MAGMA SOURCE DETERMINATION ON COLOMBIAN IGNEOUS ROCKS

Amphiboles and pyroxenes from plutonic gabbroic rocks located on both flanks of the Eastern Andes of Colombia have been analyzed using synchrotron micro-X-ray fluorescence analysis. In order to understand the processes leading to melting and crystallization of the magmatic rocks the concentrations of REE, Nb, Zr, Ba and Y were determined on these minerals.

The normalization of the trace-element concentrations to chondritic values shows that the pyroxenes as well as the amphiboles present nearly consistently a negative anomaly for K and a positive one for Ba and La. Only the samples from one locality at the eastern flank, exhibit a slightly different pattern with a well defined negative anomaly for Zr and Ti.

Despite of the similarities of the normalized values, the characteristics of the magma source can be well distinguished between the samples coming from the eastern and western flanks of the Cordillera. The Zr/Y vs. Nb/Y diagram suggests two different sources of magma, an enriched one in the east and a depleted one in the west. These ratios are insensitive to alteration, fractional crystallization, and variable degrees of partial melting. Moreover, the Ba vs. Ba/Nb diagram for the pyroxenes shows again a difference between east and west. Based on these data we suggest an evolving magma source from east to west that records contributions from the rifting system present during the Cretaceous (120 - 70 Ma) in the northern part of South America.

The REE and trace element contents of the first crystallized phases reflect very accurately the chemical trend observed on the whole rocks. Our results document that the approach used here is a feasible alternative for investigating the magmatic origin of strongly altered magmatic rocks as long as relicts are present that reflect the original composition of the rocks.