Petrochronological constraints on the origin of the Mountain Pass ultrapotassic and carbonatite intrusive suite, California

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 geochronological 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 ± 10 to 1385 ± 18 Ma, expanding 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 ± 16 and 1371 ± 10 Ma, respectively). The Hf isotope compositions of zircon in the ultrapotassic rocks are uniform, both within and between samples, with a weighted mean εHfi of 1·9 ± 0·2 (MSWD = 0·9), indicating derivation from a common, isotopically homogeneous source. In contrast, in situ Nd isotopic data for titanite in the ultrapotassic rocks are variable (εNdi = -3·5 to -12), suggesting variable contamination by an isotopically enriched source. The most primitive εNdi isotopic signatures, however, do overlap εNdi from monazite (εNdi = -2·8 ± 0·2) and bastnäsite (εNdi = -3·2 ± 0·3) in the ore-bearing carbonatite, suggesting derivation from a common source. The data presented here indicate that ultrapotassic magmatism occurred in up to three phases at Mountain Pass (~1425, ~1405, and ~1380 Ma). The latter two stages were coeval with carbonatite magmatism, revealing previously unrecognized synchronicity in ultrapotassic and carbonatite magmatism at Mountain Pass. Despite this temporal overlap, major and trace element geochemical data are inconsistent with derivation of the carbonatite and ultrapotassic rocks by liquid immiscibility or fractional crystallization from common parental magma. Instead, we propose that the carbonatite was generated as a primary melt from the same source as the ultrapotassic rocks, and that although it is unique, the Mountain Pass ultrapotassic and carbonatite suite is broadly similar to other alkaline silicate-carbonatite occurrences in which the two rock types were generated as separate mantle melts.