A revised interpretation of the Chon Aike magmatic province: Active margin origin and implications for the opening of the Weddell Sea

Late Triassic – Jurassic igneous rocks of the Antarctic Peninsula and Patagonia provide evidence for the evolution of the margin of southwestern Gondwana. We present new geochronological (LAICP-MS zircon U-Pb dates) analyses of 12 intrusive and volcanic rocks, which are complemented by geochemical and zircon (Hf) as well as whole rock isotopic (Nd, Sr) data. These are combined with similar analyses of 73 other rocks by previous studies, to constrain the magmatic evolution and Late Triassic – Jurassic tectonic setting. The distribution of crystallization ages reveals four main magmatic pulses that collectively span ~225–145 Ma, all of which have compositions that are consistent with a continental arc setting. The first episode occurred between ~223–200 Ma, and records active margin magmatism within the Antarctic Peninsula and northern Patagonia, and reveals the presence of a flat-slab that gave rise to magmatism in eastern Patagonia. After a period of magmatic quiescence during ~200–188 Ma, the second episode occurred between ~188–178Ma, with a continuation of arc magmatism above a flattened slab. The third episode spanned ~173–160 Ma, and its distribution suggests the slab was steepening, driving magmatism towards the south and west in Patagonia. Finally, the fourth period occurred during ~157–145 Ma, and was emplaced along the Antarctic Peninsula and western Patagonia, with no evidence for flat-slab subduction. The analysed rocks include the Chon Aike magmatic province, which has been considered to be genetically related to the break-up of Gondwana, the western Gondwanan subduction and the peripheral thermal effect of the Karoo plume in southern Africa. Our new data and revised compilation now suggest that the extent of the Early–Middle Jurassic Large Igneous Province in Patagonia and the Antarctic Peninsula should be re-examined, which may have implications for the primary causes of environmental changes. We also show that the final stage of Jurassic magmatism during ~157–145 was coincident with rifting that formed oceanic lithosphere of the Weddell Sea, and back-arc extension of the Rocas Verdes Basin, potentially revealing the presence of a triple junction that was located between southern Patagonia and the northern Antarctic Peninsula, and lead to the disassembly of southern Gondwana.

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