1999 GSA Annual Meeting -- Denver, Colorado

Abs. No. 51938

ELECTRON MICROPROBE AGE MAPPING AND DATING OF MONAZITE: NEW INSIGHTS ON THE TIMING OF POLYPHASE PALEOZOIC METAMORPHISM IN THE CENTRAL APPALACHIANS.

Author(s): BOSBYSHELL, H., Dept. of Geology, Bryn Mawr College, Bryn Mawr, PA 19010; hbosbysh@brynmawr.edu; WILLIAMS, M.L., JERCINOVIC, M.J., Dept. of Geosciences, University of Massachusetts, Amherst, MA 01003; CRAWFORD, M.L., Dept. of Geology, Bryn Mawr College, Bryn Mawr, PA 19010.

Keywords: Monazite, microprobe, dating, zoning, Appalachians,

Metasedimentary rocks of the Wissahickon Fm. in the central Appalachian Piedmont of SE Pennsylvania experienced two main stages of Paleozoic metamorphism. Mineral assemblages that define an early andalusite-sillimanite facies series field gradient (M1, 4-6 kb) are variably overprinted by a kyanite-bearing, Barrovian sequence (M2, 550- 650C, 7-9 kb). High-resolution X-ray composition maps reveal complex elemental zoning in all monazite crystals studied. Age maps, calculated from Th, U, and Pb X-ray maps, and chemical ages, obtained from spot analyses, show that much intracrystalline zoning occurs within single age domains, but domains and overgrowths of distinctly different ages are also present. We have analyzed monazite from areas that experienced different paths through the two-stage history. Samples with only weak M2 overprints yield consistent Ordovician ages (~480 Ma). Where M2 overprinting is more intense, monazite commonly exhibits multiple age domains. Monazite from a sample within the M1 sil + ms zone that was almost entirely reconstituted during M2, has cores which yield middle Ordovician ages and high U rims that are at least 60 Ma younger, a likely source of discordance in previously reported TIMS U/Pb isotope ages of monazite from this area (Bosbyshell et al., 1998). Overgrowths on many grains are elongate in the foliation plane suggesting that we are dating syn-deformational mineral growth. Monazite from a second location, that experienced only M2 (and is interpreted to lie outside the M1 andalusite-in isograd), shows some evidence for multiple age domains, but most grains give a chemical age of ~380 Ma. Because monazite inclusions in garnet are the same age as matrix grains, we interpret monazite growth to have occurred on the prograde path. The chemical age agrees well with the previously reported U/Pb isotope age (372 Ma) for M2 monazite in the Wissahickon Fm. and confirms the presence of Acadian-aged amphibolite facies metamorphism in the central Appalachians. Our study demonstrates the importance of the microprobe as a geochronological tool in analyzing multiply metamorphosed terrains as young as Devonian.