GEOCHEMICAL AND ISOTOPIC CONSTRAINTS ON THE AGE AND ORIGIN OF THE ELK CREEK CARBONATITE COMPLEX, SE NEBRASKA

New whole rock and mineral chemical, isotopic, and geochronologic data from core samples of the Elk Creek carbonatite and related igneous rocks were obtained to better define the age and origin of this alkaline complex. Low precision laser ablation U-Pb age determinations from individual zircon in syenite intrusive bodies range from 480 +/- 20 Ma to 540+/- 14 Ma and confirm that the silicate magmatism in this complex is early Cambrian in age and ~contemporaneous with carbonatite emplacement. Laser ablation Hf isotopic data obtained from the zircon grains yield eHf (T) ranging from 0 to +3.4. These isotopic compositions preclude the possibility that the syenite parental magmas were derived solely from upwelling, asthenospheric mantle but instead likely require a component derived from a low Lu/Hf , Precambrian continental lithospheric mantle (CLM) source. Geochemical studies of the carbonatites are ongoing but preliminary studies of “apatite beforsite” suggest that in at least some of these rocks, light rare earth elements were introduced into early Mg carbonatite during injection of a later stage, higher Fe ,carbonatititic fluid. Brecciation and apatite precipitation accompanied the latter event. Physical microsamples of whole rock brecciated carbonatite yield low measured ⁸⁷Sr/⁸⁶Sr~0.7027, which require a mantle origin for the carbonatites. One possible explanation for both the syenite and carbonatite isotopic characteristics is that their parental magmas incorporated metasomatic components introduced to the CLM from the sublithospheric mantle during the formation of the Mid-Continent Rift System in this region at ~1.1Ga. These components were remobilized during lithospheric heating and or deformation of the CLM in the Cambrian, leading to the emplacement of the Elk Creek complex.