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

Exposures of metamorphic basement in the Central Andes are scarce and recon structions of the history of the Pacific margin of Gondwanaland must rely o n a few isolated outcrops. We studied two areas of exposed basement in nort hernmost Chile (Belen) and westernmost Bolivia (Cerro Uyarani). The Belen m etamorphic complex has been known for some time and consists of fault-bound ed amphibolites, gneisses, schists, and minor quartzites overlain by folded Mesozoic to Cenozoic strata. The Cerro Uyarani is the only basement outcro p on the Bolivian Altiplano and has only recently been found and studied by geological reconnaissance. It consists of foliated mafic and felsic granul ites, charnockites, and amphibolites. How do these basement occurrences com pare and how do they relate to the other Precambrian crustal domains in the Central Andes? To answer these questions, we used geothermobarometers to r econstruct the P-T conditions of metamorphism, as well as geochemical analy ses 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 Scienc e Ltd. All rights reserved.