Volcanic rocks of the Rio Cauca Valley, Colombia, S. A
Jaramillo, Jose Maria
Rogers, John J. W.
Master of Arts
Miocene volcanic and related intrusive rocks have been investigated in the Rio Cauca valley of western Colombia. This valley is formed along the Romeral and related fault zones during tectonic adjustments between the continental plate east of the Romeral fault and dominantly oceanic rocks in the Cordillera Occidental. The Combia Formation is predominantly a sequence of volcanic and pyroclastic rocks formed in a graben-like depression along the Rio Cauca valley south of Medellin. Flows of the Combia Formation are generally tholeiitic according to most criteria, as are basic dikes that cut the formation. Other shallow intrusive rocks, however, are mostly calcalkaline by the same criteria, and Plio-Pleistocene flows in the adjoining Cordillera Central show ideal calcalkaline trends. One dike cutting the Combia Formation shows alkaline tendencies. The close association in time and space of these diverse rocks in the Rio Cauca Valley implies ease of access to volcanic source regions. Furthermore, the crust beneath the Cauca Depression is inferred to be mostly continental on the basis of the diverse chemistry of the associated suites. Continental crust presumably extends to the western edge of the Cauca Depression, where it adjoins a dominantly oceanic crust of the Cordillera Occidental.