IN-SITU MONAZITE GEOCHRONOLOGY OF MIGMATITES IN THE SHELVING ROCK QUADRANGLE: IMPLICATIONS FOR THE TECTONIC HISTORY OF THE EASTERN ADIRONDACK HIGHLANDS, NY

The timing of gneissic fabric formation in the eastern Adirondack Mountains is uncertain. The purpose of this research is to constrain the nature, number and especially timing of fabric forming events using in-situ monazite geochronology of highly strained and partially melted gneisses. The Shelving Rock Mountain – Sleeping Beauty area of the Shelving Rock Quadrangle contains exposures of metasedimentary gneisses, along with a number of metaigneous units that preserve a record of the deformation history in the eastern Adirondack Highlands. The most prevalent of the metasedimentary units is a two-feldspar, quartz ± biotite, hornblende, and garnet paragneiss, in gradational contact with a regionally mappable quartzite unit. On the southwestern slope of Sleeping Beauty Mountain, there is a NW-SE trending cliff composed of a garnet-rich, two-feldspar, quartz, and biotite migmatite. This unit, set within the main paragneiss unit, contains evidence for localized biotite dehydration melting. Quartz and feldspar within the unit display mylonitic textures including quartz ribbons and feldspar core and mantle structures in a fine-grained, and dynamically recrystallized Qtz-Fsp-Bt matrix. Also present are distinct K-spar-Pl-Qtz-bearing leucosome layers. Garnet ranges from 1-40mm in size, with some of the larger crystals displaying 2-stage compositional and textural zonation. The foliations strike approximately north and dip shallowly to the east. Lineations plunge shallowly to 090. Asymmetric sigma tails on recrystallized feldspar porphyroclasts indicate top => west (thrust) kinematics. Although of lower confidence, matrix asymmetric quarter structures on garnet suggest top => east (normal) shearing. One possibility is that the two minerals record two distinct shearing events. Preliminary in-situ monazite results from the migmatite cliff indicated at least four periods of monazite growth ca. 1170-1150, 1050, 1030, and 980-930 Ma, but the significance of each event is unclear. The Y content of monazite can link monazite to garnet growth or break-down events, as Y is strongly partitioned into garnet. This connection will be used in current work to constrain the timing of garnet growth during melting, along with dating monazite textural domains to constrain the timing of shearing in the eastern Adirondacks.