EVOLUTION OF THE MAGMATIC SYSTEM BENEATH THE SAN JUAN MOUNTAINS: COMPARING THE LAVA FLOWS OF THE CONEJOS AND HINSDALE FORMATIONS
Additionally, we use whole rock major and trace element geochemistry to characterize these two formations and track the evolution of the transitional magmatic system. The Conejos rocks have higher abundances of the incompatible LILE, HFSE and HREE, whereas the Hinsdale rocks have higher abundances of the compatible elements and LREE. This suggests varying degrees of crustal contamination and different primary magmatic compositions. The Conejos samples show a constant Ni/Rb with increasing silica. The Hinsdale samples show a decreasing Ni/Rb with increasing silica due to strong depletion of Ni from the liquid and the opposite behavior of Rb as it crystallizes out later. Ni/Rb ratios increase during evolution of the San Juan Mountains but decrease in the Hinsdale Formation as the path of fractionation removes Ni relative to Rb. The Conejos samples show a decreasing La/Yb with increasing silica. The shift from intermediate Conejos rocks to bimodal Hinsdale rocks is shown by incompatible element analyses that support earlier models of crustal contamination of subduction-related Conejos and mantle sources for the younger rift-related Hinsdale.