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Abstract

The age of the Mountain Pass carbonatite, the largest source of rare-earth elements in the world, is an important constraint in the search for similar bodies. Emplacement of the carbonatite was the last event in the intrusion of a 1400-Ma alkalic complex. Based on field relationships, the intrusive sequence was 1) shonkinite, 2) syenite and granite, and 3) carbonate bodies, dikes, and veins. Late shonkinite dikes cut syenite and granite, but not the carbonate bodies.

Alkalic igneous rocks were emplaced between 1410 and 1400 Ma. Apatite from the shonkinite has a U-Pb date of 1410±2 Ma. Phlogopite from the shonkinite has a 40Ar/39Ar plateau date of 1400±8 Ma. Arfvedsonite from the syenite has a 40Ar/39Ar plateau date of 1403±7 Ma. Zircon from the syenite are highly discordant and have Pb-Pb dates as old as 1330 Ma; their U-Pb dates were not made appreciably older by abrasion techniques.

Monazite from the carbonate body have Th-Pb dates of 1375±7 Ma and imply that the carbonatite was emplaced about 25 Ma after syenite and granite. Bastnaesite and parisite from the carbonatite contain considerable common lead and suggest that the body was open to lead migration after emplacement. Clear parisite has an older Th-Pb date (1332±7 Ma) than coexisting bastnaesite, but that date is anomalously young if compared to the Th-Pb dates from monazite. The least radiogenic common-lead ratios of galena from the carbonatite and potassium feldspar from alkalic igneous rocks (204:206:207:208 = 1:16.08:15.23;35.61) indicate that the parent magma for both rock types was

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