40Ar/39Ar ages and geochemistry of the Intersalar Range of the Bolivian Altiplano: A volcanological transect spanning the arc and reararc of the Central Andean Plateau

Morgan J. Salisbury, Néstor Jiménez, Dan Barfod

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Abstract

The volcanic Intersalar Range of western Bolivia provides a unique opportunity to examine geochemical variations spanning the arc and reararc regions of the Central Andean Plateau. In this study we report 23 new 40Ar/39Ar ages, 15 whole-rock Sr-Nd-Pb isotope analyses, and 50 whole-rock major and trace element analyses from samples collected across ∼115 km of the Intersalar Range. Most samples are classified as trachyandesites and trachydacites, with the most mafic lavas (slightly alkaline, basaltic trachyandesites) erupting from the Pliocene Coracora volcano in the central Altiplano. We identify two distinct pulses of reararc magmatism: a Miocene phase between 20 Ma and 14 Ma that corresponds with local compressional shortening, and a Plio-Pleistocene phase between 5 and 1 Ma that postdates observed structural deformation in the region. 87Sr/86Sr values (0.70512–0.70600) and 143Nd/144Nd values (0.51226–0.51255) are generally higher, and lower, respectively, in the younger phase, whereas Pb isotopes (206Pb/204Pb = 17.7315–18.5095; 207Pb/204Pb = 15.5714–15.6279; 208Pb/204Pb = 37.7862–38.6156) show little variation with age. Isotope values are only loosely correlated with distance from the modern Central Volcanic Zone. Higher Sr/Y, Dy/Yb, and [La/Yb]N values in the Plio-Pleistocene samples are consistent with homogenization at the base of a thicker continental crust compared to the Miocene samples. Nb concentrations show the strongest correlation with distance into the reararc compared to all other trace elements (arc Nb = 6–16 ppm; reararc Nb = 12–26 ppm). Nb/Nb* values (a measurement of the depth of the negative Nb anomaly) correspondingly increase into the reararc (indicating smaller anomalies), reaching a maximum at Coracora volcano before decreasing in the far rear arc region. Compiled data across the Central Andean Plateau reveal a strong correlation between Nb/Nb* and the presence of intact mantle lithosphere beneath the central Altiplano. We interpret this distinct Nb signal to reflect melting triggered by the breakdown of Nb-rich hydrous minerals within foundering (delaminating) mantle lithosphere. In conjunction with spatiotemporal data, Nb systematics provide the clearest indication of mantle lithosphere in regions where mafic samples are not present.

Original languageEnglish
Article number917488
JournalFrontiers in Earth Science
Volume10
DOIs
StatePublished - 3 Oct 2022

Bibliographical note

Funding Information:
This research was performed by MS as part of the International Junior Research Fellowship program at Durham University. The Department of Earth Sciences at Durham University provided additional funding for fieldwork, thin section preparation, and isotope analyses. Funding for 40Ar/39Ar age determinations was provided by the Natural Environmental Research Council (NERC). The Instituto de Investigaciones Geológicas y del Medio Ambiente at the Universidad Mayor de San Andrés provided vehicles for fieldwork.

Funding Information:
MS Principle Investigator. Developed the project in collaboration with NJ and Jon P. Davidson. Wrote the grants for funding, led the field investigations, performed the majority of sample preparation, coordinated all analytical analyses, interpreted the results, primary author of manuscript. NJ Collaborated with the overall development of the project. Coordinated field excursions. Participated in discussions regarding results and interpretations and contributed input and text for the manuscript. DB led the investigations of the Ar/Ar age data. Wrote sections of the manuscript regarding the Ar-Ar data. Reviewed manuscript and contributed to interpretations and conclusions of the study. 40 39

Publisher Copyright:
Copyright © 2022 Salisbury, Jiménez and Barfod.

Keywords

  • 40Ar/39Ar age dating
  • Bolivia Altiplano
  • Central Andes
  • volcanism
  • whole-rock and isotope geochemistry

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