Opaque mineral analysis of the Amaga formation and potential source rocks and its relation to uplift of the Colombian Andes

The Amaga formation lies in the Cauca-Amaga Basin between the Western and Central Cordillera of the Colombian Andes. The basin experienced a sudden change in sediment type during the mid-Cenozoic. The change from the lighter Los Palomos member to the darker El Cinco member is due to the uplift and erosion of a different source rock introduced by the subduction and collision of a volcanic arc in the Miocene. Samples of the sediment and possible source rock surrounding the Amaga Basin were collected and the opaque heavy minerals were separated and analyzed with a JEOL JXA 8900M Electron Probe Microanalyzer for chemical composition. The composition of the opaque heavy minerals was compared between the sediment and potential source rocks collected. The results did not reveal any unique matches; however, it is possible to exclude certain rock bodies as potentially significant sources because of contrasts in opaque mineral compositions. Thus the following bodies were probably minor or negligible contributors to Amaga sediments: the Amaga stock, the Arquia complex, and the Diorite of Pueblito. Analysis of the sediment of El Cinco indicates the change in sediment is related to the collided volcanic arc terrane. Samples from the Cretaceous Barroso Formation pillow basalt, considered to be part of an accreted ophiolite complex, showed that the magma was high in Mg. Such accretionary oceanic terranes may be characterized by a higher content of distinctive minerals such as chromite. Chromite-bearing gabbros are known in these accreted terranes. However, no evidence of chromite was found in the opaques analyzed in the El Cinco member.