SHRIMP chronology of the Magallanes Basin basement, Tierra del Fuego: Cambrian plutonism and Permian high-grade metamorphism.

Abstract

Five new SHRIMP U-Pb zircon ages are reported for gneisses and foliated plutonic rocks belonging to the Tierra del Fuego igneous and metamorphic basement complex (TFIMC), obtained from the bottom of borehole cores through the Magallanes Basin. Three ofthe samples yielded weightedmean 206Pbf23SU ages (523±7 Ma, 522±6 Ma and 538±6 Ma), interpreted as indicating Early Cambrian igneous crystallization ofthe host rocks. A migmatitic gneiss shows peaks at ca. 950-1,100 Ma and 560-650 Ma from inherited zircon grains in addition to two grains with ages of ca. 525 M a, suggesting involvement of Grenvillian and Brasiliano material in the protolith of a Carnbrian migrnatite. A cordierite-sillimanite-gamet gneiss contains igneous zircons ofCambrian age anda population ofU-rich metamorphic Permian zircons, indicating that a Permian high-grade metamorphic and anatectic (P=2-3 kbar, T=730-770°C) event affected the Cambrian igneous rocks or sedimentary rocks derived from them. Cambrian/Ediacaran plutonic rocks are known from the basement ofNW Argentina, the Sierra de la Ventana, the Cape Fold Belt in South Africa, and the Ross Orogen inAntarctica. The Permian metamorphic event is coeval with the deformation and low-grade metamorphism of the sedimentary successions that overlie the basement in many ofthese areas. In Tierra del Fuego at least 8 to 12 km of cover rocks were removed following the high-grade Permian metamorphic episode and the unconformable deposition of the Tobífera Formation volcanic rocks in the Middle to Late Jurassic. This eroded cover could have been an important so urce of detritus for the conglomera tic Permian and Triassic? successions of neighbouring regions in South Arnerica, Africa and Antarctica.