Precambrian and Early Paleozoic evolution of the Andean basement at Belen (northern Chile) and Cerro Uyarani (western Bolivia Altiplano)

Abstract

Exposures of metamorphic basement in the Central Andes are scarce and reconstructions of the history of the Pacific margin of Gondwanaland must rely on a few isolated outcrops. We studied two areas of exposed basement in northernmost Chile (Belen) and westernmost Bolivia (Cerro Uyarani). The Belen metamorphic complex has been known for some time and consists of fault-bounded amphibolites, gneisses, schists, and minor quartzites overlain by folded Mesozoic to Cenozoic strata. The Cerro Uyarani is the only basement outcrop on the Bolivian Altiplano and has only recently been found and studied by geological reconnaissance. It consists of foliated mafic and felsic granulites, charnockites, and amphibolites. How do these basement occurrences compare and how do they relate to the other Precambrian crustal domains in the Central Andes? To answer these questions, we used geothermobarometers to reconstruct the P-T conditions of metamorphism, as well as geochemical analyses and petrological methods to study these rocks. The two basement blocks were found to have distinct geological histories and are probably separated by a major crustal domain boundary. Isotopic fingerprinting by Pb-isotopes clearly exclude Laurentian crustal components either in the protoliths or as reworked material. This signature is quite distinct from basement rocks farther south in Chile and northwestern Argentina. (C) 2000 Elsevier Science Ltd. All rights reserved.