Abstract
The Crucero Basin, located north of Lake Titicaca (70°W, 14°20's), was probably connected with the Altiplano endoreic basin since Oligocene time. During the Oligocenne and Miocene, this basin was filled by nearly 1000 meters of fanglomerate and lacustrine sediments —e.g., the Cayconi Formation, which unconformably overlies upper Paleozoic and Cretaceous rocks and is unconformably overlain by fanglomerates and palustrine sediments of Pliocene age. Basic and silicic volcanic rocks are interbedded and associated with the Cayconi Formation. Major element analyses indicate intracrustal melting for the silicic volcanic rocks, whereas the basic volcanic seem to be mantle derived. K-Ar dating of these volcanic rocks yielded ages ranging from 25.9 Ma to 15.5 Ma. Thus, in the Cordillera Oriental, magmatism is at least as old as late Oligocene.
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