Magmatism in the Huarina belt, Bolivia, and its geotectonic implications

Abstract

The Huarina belt is a tectono-stratigraphic unit extending along the western side of the Eastern Cordillera and southern Altiplano. It includes the well-known Bolivian tin belt. Both geophysical and isotopic evidence suggest that the Huarina belt is located between three cratonic blocks in a major lithospheric weakness zone that has probably been inherited from the Proterozoic. Most of the Phanerozoic backarc magmatism in the Bolivian Andes occurred along this belt. Local Ordovician basalts and andesites, and clasts of Devonian granitoids occurring in Late Devonian–Early Carboniferous sedimentary units, are testimonies of a scarce Palaeozoic igneous activity. During the Mesozoic, rift-related magmatism resulted in peraluminous to metaluminous, high-K calcalkaline Triassic granitoids, as well as discrete Cretaceous centers of alkaline and carbonatitic rocks. In the Cenozoic, huge high-K, generally peraluminous ignimbrite sheets, along with numerous volcanic and intrusive centers, were generated along the belt. Shoshonitic, alkaline and ultrapotassic rocks have been locally described. On the basis of both geochemical and geophysical evidence, a general evolution model is proposed for the Huarina belt. During Paleozoic and Mesozoic times, the predominant extensional regime of the continental crust led to lithospheric thinning, basin generation, and production of within plate magmas. During the Cenozoic, prevailing compressional conditions led to a gradual thickening of the lithosphere, inducing delamination of its denser and less viscous root. The mantle-generated magmas interplayed widely with crustal melts, giving rise to the dominantly peraluminous magmas.