Geology, geochronology, and geochemistry of the Pennington Mountain trachyte in the northern Appalachians of Maine, USA: Constraints on tectonic setting, petrogenesis, and rare earth element-Nb-Zr mineralization

The recently discovered Pennington Mountain trachyte, a rare earth element (REE)-Nb-Zr-mineralized subvolcanic intrusion within volcanic rocks of the Winterville Formation in the northern Appalachians of Maine, USA, records a critical mineral enrichment process directly tied to volatile-driven explosive magmatism. Mineralization is exclusively hosted in the thoroughly microbrecciated eastern lobe of the trachyte, where a crypto-explosive event generated tuffisite fragments 0.05−1.0 cm in diameter. This pervasive brecciation facilitated infiltration of a K-rich, Si-poor residual melt or fluid, precipitating 10−15 vol% fine-grained REE-Nb-Zr phases (monazite, bastnäsite, columbite, and zirconium silicates) together with minor sphalerite, cassiterite, pyrite, barite, and albite. The mineralized trachyte averages 0.56 wt% ΣREE + Y oxides, 1.18 wt% Zr, and 0.17 wt% Nb, exhibiting a consistent heavy REE enrichment over light REEs. Zircon U-Pb laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) geochronology constrains the Winterville Formation and the Pennington Mountain trachyte to a coeval age of 462−460 Ma. The tight temporal and petrogenetic link between the trachyte and the surrounding basalt is evidenced by a shared anorogenic, extension-related alkalic affinity; the trachyte is chemically part of an A1-type ferroan series. Integrated Nd, Sr, and Pb isotopic data are most consistent with a depleted mantle-dominated source containing a minor Grenvillian crustal component, with compelling μ (238U/204Pb) values indicating a Grenvillian inheritance. Decoupled initial εNd and elevated 87Sr/86Sr values are interpreted to reflect preferential Sr mobility during alkali metasomatism subsequent to primary mineralization. This study establishes that high-pressure, crypto-explosive brecciation within a cooling, fractionating subvolcanic magma can be a primary mechanism for generating REE-Nb-Zr mineralization. It provides a definitive petrogenetic model linking A-type alkaline magmatism in an extensional setting to critical metal enrichment. Our findings underscore the importance of intracrustal volatile exsolution and explosive fragmentation as a key, underappreciated process in the metallogeny of REEs and high field strength elements, offering a targeted exploration framework for similar alkaline provinces within extensional settings regionally and globally.