The Antarctic Peninsula and the southern Andean continental margin have been the sites of extensive and varied magmatic activity since at least late Palaeozoic time. The dominant activity has been of calc-alkaline character and, although andesite and tonalite are the main rock types, there are well-marked transverse variations in the major and trace element chemistry of the magmas erupted in both areas. Thus, calc-alkaline magmas erupted farthest from the ancestral trench are generally more silicic, potassic and light-REE enriched than magmas erupted closer to the trench. In both regions, the locus of calc-alkaline activity has tended to move trenchwards since the Palaeozoic, although activity began to wane during the mid-Cenozoic when subduction of the Pacific Plate slowed down and ceased. A northward cessation of calc-alkaline magmatism along the Antarctic Peninsula can be linked in broad terms to the progressive northwards subduction of sections of the Aluk Ridge during the Cenozoic. Extensional tectonics within and behind the main igneous belts caused the development of narrow back-arc basins in southern Chile during the Mesozoic and in Bransfield Strait along the northern Antarctic Peninsula during Recent times. Tholeiitic magmas with transitional calc-alkaline characteristics were erupted in these basins. In addition, alkalic basalts and related undersaturated magma types were erupted onto the stable continental platform well behind the main calc-alkaline and marginal basin magmatic belts during Cenozoic and Recent times, probably in response to extensional tectonics as subduction slowed or ceased.

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