AGES AND PB-SR-ND ISOTOPES OF SILICATE ROCKS AT MOUNTAIN PASS, SOUTHERN CALIFORNIA

New SHRIMP-RG U-Pb zircon ages for nine samples of syenite, shonkinite, and leuco-granite from the Mountain Pass (MP) area yield overlapping ages ranging from 1422 to 1410 Ma (weighted mean = 1417.3 ± 4.6 Ma), suggesting their emplacement during one continuous magmatic episode. These ages are slightly overlapping yet notably older than two ⁴⁰Ar/³⁹Ar ages between 1403 and 1400 Ma (± 7 Ma) on arfvedsonite (syenite) and phlogopite (shonkinite), respectively. These age constraints suggest either an anomalously slow cooling rate of ~20 degC/m.y. for the MP silicate complex or the possibility of a second thermal (perhaps hydrothermal) event that affected these silicate rocks just prior to 1403 Ma.

With the exception of overgrowths on zircons from two samples, trace element patterns of zircon from these silicate rocks are normally magmatic with pronounced Ce and Eu anomalies. Ti-in-zircon thermometry indicates temperatures between 780 and 933 degC with an average trace-element closure temperature of 854 deg C.

Age-corrected (initial) Pb isotopic signatures for MP silicate rocks are in all cases slightly more enriched than the galena Pb compositions that describe a trend from model mantle values at ca. 1400 Ma (e.g. 6/4 = 16.05, 7/4 = 15.32, 8/4 = 35.5) to typical crustal values at ca. 1400 – 1550 Ma (6/4 = 16.3, 7/4 = 15.5), and appear to originate from either an enriched mantle or orogenic environment. They are also coincident with the least-radiogenic, initial Pb compositions for ca. 1700 – 1800-Ma Mojave and transitional zone basement rocks.

Sm-Nd data for MP silicate and carbonate rocks yield an ill-defined isochron age of 1458 ± 59 Ma that at least lends confidence for the stability of this isotopic system in these rocks. Initial whole-rock Sr and Nd signatures for the MP silicate and carbonate rocks range between 0.7045 and 0.7068, and -1.5 and -6, respectively. These signatures are similar to those for the Phalaborwa and Spitskof complexes of South Africa that indicate a hybrid mantle source and are notably more enriched than many other carbonatites worldwide, including Iron Hill, Jacupiranga, Bayan Obo, that typically exhibit initial Sr-Nd values between 0.702 and 0.7055, and -1.0 and +4, indicating more of a depleted mantle source.