Crustal evolution at the Central Andean continental margin: a geochemical record of crustal growth, recycling and destruction.

FRANZ, G.; LUCASSEN, F.,; KRAMER, W.; TRUMBULL, R.; ROMER, R.; WILKE, H.; WILKE, H.; VIRAMONTE, J. G.; BECCHIO, R.; SIEBEL, W.

The Andes from Top to Bottom : Structure and Processes of a Subduction Orogen

Springer

Lugar: Berlin; Año: 2006; p. 45 - 64

Active continental margins are considered as the principal site of growth, recycling and destruction of continental crust. The Andean continental margin is periodically active at least since Early Paleozoic and allows evaluating the long-term relevance of these processes. The principal constituent of the Cenozoic, uneroded orogen is Early Paleozoic, felsic metamorphic, magmatic and sedimentary crust inferred from surface distribution and chemical and isotopic composition of Paleozoic to Recent magmatic rocks. The Early Paleozoic crust recycled and homogenized ca. 2.0 Ga old Early Proterozoic crust of the Brazilian Shield, which already underwent an orogenic cycle at ca. 1 Ga. Crustal growth by accretion of exotic terranes or major juvenile additions from a mantle source (arc magmatism or island arc accretion) is ruled out by metamorphic and magmatic evolution and the isotopic signatures of the Early Paleozoic rocks. The dominant observed process in the Paleozoic was conservation of pre-existing crust. Destruction of continental crust by subduction erosion is prominent in sections of the present active margin and should have occurred in the past considering that the prerequisites favouring such process are not unusual. Global models of crustal growth indicate a near constant or slightly increasing mass of continental crust in the Phanerozoic, i.e. substantial losses should be compensated by juvenile magmatic additions. Voluminous juvenile magmatism is only observed in the Jurassic – lower Cretaceous extensional magmatic arc.  Compositions of mantle derived magmas and late Cretaceous mantle xenoliths indicate the presence of depleted mantle already beneath the Early Paleozoic orogen. The old subcontinental mantle was replaced by asthenospheric mantle in the subduction systems and compositional variations of radiogenic isotopes reflect metasomatism process, the subduction component dominated by input of the local continental and –after the cease of subduction - separation of the mantle wedge from the convective mantle