ZIRCON CHRONOLOGY OF THE CHIMNEY MOUNTAIN METASEDIMENTARY SEQUENCE, ADIRONDACK HIGHLANDS

A shallowly dipping sequence of relatively intact metasedimentary rocks lies in structural contact with strongly lineated and steeply foliated granitic gneisses on the western side of Chimney Mountain in the central Adirondack Highlands. The rocks are noteworthy for their apparent state of preservation including intact compositional layering. Despite metamorphism and the anatexis of underlying calc-silicate gneisses, they are not intruded, folded, or otherwise disrupted. Compositionally the rocks include layers ranging from 43-82% SiO2 with abundant CaO (6-26%) and MgO (4-11%). They contain variable amounts of quartz, plagioclase, diopside, phlogopite, and titanite. When normalized to the upper continental crust, rare earth element patterns are similar and primarily flat with HREE enrichment shown in less siliceous samples. Their REE concentrations are typical of upper crustal sedimentary rocks and decrease steadily with increasing quartz content. Slightly rounded to euhedral, elongate, clear to dark zircons up to 300 µm in diameter were separated from quartz-rich layers (0.25 - 1.0 m thick) and yield U-Pb SHRIMP ages of 1042+/-4 Ma (n = 31) and 1073+/-15 Ma (n = 4). Most of the zircons show little response to CL, lack visible cores or overgrowths, have U/Th ratios of 5.3+/-1.5 (n = 39), and U contents of 1093+/- 483 ppm. Two possibilities for the origin of these rocks will be evaluated: 1) they are part of the surrounding Grenville sedimentary sequence; or 2) they represent Neoproterozoic or younger supracrustal rocks. Option #1 implies that all the zircons analyzed are metamorphic in origin and detrital zircon grains, if originally present, have been extensively recrystallized. Option #2 implies that the rock contains abundant Ottawan detritus, was deposited and metamorphosed after the Ottawan event, and was structurally emplaced into its current location. Uranium-lead titanite chronology will be used to constrain the geologic history of the sequence.