GEOCHEMISTRY, TECTONIC SETTING, AND EMPLACEMENT AGE OF THE REE-NB-ZR-MINERALIZED PENNINGTON MOUNTAIN TRACHYTE OF NORTHERN MAINE

Located in the Winterville inlier of northern Maine, the Pennington Mountain trachyte occurs as a stock (~1.129 km²) in the Winterville Formation. Its East Lobe is pervasively fragmented and significantly mineralized in REE, Nb, and Zr as recently discovered, making the Pennington Mountain trachyte the only REE-Nb-Zr-mineralized trachyte ever reported in North America. Wherever it is fragmented, it is mineralized, suggesting that the volcanic-induced subsurface fragmentation was a major controlling factor for the REE-Nb-Zr mineralization. The trachyte is microcrystalline and mostly also micro-porphyritic with perthite phenocrysts set in a microcrystalline matrix composed mostly of perthite and albite that account for 95% of the trachyte. Part of the trachyte in the non-fragmented, non-mineralized West Lobe is vesicular and amygdaloidal, indicating a shallow emplacement. TAS and alkalinity plotting shows trachyte-trachydacite and calc-alkaline affinity. Spider diagrams show depletions in Ba, K, Sr, Eu, P, Ti, and Li and abnormally high enrichment of U, Th, Nb, Ta, Pb, Zr, Hf, and Y, particularly in the mineralized trachyte. REE distribution patterns show slightly higher LREE than HREE with a significant negative Eu anomaly. Discrimination diagrams show alkaline “anorogenic domain” and “MORB-OIB array”. All the non- and weakly-mineralized samples show A1-type “granitoids”. The surrounding basalt is geochemically trachy-andesite or alkali basalt or trachy-basalt and also calc-alkaline. Both the trachyte and the hosting basalt share similar geochemical features and are non-orogenic, implying their petrogenetic association. A non-mineralized trachyte sample collected from the West Lobe yielded 40 igneous zircon crystals that gave a U-Pb concordia age of 462.8±1.4 Ma for the emplacement and REE-Nb-Zr mineralization of the trachyte. Zircon dating on a dacite from Hedgehog Mtn and on a trachyte from Haystack respectively gave U-Pb ages of 460.7±2.7 Ma and 465.4±3.4 Ma for the Winterville Formation that is dominated by volcanic rocks produced in an ensialic volcanic arc setting. The non-orogenic setting for the trachyte and the hosting basalt therefore indicates a short period of arc-rifting during the arc development history of the Winterville inlier of the Ordovician Munsungun-Winterville belt.