J OURNAL OF Journal of Petrology, 2016, Vol. 57, No. 8, 1555–1598 doi: 10.1093/petrology/egw050 P ETROLOGY Original Article Petrochronological Constraints on the Origin of the Mountain Pass Ultrapotassic and Carbonatite Intrusive Suite, California Downloaded from https://academic.oup.com/petrology/article-abstract/57/8/1555/2413438 by guest on 31 March 2020 Jacob E. Poletti1*, John M. Cottle1, Graham A. Hagen-Peter1 and Jade Star Lackey2 1Department of Earth Science, University of California, Santa Barbara, CA 93106-9630, USA and 2Geology Department, Pomona College, Claremont, CA 91711, USA *Corresponding author. Telephone: 805-748-2549. Department telephone: 805-893-4688. Department fax: 805-893-2314. E-mail:
[email protected] Received October 21, 2015; Accepted August 3, 2016 ABSTRACT Rare earth element (REE) ore-bearing carbonatite dikes and a stock at Mountain Pass, California, are spatially associated with a suite of ultrapotassic plutonic rocks, and it has been proposed that the two are genetically related. This hypothesis is problematic, given that existing geochronolo- gical constraints indicate that the carbonatite is 15–25 Myr younger than the ultrapotassic rocks, requiring alternative models for the formation of the REE ore-bearing carbonatite during a separate event and/or via a different mechanism. New laser ablation split-stream inductively coupled plasma mass spectrometry (LASS-ICP-MS) petrochronological data from ultrapotassic intrusive rocks from Mountain Pass yield titanite and zircon U–Pb dates from 1429 6 10 to 1385 6 18 Ma, ex- panding the age range of the ultrapotassic rocks in the complex by 20 Myr. The ages of the youngest ultrapotassic rocks overlap monazite Th–Pb ages from a carbonatite dike and the main carbonatite ore body (1396 6 16 and 1371 6 10 Ma, respectively).