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

first_imgSubduction-related Mesozoic mafic dykes in eastern Palmer Land, Antarctic Peninsula, record the developmentof an intra-oceanic arc terrane, the Dyer Arc, probably of late Permian–Triassic age, represented by atholeiitic dyke group. Arc rocks were deformed in late Triassic–early Jurassic and mid-Cretaceous times onthe Gondwana margin. Eruption of syn- to post-mid-Cretaceous orogenesis magmatism is represented by acalc–alkaline group of dykes. The tholeiitic dykes intruded immature granitic crust of the Eastern Zonesub-terrane of the Central Domain, one of the magmatic terranes of the Pacific margin of Gondwana. Thecalc–alkaline dykes straddle the major tectonic boundary between the Central Domain and the continentalmargin 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 intoparallelism with the strike of the tectonic boundary during Cretaceous orogenesis whereas calc–alkalinedykes strike tightly ESE–WNW at a high angle to the boundary. The tholeiites pre-date late Triassic deformationand metamorphism, and are interpreted as dominantly Triassic in age, whereas Ar–Ar dated calc–alkalinedykes are younger (~100 Ma and ~97 Ma) and field relations indicate that they overlap with the waning phaseof the mid-Cretaceous Palmer Land Event. The tholeiites have trace element abundances similar to, but moredepleted than, those of modern intra-oceanic arcs, as recorded by Zr/Hf ratios. Nb/Yb versus TiO2*/Yb andSm/Yb versus La/Sm plots are used to model depths of partial melting. The tholeiites were mostly generatedat shallow depths corresponding to 2.5 and certainly less than 3.0 GPa in largely garnet-free mantle. Thecalc–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 beencaused by deformation of the intra-oceanic arc terrane on the Gondwana margin, and lithospheric thickeningduring the Palmer Land Event.last_img

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