GEOCHEMICAL STUDIES OF EDIACARAN-ORDOVICIAN PLUTONIC ROCKS IN SOUTHERN COLORADO POSSIBLY RELATED TO THE SOUTHERN OKLAHOMA AULACOGEN

Ediacaran-Ordovician intraplate plutonic rocks and dikes along a NW trend in southern Colorado possibly formed in a continuation of a major rift defined by the Ediacaran-Cambrian Southern Oklahoma Aulacogen (SOA). The plutonic rocks include the Powderhorn Complex near Gunnison, as well as the Gem Park, McClure Mountain and Democrat Creek complexes in the Wet Mountains to the east.

Nepheline (nph) syenite samples from the Powderhorn Complex are separated by a wide compositional gap from carbonatite in the center of the complex, which shows strong LREE enrichment. Carbonatite samples show significant variations in MgO and P₂O₅, suggesting that this unit is a composite intrusion. The McClure Mountain complex (MMC) contains layered mafic-ultramafic rocks intruded by syenite, nph syenite and ijolite. Olivine gabbro layers in the mafic-ultramafic unit have high Ni and Cr, consistent with a cumulate origin. Diopside pyroxenite intruding the layered rocks has much higher Ni and Cr, suggesting that the pyroxenite magma formed at a different level before injection into the host rocks. Five nph syenites show a wide range in SiO₂ contents but no obvious trends in other elements, indicating these rocks do not represent magmas derived from a single parent. Three of these samples from an area < 40 m across show marked differences in major and trace elements between samples and likely formed from discrete small magma batches injected into the growing pluton. Syenite shows positive Eu and Ba anomalies, suggesting it records feldspar accumulation during fractionation processes.

Diopside pyroxenite from the layered mafic-ultramafic Gem Park Complex has MgO and CaO comparable to the MCC layered rocks but has much higher contents of TiO₂, FeOT, and Ni, suggesting the two intrusive bodies may be unrelated. The Democrat Creek Complex contains tholeiitic gabbro but is dominated by quartz syenite with highly fractionated trace element patterns, including a strongly negative Eu anomaly. Except for the Democrat Creek Complex, the Colorado plutons are compositionally distinct from rhyolites, granites, and tholeiitic mafic-ultramafic rocks in the SOA and must have a different petrogenetic history. Isotopic dating is in progress to determine possible age relations between the intraplate igneous rocks in the two areas.