Crustal thickening along the West Antarctic Gondwana margin during mid-Cretaceous deformation of the Triassic intra-oceanic Dyer Arc

Subduction-related Mesozoic mafic dykes in eastern Palmer Land, Antarctic Peninsula, record the development of an intra-oceanic arc terrane, the Dyer Arc, probably of late Permian–Triassic age, represented by a tholeiitic dyke group. Arc rocks were deformed in late Triassic–early Jurassic and mid-Cretaceous times on the Gondwana margin. Eruption of syn- to post-mid-Cretaceous orogenesis magmatism is represented by a calc–alkaline group of dykes. The tholeiitic dykes intruded immature granitic crust of the Eastern Zone sub-terrane of the Central Domain, one of the magmatic terranes of the Pacific margin of Gondwana. The calc–alkaline dykes straddle the major tectonic boundary between the Central Domain and the continental margin Eastern Domain. 132 dykes were studied, 82 of which are calc–alkaline, 48 tholeiitic and two shoshonitic, across an area of approximately 4000 km2. Tholeiitic dykes strike broadly NNW–SSE, transposed into parallelism with the strike of the tectonic boundary during Cretaceous orogenesis whereas calc–alkaline dykes strike tightly ESE–WNW at a high angle to the boundary. The tholeiites pre-date late Triassic deformation and metamorphism, and are interpreted as dominantly Triassic in age, whereas Ar–Ar dated calc–alkaline dykes are younger (~100 Ma and ~97 Ma) and field relations indicate that they overlap with the waning phase of the mid-Cretaceous Palmer Land Event. The tholeiites have trace element abundances similar to, but more depleted than, those of modern intra-oceanic arcs, as recorded by Zr/Hf ratios. Nb/Yb versus TiO2*/Yb and Sm/Yb versus La/Sm plots are used to model depths of partial melting. The tholeiites were mostly generated at shallow depths corresponding to 2.5 and certainly less than 3.0 GPa in largely garnet-free mantle. The calc–alkaline magmas were generated at pressures greater than 3.0 to possibly >3.5 GPa, in the garnet zone. The deepening of the mantle source, and onset of calc–alkaline magmatism are interpreted to have been caused by deformation of the intra-oceanic arc terrane on the Gondwana margin, and lithospheric thickening during the Palmer Land Event.

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