GEOCHEMICAL, GEOCHRONOLOGICAL, AND ISOTOPIC TRAITS OF A NEWLY DISCOVERED 1.36 GA LAYERED MAFIC INTRUSION IN THE WET MOUNTAINS, CO: IMPLICATIONS FOR MESOPROTEROZOIC TECTONICS

In the southwestern U.S., the mid-Mesoproterozoic (~1.4 Ga) is known for prolific ferroan granitoid magmatism, interpreted as a result of extension. Conversely, evidence exists for coeval regional metamorphism, shear zone reactivation, and folding, which are indicative of a convergent setting. Mid-Mesoproterozoic mafic intrusions of significant size, which may be indicators of ~1.4 Ga mantle conditions, are rare in the southwestern U.S. The few known examples include the 1.43 Ga Electra Lake gabbro in southwest Colorado and the 1.43 Ga Laramie anorthosite complex in southeast Wyoming. A newly discovered mafic-ultramafic layered complex in the Wet Mountains of southern Colorado is interpreted to be mostly buried beneath the 1362 Ma ferroan San Isabel batholith, based on airborne magnetic data from Earth MRI, ground gravity data, and preliminary geologic mapping of local exposure of mafic rocks. Based on LA-ICP-MS U-Pb zircon analyses from gabbro and orthopyroxenite samples, the interpreted crystallization age of the complex is 1361 ± 31 Ma, consistent with the presence of inclusions of San Isabel granite in the mafic rock in outcrop. Whole-rock geochemical data show LREE and LILE enrichment trends and negative primitive mantle-normalized Nb-Ta-Ti-Zr-Hf anomalies similar to the San Isabel granite and typical of melting of metasomatically enriched lithospheric mantle. Nd-Sr-Pb TIMS data have primitive/chondritic isotopic signatures (εNd₁₃₆₀ = -0.69– +0.49; ⁸⁷Sr/⁸⁶Sr_i = 0.70265–0.70361; model source μ = 9.38–9.62), which agree with previously published data from the San Isabel granite (εNd₁₃₆₀ = -1.1– -1.4; ⁸⁷Sr/⁸⁶Sr_i = 0.7031; μ = 9.53–9.54). These geochemical traits can be explained by melting of enriched lithospheric mantle or primitive asthenospheric mantle. Zircon trace element signatures are typical of both ocean island and continental arc magmas. Combined, the datasets indicate a genetic relationship between the layered complex and the overlying and slightly older San Isabel granite and melting of both asthenospheric and enriched lithospheric mantle. Melting of both mantle sources supports previous tectonic models for ~1.4 Ga tectonism in the southwestern U.S., including back-arc extension and/or delamination of an overthickened lithosphere inboard of a long-lived convergent margin to the southeast.