LAMPROPHYRE AND KIMBERLITE INTRUSIONS FROM THE GRASSRANGE FIELD, CENTRAL MONTANA

The Grassrange intrusion field of central Montana consists of more than thirty Eocene age dikes, plugs, diatremes, and sills scattered over an area of 42 x 27 km. The least-altered, primary igneous rock type in most intrusions is aillikite, a carbonate-bearing ultramafic lamprophyre. The aillikite consists of olivine (Fo 90-95) and phlogopite (TiO2 < 2 wt. %) phenocrysts or xenocrysts in a matrix of spinels (Fet/(Fet+Mg) > 0.85), carbonate (SrO up to 5.0 wt. %), chlorite, serpentine, apatite, ± perovskite, and ± amphibole. Recent discoveries (Ellsworth, 2000) have demonstrated that kimberlite and olivine monchiquite also exist at Grassrange. The Homestead Kimberlite contains abundant mantle nodules (e.g., harzburgite, lherzolite, and dunite) up to 75 cm in diameter and kimberlite indicator minerals (sub-calcic G10 pyrope, chromite, and Cr-diopside). A microdiamond was recovered from one Homestead Kimberlite outcrop grab sample (Ellsworth, 2000). Most exposures of the above rock types have been altered to yellow-orange, carbonate-rich rocks and now consist of calcite, dolomite, phlogopite, and secondary oxide minerals. Carbonate-free alnöite forms the Winnett sills of Grassrange.

Grassrange aillikites, kimberlites, and monchiquites have generally similar REE/Chondrite and spidergram patterns, with, for example, high La/Yb ratios (≥100) and negative K anomalies, respectively. The presence of ultramafic mantle nodules, kimberlite indicator minerals, and a microdiamond in the Homestead Kimberlite supports a deep-seated origin for the carbonate-bearing Grassrange magmas. Relative to these rocks, the alnöites have depleted incompatible element (IE) concentrations and distinct IE ratios, resulting from (1) a lack of igneous carbonate minerals, and (2) a magma source region different from that of the carbonate-bearing rocks. Carbonatized aillikites, kimberlites, and monchiquites of Grassrange exhibit significant enrichments in REE and other IE with increasing modal carbonates and apatite, reflecting the effects of late-stage metasomatic events.

Ellsworth, Peter C., 2000, Homestead Kimberlite: New discovery in central Montana: Guidebook, 25th Annual Tobacco Root Geol. Soc. Field Conf., Butte, MT, p. 14-20.