UPPER OLIGOCENE BACK ARC MAGMATISM AND THE ONSET OF THE PLATEAU UPLIFT IN THE BOLIVIAN ANDES

In Late Oligocene, two discontinous belts of magmatic rocks formed in the Bolivian Andes. One of these developed in the northern Altiplano, with younger prolongation to the southern Altiplano and southern Eastern Cordillera. This belt is composed by basalts and andesites (SiO2: 43-59%) of alkaline affinities intercalated and intruded in continental sediments. K–Ar and Ar–Ar ages of these rocks range between 27.8 and 22 Ma. The second belt, developed in the Eastern Cordillera, is mainly constituted by small granitoid plutons which intrude paleozoic marine rocks. Minor tuffs and lavas intercalated in syn-tectonic conglomerates are also part of this belt. All of these are high–K calkalkaline and shoshonitic, mainly of metaluminous character. Although there is a wide range of compositions (SiO2: 48-76%), intermediate rocks, granodiorites, dacites, and andesites, are clearly predominant. The available K–Ar and Ar–Ar ages vary between 28,4 and 22,8 Ma however some older ages were also reported. Rocks of both belts are rich in LILE and have variable contents of HFSE, as well as K2O/Na2O ratios. Both belts started to develop coevally to the onset of the uplift of the whole plateau, in a time when, apparently, there was no a true active volcanic arc in the continental margin, and when an increase in the subduction speed of the oceanic slab occurred. Trace elements compositions of these rocks suggest a within plate origin from a heterogeneous mantle lithosphere, variably affected by the long subduction process occurred during Mesozoic-Palaeogene and maybe other events of deep mantle fluids metasomatism. The proposed models favour the origin of this magmatism as a consequence of a change in the subduction angle of the oceanic plate which creates vigorous convection in the asthenospheric mantle. This convection can melts, and even delaminates, the base of the lithosphere giving origin to both magmatism and plateau uplift.