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

Abstract

The Cordillera Real granitoids are a suite of Triassic and Oligocene plutons in the core of the Eastern Cordillera of the Central Andes of Bolivia. Their tectonic setting, chemical and ore compositions make them part of the so-called “Inner Magmatic Arc”, which differs from the current “Magmatic Arc” located immediately to the west. U-Pb SHRIMP data were obtained in order to constrain crystallization ages. The Triassic group yielded the following results: 239±2 Ma for the Huato granite, 231±1 Ma for the Illampu granodiorite, 221±3 Ma for the Huayna Potosí granite and 223±2 Ma for the Taquesi granodiorite. For the Oligocene group we obtained ages of 27 Ma for two samples of the Quimsa Cruz granite. 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) anomalous data plot in the Concordia diagram by reorganization of U-Pb isotopic ratios. As noted in previous studies, most zircon analysed from the Zongo/Kuticucho Triassic granite exhibited extremely high U enrichment, producing reverse discordia curves that obscure the true crystallization age. Relatively abundant zircon inheritance was found in these “cold” granitoids, with ages suggesting provenance from early Paleozoic metapelites that also contained recycled older sources. This relatively abundant xenocrystic inheritance probably records the influence of the subduction process acting during 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).