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Abstract

Shonkinite, syenite, and granite at Mountain Pass, California, differ from typical alkalic rocks associated with carbonatites in other areas by being saturated to oversaturated in silica rather than undersaturated, displaying strong light rare-earth element (LREE) enrichment, and by being enriched in K rather than in Na (by later fenitization?). New major and REE data suggest that the shonkinite and syenite are probably comagmatic and related by crystal fractionation. The granite, however, is spatially, but not genetically related to either the syenite or shonkinite.

Shonkinite is characterized by high total REE abundances (REE = 715-1214 ppm) and high Ce/YbN (22–37). Syenite has similar REE contents (REE = 900 ppm, Ce/YbN = 24), but has lower CaO, P2O5, MgO, TiO2, Ba, and Cr, and higher Al2O3, FeO, Hf, Zr, and Ta concentrations than shonkinite. This elemental distribution can be explained by mineralogic differences due to crystal fractionation. Granite is lower in total REE (REE = 397–488 ppm), but shows greater LREE enrichment (Ce/YbN = 37–50). The lack of a Euanomaly suggests that feldspar fractionation was not important in the formation of the granites. The following petrogenetic model is proposed: (1) generation of shonkinite liquid by 1% partial melting of a REE enriched mantle (garnet peridotite), (2) 20% crystal fractionation of the shonkinite to produce syenite, and (3) generation of an alkali-granite liquid by the partial melting of garnet pyroxenite in the upper mantle.

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