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The Cenozoic calc-alkaline volcanism of the western Andes in Bolivia (17° to 23°S) defines an elongated Inner Volcanic Arc parallel to the Chile trench. This arc was initiated during the late Oligocene (ca. 23 Ma) with the eruption of the Rondal Lavas in the south Lipez region. During the upper Miocene–lower Pliocene, large volumes of ignimbrites and welded tuffs were erupted; these pyroclastic units underlie the Quaternary andesitic–dacitic stratovolcanoes outcropping along the Bolivia–Chile border.

Four main periods of uplift and gentle folding are recognized in the Inner Arc. These were caused by the development of the “Bolivian orocline,” which itself is a consequence of an A-type subduction (backthrusting) of the Nazca plate near or around the Arica Bend.

The Cenozoic volcanics of the western Andean Cordillera consist of mainly high-K calc-alkaline andesites, dacites, and rhyolites with high K2O/Na2O ratios (as much as 1.0). These data, combined with isotopic evidence, suggest that the parental magmas generated in the asthenospheric mantle underwent a combined assimilation–fractional crystallization process at crustal levels. Minor TiO2-rich alkaline suites (mugearites, alkali-basalts) are present in the northern Sajama region. These suites are linked to the counterclockwise rotation of the Arica lithospheric block.

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