UNUSUAL MINERALS AND ASSOCIATIONS IN PRIMITIVE ULTRAPOTASSIC MICROSHONKINITE DIKES, MOUNTAIN PASS RARE EARTH DISTRICT, SOUTHERN CALIFORNIA

The Mountain Pass district is known for Ba- and LREE-rich carbonatite, a bizarre carbonate-sulfate igneous rock with phenocrysts of barite. Coeval (Poletti et al. 2016: J. Petrology) unusual silicate igneous rocks share some compositional features of the carbonatite. In particular, primitive (mg = 0.76) microshonkinite (MSH) dikes that represent the parental silicate magma of the district are highly enriched in F (1 %), Ba (8400 µg/g), LREE (2500 µg/g), and Th (140 µg/g). Major constituents of these fine- to medium-grained dike rocks are alkali feldspars, saginitic magnesian biotite, amphibole, and diopside. Accessory and minor minerals include apatite, titanite, magnetite, allanite, monazite, zircon, thorite, barite, and eudialyte(?). SEM and EMP analysis reveal several unusual compositions and associations.

Surprisingly, MSH contains two, apparently mutually stable, alkali feldspars: albite (Ab₉₉) and barian K feldspar (Or_90-94Ab_2-5Cs_0-6). We find no intermediate feldspars. Both form small anhedral grains; K feldspar also forms large (several mm) oikiocrysts enclosing most other major and minor minerals.

MSH amphibole is pale-blue to -greenish-blue, and compositionally peculiar, best described as Na-K actinolite, with wt. % Al₂O₃ 0.80, Na₂O 2.3, K₂O 0.70; mg = 0.75 (cf. Aoki et al. 1972: Am. Min.). Some actinolite rims diopside, but most forms discrete aggregates of subhedral, blocky to lath-like crystals that show no evidence of having replaced another mineral, and apparently are primary.

Blebs of thorite occur within zircon and as closely associated anhedral crystals. These textures presumably result from exsolution from an initially thorium-rich zircon.

Perhaps the most unexpected feature of MSH is ubiquitous small (<10 µm) irregular, blocky, or amoeboid crystals of barite, or barite-celestine. These occur as inclusions in at least six primary minerals: feldspar, mica, diopside, apatite, titanite, and magnetite. Presence of barite within these minerals appears to owe to mechanical inclusion, not co-crystallization. This apparently xenocrystic barite suggests to us that MSH magma interacted with barite-bearing carbonatite or carbonatite magma.

Some of these unusual minerals and associations must owe to the composition of MSH, others possibly to its connection with carbonatite.