EVOLUTION OF THE MAGMATIC SYSTEM BENEATH THE SAN JUAN MOUNTAINS: COMPARING THE LAVA FLOWS OF THE CONEJOS AND HINSDALE FORMATIONS

The Eocene-Oligocene Conejos and Oligocene-Miocene Hinsdale formations consist of lava flows and volcanic deposits with similar areal distributions in the San Juan Mountains of southern Colorado and northern New Mexico. The Conejos is a series of intermediate-composition lavas, breccias, and volcaniclastic deposits interpreted as subduction-related, whereas the Hinsdale consists of mafic and silicic lava flows and is interpreted as volcanism associated with rifting. These formations present an excellent opportunity to evaluate the evolution of the magmatic system beneath the San Juan Mountains prior to the Oligocene ignimbrite eruptions of the San Juan volcanic locus and afterwards with the transition to early Rio Grande rift-related magmatism. We present new U-Pb LA-ICP-MS zircons dates from two Conejos samples and one Hinsdale sample. Ten zircon grains from one Conejos sample yielded a ²⁰⁶Pb/²³⁸U weighted mean age of 33.22 ± 0.24 Ma (2s, n=18) and another sample yielded an age of 33.01 ± 0.18 Ma (2s, n=31) from 13 zircons. Nineteen zircons from one Hinsdale sample yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 22.223 ± 0.043 Ma (2s, n=28).

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.