Distribution and characteristics of Tonian metamorphism in Ediacaran-Cambrian collisional orogenic belt of Lützow-Holm Complex, East Antarctica

We conducted sensitive high-resolution ion microprobe U-Pb zircon dating of the unexplored outcrop Chijire Rocks and the previously undated Cape Ryûgû and estimated the metamorphic conditions of each region, which belong to the Ediacaran-Cambrian collisional orogenic belt of the Lützow-Holm Complex, East Antarctica. Sample analysis of Chijire Rocks and Cape Ryûgû confirmed that metamorphic rocks in these areas were also subjected to Tonian metamorphism (ca. 984–969 Ma). Core ages of zircons from both areas are consistent with the protolith age of the metamorphic rocks in Akebono Rock. We could not date zircons older than 1100 Ma. The main metamorphism in Cape Ryûgû occurred around 546–545 Ma; however, Tonian metamorphism was preserved in the core and mantle, characterized by low Th/U ratios. This fact implied the eastward extension of Tonian metamorphism in Cape Ryûgû. The Chijire Rocks were metamorphosed under conditions of >7.5 kbar and 680–700 °C, where kyanite is stable. Cape Ryûgû is considered to have undergone low-pressure metamorphism (sillimanite stable) in pseudosection analysis. Differences in the protolith ages and metamorphic conditions of basement rocks were recognized between the Akebono Rock and Chijire Rocks (AKB–CHJ) in the northeast and the west Cape Hinode and Niban-nishi Rock in the southwest, centered around Cape Hinode. The Tonian metamorphism in the AKB-CHJ region, accompanied by dynamic recrystallization and relatively high-pressure conditions, probably resulted from the subduction of a hot, young oceanic crust and a hot mantle at that time. Basement rocks metamorphosed at the Tonian exposed over a distance of approximately 90 km from Niban Rocks to Cape Ryûgû on the Prince Olav Coast, and are thought to be composed of multiple subterranes with different protolith ages. We temporarily propose to classify this as the “Tonian Prince Olav terrane” within the Lützow-Holm Complex.