GEOCHEMICAL AND AGE MAPPING OF MONAZITE FROM THE LUCERNE GRANITE, MAINE

Monazite from the Lucerne Granite, Maine is being investigated using high-resolution compositional/age mapping electron microprobe techniques. Geochemical scans of a thin section of Lucerne granite located 14 monazite crystals »14-200µ in size. Detailed investigation of three crystals identified distinct geochemical domains (based on Y, Th, U, and Pb abundance). Two crystals contain distinct low Y (4000-6000 ppm) cores overgrown by high Y (10,000-20,000 ppm) rims. Cores retain relict crystal forms modified by extensive resorption. Rim overgrowths exhibit subtle oscillatory zonation in Y and Th abundance. The third crystal is irregular in form and complexly zoned. All three crystals preserve distinct age information. Cores yield ages of 449±5 Ma; 446±5 Ma, and 434±4 Ma (n=5-6 analyses per crystal). Single spot analyses from the rim yield younger ages of 398 Ma, 389 Ma, and 375 Ma (»±11 Ma.). Crystallization ages for Lucerne granite of 380±4 Ma (²⁰⁷Pb/²⁰⁶Pb on zircon; Zartman and Gallego, 1979) and 385±3 Ma (w.r. Rb/Sr n=10; Hogan & Sinha, unpub. data) are consistent with monazite rims forming during crystallization of Lucerne granite. With refinement, these dates will place a minimum age constraint on the timing of terrane accretion in the Penobscot Bay area. Common Pb isotopic systematics suggest Lucerne granite originated by partial melting of »1.3 Ga old source (Hogan and Sinha, 1991). The SLREEs (382 ppm) and estimated melting conditions (»850^oC) for Lucerne granite suggest the melt was saturated with respect to monazite; the presence of "inherited" monazite crystals was expected. However, the late Ordovician to early Silurian ages suggest the xenocrystic cores were incorporated at higher structural levels, during ascent, rather than originating from the »1.3 Ga source region as restite. Monazite mapping and dating technique is a powerful tool for evaluating complex, multi-stage histories of granitic bodies that are key to the building of continental crust.