Volcanism And Structure Of Southwestern Bolivia

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Journal Article: Volcanism And Structure Of Southwestern Bolivia

Cenozoic volcanic activity started in southwestern Bolivia during the Oligocene. From the Early Miocene up to the Holocene, volcanism produced about 8000 km3 of lavas and pyroclastic rocks. Five major cycles could be distinguished. North-south-trending fissures in the eastern part of southwestern Bolivia were sources of Miocene ignimbrites. The eruptive centers of Pliocene ignimbrites are scattered throughout the investigated area, but are concentrated in the western part. During Pleistocene times small ignimbrite flows were formed by collapse of composite volcanoes. The oldest lavas (Oligocene) are alkaline and differ considerably from younger rocks, which are rhyodacites and dacites with only a small limited range of compositions. Based on the K2O/Na2O ratios and the mineralogical composition, the lavas can be subdivided into a calc-alkaline and a high-K calc-alkaline (shoshonitic) association. Hornblende is absent from lavas with high K2O/Na2O ratios. In southwestern Bolivia a westward migration of volcanic activity is apparent. The potassium content of the lavas decreases from the Miocene to the Holocene, whereas the sodium content increases. The potassium content cannot, therefore, be correlated with the depth of the Benioff zone. It is suggested, that the lavas and ignimbrites were formed by partial melting of material from different crustal levels.

S. Kussmaul, P. K. Hormann, E. Ploskonka and T. Subieta

Published Journal 
Journal of Volcanology and Geothermal Research, 1977

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S. Kussmaul,P. K. Hormann,E. Ploskonka,T. Subieta. 1977. Volcanism And Structure Of Southwestern Bolivia. Journal of Volcanology and Geothermal Research. (!) .