GEOCHEMICAL AND PETROLOGIC ANALYSIS OF A NEWLY DISCOVERED, LAYERED MAFIC-ULTRAMAFIC COMPLEX IN THE WET MOUNTAINS, COLORADO

The Wet Mountains are a northwest-trending exposure of Proterozoic metamorphic rocks and foliated granites intruded by Ediacaran-Cambrian bimodal alkaline complexes. New USGS geophysical surveys define an elliptical magnetic and gravity high in the southern Wet Mountains, spatially coincident with local mafic outcrops that are adjacent to the 1.36 Ga ferroan San Isabel granite. The geophysical anomalies are interpreted to represent a subsurface mafic complex, similar to mafic portions of the alkaline complexes to the north. Petrographic and whole-rock major and trace element analyses were carried out on nine samples from the mafic outcrops to describe and characterize the intrusion, and to determine its relationship with other intrusions in the area. Hand sample, microscopic, and automated mineralogy analyses define three igneous rock types (orthopyroxenite, norite, and anorthosite) and a hornblende hornfels peripheral to the San Isabel granite. Most samples have a cumulate texture with plagioclase and orthopyroxene, the latter partly replaced by actinolite, as the main cumulate phases. Intercumulus phases include actinolite replacing orthopyroxene and Fe-Ti oxides with minor biotite, quartz, and apatite. On a TAS major element diagram, the samples plot near the alkaline-subalkaline boundary, with the presence of orthopyroxene indicating a subalkaline composition. On a chondrite-normalized REE plot, samples show LREE enrichment with (La/Lu)_N of 3.2-11.6 and ∑REE of 45.9-315.9 ppm. Most samples except the orthopyroxenite and hornfels show positive Eu anomalies, with (Eu/Eu*)_N = 0.8-1.6. Based on these results and the variety of cumulate rock compositions, the geophysical anomalies are interpreted as a largely buried, layered, subalkaline mafic-ultramafic intrusion. A primitive mantle-normalized trace element plot reveals positive Pb and U and negative Nb, Ta, Ti, Zr, and Hf anomalies in most samples. This could suggest an arc affinity or could reflect interaction of mantle-derived magmas with continental lithosphere previously modified by subduction. Future geochronology is necessary to determine if the intrusion is equivalent to the ca. 551-511 Ma alkaline layered mafic intrusions or related to the San Isabel granite, which is also at the subalkaline boundary and shows LREE enrichment.