ISOTOPIC EVIDENCE ON THE AGE AND ORIGIN OF THE ELK CREEK CARBONATITE, SE NEBRASKA

Whole rock and mineral isotopic 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 grains separated from carbonate-rich “syenites” range from 480 +/- 20 Ma to 540+/- 14 Ma. Laser ablation Hf isotopic data from these same zircon grains yield e_Hf (T) ranging from 0 to +3.4. Whole rock samples show a wide range of measured Nd isotopic compositions, with light rare earth element enriched apatite and barite beforsites having the lowest e_Nd (0) values (-4.5 to -9.0) and middle and heavy rare earth element enriched magnetite beforsites having significantly higher values (+6.4). Taken together, the whole rock Sm-Nd data define an “errorchron” age of ~500 Ma with initial e_Nd value of ~1.7. Whole rock Rb-Sr data also align on an ~500 Ma errorchron, with an initial ⁸⁷Sr/⁸⁶Sr~0.7035. These data are all consistent with a Cambrian emplacement age for the various lithologies comprising the Elk Creek carbonatite complex. The low initial ⁸⁷Sr/⁸⁶Sr ratios suggest that the carbonatite parental melts were likely mantle derived, but the initial Nd and Hf isotopic compositions preclude the possibility that the melts were derived directly from the sublithospheric mantle. Instead, the melts likely originated in low Lu/Hf, Sm/Nd, and Rb/Sr Precambrian lithospheric mantle (CLM) beneath this portion of the mid-continent. Given the whole rock Nd and Hf model ages of ~1.1 Ga, it is possible that these melts were derived from portions of the mantle lithosphere that were carbonated during the formation of the 1.1 Ga Mid-Continent Rift. These components were then remobilized by heating of the CLM in the Cambrian and resulted in the emplacement of the Elk Creek carbonatite. However, the exact circumstances that led to heating of the lithospheric mantle in this region in the Cambrian remain enigmatic.