AGE AND ORIGIN OF PYROXENITES FROM THE IRON HILL CARBONATITE COMPLEX, GUNNISON, COLORADO: EVIDENCE FROM PB-SR-ND ISOTOPES

The Iron Hill carbonatite complex (IHCC) crops out near the town of Powderhorn, south of Gunnison, Colorado. Primary rock types include from oldest to youngest: pyroxenite, uncompahgrite, ijolite, nepheline syenite, and carbonatite. Zones of fenite were formed by the interaction of fluids from the alkalic intrusive rocks with the Paleoproterozoic granitic basement rocks. Mixed rock assemblages were formed during the emplacement of the carbonatite into pyroxenite and uncompahgrite. Among fifteen drill core samples, six pyroxenites (SiO₂ = 23 - 36 wt. percent) and one carbonatite were analyzed for their Pb-Sr-Nd isotopic systematics to characterize their source(s). The carbonatite and two mixed-rock samples were leached in order to evaluate any potential isotopic heterogeneity between silicate (residue) and carbonate (leachate) components.

Five of six pyroxenite (previously dated at ~570 Ma by mainly K-Ar and Rb-Sr methodology), one mixed rock residue, and two leachates (N = 8) define a ²⁰⁶Pb/²⁰⁴Pb - ²⁰⁷Pb/²⁰⁴Pb whole-rock (WR) isochron age of 603 ± 21 Ma (95% CL; MSWD = 1.18). The carbonatite residue lies just off this isochron. Most of the same pyroxenite separates (from the same dissolution as the Pb results) produced a Rb-Sr isochron age of 617 ± 35 Ma (95% CL; MSWD = 3; N = 4) and an initial ⁸⁷Sr/⁸⁶Sr value of 0.70343 ± 0.00010 (2-sigma). Three of five fenite samples lie on or near this isochron. These same age results were poorly defined using the Sm-Nd isotopes (~550 - 655 Ma), again on the same separates.

Initial Sr-Nd isotopic signatures for all pyroxenite, mixed rock (both residues and leachates), and carbonatite residue, that the isochron age data indicate have not been significantly disturbed since their formation, describe a discrete field between ⁸⁷Sr/⁸⁶Sr = 0.7026 to 0.7038 and epsilon Nd = +1.4 to +3.1 that are indicative of a slightly contaminated mantle source at ca. 600 Ma (enriched relative to MORB with model Sr and Nd values of ~0.7024 and +6.8). The carbonatite residue is the most enriched of the IHCC samples with Sr-Nd of 0.7038 and +1.4, respectively. All fenite samples and the carbonatite leachate plot between the IHCC field and known Sr-Nd values (at 600 Ma) for Paleoproterozoic basement rocks in the area, indicating the incorporation of older crust into the sources for the carbonate components of the IHCC.