1988 Volume 38 Issue 208 Pages 121-130
Mesozoic-Cenozoic granitoids in Eastern Cordillera of the Bolivian Andes have been studied in terms of modal composition, major-element chemistry and magnetic properties. The Mesozoic granitoids found exclusively in the northern area of Eastern Cordillera belong to the ilmenite-series. Almost all the Cenozoic granitoids found in the central and southern areas belong to the ilmenite-series as well, with a few exceptions of those of the magnetiteseries.
The variation trends in chemistry of the Mesozoic granitoids are similar to those of the Cenozoic ones except K2O. However, the Mesozoic granitoids tend to be enriched in SiO2 and are characterized by lower K2O/Na2O ratio than that of the Cenozoic granitoids. Compared with the variation trends of the Japanese granitoids, the granitoids from Eastern Cordillera are rich in K2O and poor in AL2O3, MgO, CaO and total Fe as Fe2O3. Both the Mesozoic and Cenozoic granitoids in Bolivia belong to the I-type of CHAPPELL and WHITE (1974).
It is recognized that there are more than three hundred polymetallic hydrothermal ore deposits in Eastern Cordillera of the Bolivian Andes. They are considered to be genetically related to the Mesozoic and Cenozoic granitoids: That is, mesothermal and hypothermal tin-tungsten veins were formed by postmagmatic activities of the Mesozoic granitoids, whereas xenothermal polymetallic veins were derived from the Cenozoic acidic magma.
