PETROGENESIS OF THE MESOPROTEROZOIC MOUNTAIN PASS IGNEOUS SUITE, CALIFORNIA

The ~1.4 Ga ultrapotassic (UP) and carbonatitic igneous suite of Mountain Pass intrudes high-grade gneiss, amphibolite, and pegmatite at the southern end of the Clark Mountains, San Bernardino County, California. Stocks of UP rocks range from phlogopite melasyenite (or shonkinite) through amphibole-biotite syenite and leucosyenite to granite. The main mass of carbonatite (Sulfide Queen) is a tabular, moderately dipping plug-like intrusion and consists of a heterogeneous assemblage of bastnaesite-, barite-, parisite- and monazite-bearing calciocarbonatites and magnesiocarbonatites (Castor, 2008). Recent U-Th-Pb mineral geochronology (Poletti et al., 2016; Watts et al., 2021) implies protracted magmatism with a peak at 1415-1410 Ma (main volume of UP rocks) and subsequent, yet partly coeval emplacement of carbonatite at ~1380 Ma. Accessory magmatic phases bear evidence for contamination of the UP melts by Paleo-Mesoproterozoic crustal rocks (inheritance and heavy oxygen isotopes in zircon, unradiogenic Nd isotopes in titanite). Very high contents of incompatible elements in the UP and carbonatitic melts have, however, made certain elemental systems inert to the effects of crustal contamination. Whole-rock Sm-Nd and Rb-Sr isotope compositions, for example, offer a robust window to the mantle sources and magmatic evolution of the UP and carbonatitic melts. The UP rocks of Mountain Pass (our data; Poletti et al., 2016) have ε_Nd (1410 Ma) values of -4.2 ± 0.5 (1SD, n=10) and show no change across a very wide fractionation window (molar MgO/(MgO+FeOtot) of 0.77 to 0.18). The carbonatites (our data; Poletti et al., 2016; Verplack et al., 2016) have ε_Nd (1380 Ma) values of -3.1 ± 0.4 (1SD, n=9) and show no dependence on the lithologic variation observed (calciocarbonatite vs. magnesiocarbonatite). Initial ⁸⁷Sr/⁸⁶Sr ratios (our data; Verplanck et al., 2016) of the carbonatites (~0.705 to 0.706) are less radiogenic than those of the UP rocks (~0.708 to 0.709). The subtle, yet discernible difference in the Nd-Sr isotope composition of the UP and carbonatitic rocks of Mountain Pass implies compositional variation in the metasomatized subcontinental lithospheric mantle beneath Mojavia and that the alkali and carbonate melts were probably formed in separate melting events.