TIMING AND SIGNIFICANCE OF DEFORMATION WITHIN THE SHELVING ROCK QUADRANGLE, EASTERN ADIRONDACK HIGHLANDS, SOUTHERN GRENVILLE PROVINCE

The Adirondack Highlands (AH) show evidence for multiple stages of tectonism. These include: the 1170 Ma Shawinigan orogeny, emplacement of the ca. 1150 Ma AMCG igneous suite, the ca. 1070 Ma Ottawan orogeny, and subsequent 1050-1030 Ma extensional collapse exemplified by the Carthage-Colton mylonite zone and recently described shearing along the eastern edge of the AH (Eastern Adirondack shear zone). The nature, extent, and tectonic significance of eastern Adirondack extensional shearing are uncertain. It is critical to distinguish late extensional deformation from earlier events using structural and microstructural analysis, forward petrologic modeling, and in-situ geochronology in carefully chosen localities inside and outside of the proposed shear zone. One such locality is in the central part of the Shelving Rock quadrangle where a well-exposed ridge of aluminous mylonitic paragneiss contains abundant monazite and excellent top-west, thrust-sense kinematic indicators. Interestingly, the composition, zoning, and ages of monazite vary dramatically from layer to layer within the gneiss. Some layers contain only Shawinigan monazite whereas other layers contain Ottawan cores with or without post-Ottawan rims. Current work involves P-T-t-D analysis of whole rocks and individual layers to determine the significance of the west-directed mylonitization and to constrain monazite-producing reactions in these heterogeneous rocks. These data will be compared to analyses from the Dresdon Station metagabbro locality, where a ca. 1150 Ma coronitic metagabbro appears to cross cut gneissosity in surrounding paragneisses. Preliminary analyses of monazite from the paragneiss at the Dresdon Station outcrop yield few (if any) Ottawan ages, and contain a large Shawinigan (ca. 1170 Ma) and AMCG (ca. 1150 Ma) populations. Reconciling the timing and conditions of these two localities is critical in deciphering the polyphase history of the AH. Linking in-situ monazite geochronology to P-T-t-D histories will further constrain the development of regionally extensive granulite facies deformation within the AH, and will also provide insight into metamorphic and deformational control on monazite generation in polyphase tectonites.