Geochronology of the Real Cordillera, the inner magmatic arc of Bolivia

Abstract

The Real Cordillera granitoids are a suite of Triassic and Oligocene plutons located at the core of the Eastern Cordillera of the Central Andes of Bolivia. Its geotectonical setting, chemical and ore composition make them part of the so called “Inner Magmatic Arc” which differs from the actual “Magmatic Arc” located immediately to the west. U-Pb SHRIMP ages were obtained in order to constrain their crystallization ages. The Triassic group yielded the following results: 240 ± 2 Ma for the Huato granite, 230.7 ± 1.3 Ma for the Illampu granodiorite, 222.2 ± 2.4 Ma for the Huayna Potosí granite and 221.9 ± 1.5 Ma for the Taquesi granodiorite. For the Oligocene group we obtained two ages of 26.87 ± 0.26 and 26.88 ± 0.21 Ma both for the Quimsa Cruz granite. Mafic enclaves from the Illampu and Taquesi granodiorites report ages that were older than their respective granitoid hosts, yielding 234.1 ± 1.3 Ma and 227 ± 1.3 Ma, respectively. Secondary processes related to regional thermal anomalies and magmatic melt-enrichment, reset the K/Ar and U/Pb isotopic systems, producing: a) younger ages by Ar loss and b) older ages by U/Pb isotopic ratios reorganization. As noted in previous studies, the Zongo/Kuticucho Triassic granite yielded extremely high U enrichment in most zircon analysed, producing reset of U/Pb ratios, wide span in age ranges and reverse discordia curves that obscure its actual crystallization age. Relatively abundant zircon inheritance was found in these “cold” and inheritance-rich granitoids, with ages suggesting provenance from early Paleozoic metapelites that also recycled older sources. This relatively abundant xenocrystic inheritance records the influence of the Gondwanide orogeny (336-205 Ma) as an overall subduction arc environment, punctuated at its final stage with the imprint of a continental rifting (245-220 Ma).