ORIGIN OF THE REE-BEARING CAMBRIAN-ORDOVICIAN EPISYENITES AND CARBONATITES IN SOUTHERN AND CENTRAL NEW MEXICO, USA

Brick-red, K-feldspar-rich rocks, called episyenites (altered rocks that were desilicated and metasomatized by alkali-rich fluids solutions) are found in several areas southern and central New Mexico. These rocks contain anomalous concentrations of REE (<2,329 ppm), U (<9,721 ppm), Th, (<1,378 ppm), and high HREE (<133 ppm Yb and <179 ppm Dy), and are thought to be part of a Cambrian-Ordovician magmatic event that is documented throughout southern Colorado and New Mexico (McMillan and McLemore, 2004). This magmatic event is characterized by the intrusion of carbonatites, monzonites, alkaline granites, and mafic dikes, and is associated with K-metasomatism (i.e. fenites, episyenites) and Th-REE±U mineral deposits. In the Caballo Mountains, timing of metasomatism is older than late Cambrian as episyenite clasts occur in the C-O Bliss Sandstone that unconformably overlies episyenites. Dating of the metasomatism by ⁴⁰Ar/³⁹Ar methods on K-feldspar yield complex and intriguing age results that may be related to multiple fluid-alteration events. In the Caballo Mountains, age spectra range from nearly flat to highly disturbed with total gas ages (TGA) between ~40 and 460 Ma. However, one sample from the Burro Mountains yields a plateau age at approx. 540 Ma. The Lemitar carbonatite yields a ⁴⁰Ar/³⁹Ar age of 517.7±0.7 Ma (1 sigma, relative to FC-2 at 28.201 Ma). Synchysite is a major host of LREEs in the episyenites (63 wt%), while HREEs are concentrated predominantly in xenotime (16 wt.%) and priorite (9 wt%). Texture evidence suggests that LREE and HREE-bearing phases co-precipitated during metasomatism. Neodymiun isotopic data indicate that the Lemitar carbonatite is from a relatively strongly depleted mantle (eNd at 520 Ma +4.8, depleted mantle model age 678 Ma), whereas the Caballo episyenites are much less radiogenic (eNd at 520 Ma ca. -5 to -8, model ages ca. 1480-1780 Ma) and record a drastically difference source (probably cratonic subcontinental lithosphere).