rocks, these zircons are quite common in the lowest sedimentary units of the Heritage Group, suggesting that this magmatic event was significant. This observation further supports a connection between the Ellsworth-Whitmore Mountain crustal block and East Antarctica before the amalgamation of Gondwana. A second magmatic event in the Cambrian at 516 ± 7 Ma is recorded through zircons from a basaltic andesite within the upper Heritage Group. This magmatism is associated with an extensional setting, distinct from that of the Cryogenian micro-diorite, as evidenced by the Hf and O isotopic signature of their zircons, showing elevated δ18O values. These values indicate a strong sedimentary influence on the magma source and suggest crustal recycling. Whole-rock geochemistry of the igneous rocks reveals two distinct groups, one with E-MORB signatures and the other with subduction signatures. The interpretation of the Cambrian magmatism remains inconclusive but could be related to the tectonic escape following the collision between the Australo-Antarctic and West Gondwana/Indo-Antarctic plates, leading to the formation of Gondwana, or to a back arc extension.