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Description
There is evidence to suggest that both mantle and crustal source regions played a significant role in the development of the Triassic-Jurassic granitoids of the Cordillera Real, the most eastern cordillera of the Bolivian Andes. On the basis of petrographical, chemical and oxygen isotope characteristics, eight plutons of the Cordillera Real were found to possess two significantly different signatures. Yani, Fabulosa and Zongo batholiths have S- (sedimentary) type characteristics whereas the Huato, Sorata, Huayna Potosi, Unduavi and Taquesi batholiths have I-(igneous) type characteristics but with crustal overprints. The S-type plutons are characterized by their altered appearance and their biotite-muscovite granite composition. Petrographically they are typified by their abundance of muscovite over biotite, ubiquitous apatite crystals, and their tourmaline and ilmenite content. Geochemically, the different major element oxides plot within scattered fields of binary variation diagrams. The rocks have high peraluminous indices ranging from 1.2 to 1.7. The S-type characteristics are further substantiated by their whole rock oxygen isotope compositions. δ180 for the Yani, Fabulosa and Zongo plutons range from 11.6 to 13.40/00 Quartz and biotite 180 analyses are consistent with whole rock determinations precluding the possibility of disequilibrium between whole rock and the analysed mineral separates, due to pre- or post- crystallization alteration of the plutons. The crustally over printed I-type plutons typically have a fresh appearance and are generally composed of +/- hornblende-biotite granodiorites and high plagioclase biotite granites. Petrographically, the Huato, Sorata, Huayna Potosi, Unduavi and Taquesi batholiths are characterized by their lack of primary muscovite and by their hornblende and magnetite content. Geochemically, the plutons are typified by the relatively tight field in which major element oxides plot in binary variation diagrams. The peraluminous indices of these rocks range from 1.0 to 1.3. Evidence for crustal contamination of the rocks is substantiated by their whole rock oxygen isotope compositions. δ180 values for these plutons range from 8.9 to 9.3 per mil. For an uncontaminated I- type granite, the δ180 values range from 5 to 6 per mil. From field observations, as well as the lack of associated volcanic equivalents, it appears that the "quasi" I-type granitoids were emplaced at depths greater than 5 km. The presence of sedimentary roof pendants on these granitoids, as well as the intimate association of mineralized quartz veins and pegmatites, suggest that presently exposed plutons are at relatively shallow erosional levels. It is thus possible that the high peraluminous indices of some of these rocks are due to interactions of the cooling carapace with the overlying sedimentary rocks. The tin content of both the S- and I-type rocks ranges from 5 to 30 ppm. These values are higher than those of normal granites in other parts of the world by a factor of 5 to 7. Analyses on mica separates suggest that most of the tin in these granitic rocks is incorporated into the lattice structure of both biotite and muscovite. From the textural differences between the S- and I-type rocks, it is postulated that the S-type granitoids are syntectonic relative to the "quasi" I-type granitoids. Emplacement of the I-types appears to have been such that these granitoids intruded the already crystallized S-type granitoids.
