The crystalline pre-Andine basement outcrops only in the Cordillera Darwin (Tierra del Fuego) in the extreme southern part of Andean belt. It is located between the Antarctic, Nazca and Scotia Plate. Cordillera Darwin breaks the long ophiolitic belt constituted by the Sarmiento Complex and Rocas Verdes; many authors agree that the present setting is the result of the Andean orogenesis, that yielded to the closure of a Jurassic marginal basin (now represented by ophiolites) with its overlying sequence of rhyolitic volcanic and volcanoclastic rocks, now represented by the Tobifera Formation and metamorphism during Cretaceous (Cunningham, 1995 and references). The studied rocks, sampled by A.D. in the Sarmiento Peninsula during the 1955 De Agostini expedition, are made of phyllitic quartzites and micaschists with interlayered amphibole-chlorite schists, granitic gneisses and mafic and acid dykes. Texturally two different groups may be distinguished: rocks with one and two schistosity surfaces respectively. Rocks with S2 are quartzites and phyllitic quartzites (metablack-shales and metacherts) and amphibolites with S1 recumbent folded into axial plane S2. The syn-late S1 metamorphism ranges from the greenschist facies to the garnet-staurolite paragenesis. Occurrence of a thermal event is shown by growing of large garnets with post-kynematic rim, micas with decussate texture, poykilitic staurolite, fibrous sillimanite and hornblende substitution of fibrous amphibole. All these minerals are pre-kynematic respect to S2. Based on trace elements geochemistry the amphibolite layers show features typical of island arc tholeiites. The whole S2 sequence is crossed by veins of metamorphosed (S1) acid intrusions and mafic dykes of MORB type. We confirm the presence of a pre-Andean metamorphic basement (with S1 developed during Late Palaeozoic-Lower Triassic, and thermal event during the Mesozoic rifting) and its unmetamorphosed terrigenous cover, all involved in the Cretaceous Andean orogenesis, producing the formation of recumbent folds of the cover on its basement and the formation of a S2 in the basement, a S1 in the Tobifera Formation, in the mafic dykes (related to Sarmiento and Rocas Verdes ophiolites) and in the granitoids rocks (related to Darwin Granite Suite). A part the absence of ophiolites the last Andean scenario is curiously very similar to the pre-Triassic basement and cover relation in the polymetamorphic pre-alpine Calabrian-Peloritan arc, South Italy.
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|Stato di pubblicazione||Published - 2008|