MAGNETITE-APATITE MINERALIZATION IN A GRENVILLE INLIER: TEMPORAL CONSTRAINTS ON REGIONAL METAMORPHISM AND METASOMATISM

The wide-spread occurrence of magnetite-apatite (MtAp) deposits hosted mainly by ca. 1.25 Ga magmatic arc rocks in Mesoproterozoic Appalachian inliers like the New Jersey (NJ) Highlands and contiguous Hudson Highlands of New York intimate regional mineralization events that both pre-date and post-date Grenvillian high-grade metamorphism. Establishing an absolute timeline for the formation and modification of these deposits should provide important insight into the tectonic evolution of the eastern Laurentian margin and, more generally, into the crustal evolution of large hot orogenic systems. We present preliminary zircon and apatite ID-TIMS U-Pb results for crystals separated from MtAp ores from the Baker, High Ledge, and Mariot's mines of the central NJ Highlands. High-pressure metamorphic zircon with round crystal morphologies from the Baker mine yield ²⁰⁶Pb/²³⁸U crystallization ages from ~1012 to 997 Ma, coincident with the expected timeframe for the Rigolet phase of the Grenville Orogeny. Blocky, metasomatic zircon grains from all three sample localities and zircon crystal rims from the High Ledge mine form a ~40 Myr spread along Concordia with ²⁰⁶Pb/²³⁸U dates ranging from ~986 to 942 Ma, demonstrating that these deposits were extensively modified by protracted or episodic mid-crustal fluid flow during extensional collapse of the orogen. Younger apatite ²⁰⁶Pb/²³⁸U dates (~917 to 900 Ma) likely reflect regional cooling below ~520-550 °C, signifying the end of high-temperature metamorphism. U-Pb dating of zircon cores from the High Ledge mine and geochemical analyses of all dated grains paired with detailed petrographic analysis will likely offer additional clues into the nature of primary MtAp mineralization and the composition of metasomatic fluids.