SINGLE-CRYSTAL AND WITHIN-GRAIN VARIATION IN U-TH-PB MONAZITE AND 40AR/39AR MUSCOVITE AGES FOR SAMPLES OF THE GASSETTS-TYPE SCHIST, VERMONT APPALACHIANS

Retention of radiogenic 40Ar in muscovite in high-grade metamorphic rocks has been viewed as the result of isotopic closure during cooling through the temperature range of ca. 300-400 °C, whereas Pb diffusion in monazite is sufficiently slow to indicate that U-Th-Pb monazite microprobe ages likely correspond to mineral growth or recrystallization events. Thus if monazite and muscovite in a rock crystallized above 400 °C, then 40Ar/39Ar ages for all muscovite crystals of that same rock should all be systematically younger than the U-Th-Pb age of the monazite. We have analyzed several samples of the ‘Gassetts-type’ high-Al schists of southeastern Vermont in order to test this traditional view of isotopic closure in muscovite and the relationship to U-Th-Pb monazite ages. These samples show a complex textural history, with early-formed porphyroblasts that are generally deformed and/or resorbed, and superimposed foliations defined by coarse, relatively undeformed micas. Laser 40Ar/39Ar ages for muscovite in single rocks have a range of about 60 million years with distinct age populations: single-crystal ages for muscovite vary from ca. 360 Ma to 320 Ma, whereas spot fusion ages within the cores of porphyroclastic muscovite range up to ca. 390 Ma. Microprobe ages for monazite in these samples also range over about 60 million years, also with distinct age populations. Matrix monazite cores, and particularly monazite inclusions in garnet, yield microprobe ages of ca. 425 and 388 Ma, whereas matrix monazite rims yield age populations of ca. 360 Ma. In contrast to conventional thinking that Ar ages should reflect closure during cooling in such rocks, white-mica 40Ar/39Ar and monazite U-Th-Pb age results indicate that each chemical system in these rocks is primarily influenced by mineral growth and recrystallization events that accompanied thermal, deformational or fluid-infiltration episodes at ca. 425, 390, and 360 Ma. Mineral relics that formed at 425 Ma survived, but were extensively modified by, the younger events, and most of the fabric-forming micas in these rocks are interpreted to have formed at 360 Ma. The enhanced spatial resolution and in-situ capability of U-Th-Pb microprobe dating of monazite and laser 40Ar/39Ar dating of micas offer complimentary and powerful approaches to evaluating metamorphic history.