A revised interpretation of the chon aike magmatic province: active margin origin and implications for the opening of the weddell sea

Abstract

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 (LA-ICP-MS zircon U-Pb dates) analyses of 12 intrusive and volcanic rocks, which are complemented by geochemical and zircon isotopic (Hf) as well as whole rock isotopic (Nd, Sr) data. These are combined with similar analyses of 73 other igneous rocks by previous studies, to constrain the magmatic evolution and Late Triassic - Jurassic tectonic setting. The distribution of crystallisation ages reveals four main magmatic pulses that collectively span similar to 225-145 Ma, all of which have compositions that are consistent with a continental arc setting. The first episode occurred between similar to 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 (similar to 200-188 Ma), the second episode occurred between similar to 188 and 178 Ma, with a continuation of arc magmatism above a flattened slab. The third episode spanned similar to 173-160 Ma, and its geographic distribution suggests the slab was steepening, driving magmatism towards the south and west in Patagonia. Finally, the fourth period occurred between similar to 157 and similar to 145 Ma, during which time magmas were 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 have been influenced by the break-up of Gondwana, via heating associated with the Karoo plume in southern Africa and the active margin in western Patagonia and the Antarctic Peninsula. Our new data and revised compilation now suggest that the Early - Middle Chon Aike Jurassic silicic magmatic province in Patagonia and the Antarctic Peninsula can be entirely accounted by active margin processes. We also show that the final stage of Jurassic magmatism (similar to 157-145 Ma) 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 located between southern Patagonia and the northern Antarctic Peninsula that led to the disassembly of southern Gondwana.