MONAZITE DATING IN METASEDIMENTARY ROCKS OF PUZZLE MOUNTAIN, WESTERN MAINE

The Puzzle Mountain area in northwest Maine is composed of Paleozoic metasedimentary rocks underlain by granitic plutons. Metamorphism and deformation in this region are associated with the 386 Ma Acadian orogeny. Finer-grained pelitic schists at the top of the hill change into coarser-grained gneiss and migmatite at the base. These meta-pelites range in grade from the Lower Sillimanite Zone (LSZ) at the top of Puzzle Mountain to the sillimanite + K-feldspar zone (SKZ) in the southern part of the area and contain AFM (+ Qtz + Musc) assemblages of sillimanite + staurolite + biotite + garnet (LSZ), sillimanite + biotite + garnet (USZ), and sillimanite + K-feldspar + biotite ± garnet (SKZ). Multiple events are implied by mineral growths over fabrics, overprinting of earlier foliations by later cleavages, and muscovite pseudomorphs of staurolite.

The purpose of this study is to use U-Th-Pb monazite dating in order to constrain the age of metamorphism at Puzzle Mountain. Preliminary data from four samples along a north-south transect from Puzzle Mountain to the Sebago Batholith have been obtained using the UCLA ion microprobe. One to five spot analyses were run in two to four grains from each sample. The standard error for each spot age typically ranges from 3 to 15 Ma. Average ages range from 420 to 354 Ma with a few older (~435 Ma) spots and two younger (~260 Ma) ones. These data are consistent with 40Ar/39Ar white mica cooling ages of 290 Ma (Kinner, 1996) to 280 Ma (McMillan, 1997) from the same samples. Thus, monazite and white mica ages from the Puzzle Mountain area are inconsistent with metamorphic growth caused by the 292 Ma Sebago Batholith and instead reflect true Acadian (400-380 Ma) growth during metamorphism.