NEW ZIRCON AND APATITE U-PB GEOCHRONOLOGY FROM THE CARTHAGE-COLTON SHEAR ZONE: CONSTRAINING THE TIMING OF DEFORMATION IN A MAJOR CRUSTAL BOUNDARY IN THE NORTHWEST ADIRONDACKS

The Adirondack Mountains represent a southern extension of the Grenville Province, where poly-deformed and metamorphosed mid-to-lower crustal rocks are exposed. These units recorded subduction and collision-related crustal thickening and magmatism, lithospheric delamination, and the extensional collapse of the orogenic belt in the Mesoproterozoic during the amalgamation of Rodinia. The Adirondacks are morphologically and geologically divided into two domains; the Highlands that consists of granulite-facies meta-igneous rocks, and the Lowlands, where mostly amphibolite-facies metasedimentary rocks crop out. Both domains were affected by magmatism and metamorphism during the Elzevirian (ca. 1240-1220 Ma) and Shawinigan (ca. 1190-1140 Ma) orogenic phases. Large-scale thermal effects of the Ottawan orogeny (ca. 1090-1020 Ma), however, only described in the Highlands as the maximum temperatures didn’t exceed 400 °C in the Lowlands during this phase. The Carthage-Colton Shear Zone (CCSZ), an SW-NE trending 1-15 km wide ductile shear zone, forms the boundary between these tectonic domains and its thermal history and role during the Grenville Orogeny is highly debated. We present new U-Pb geochronology data from the CCSZ to better differentiate distinct orogenic phases and document the deformational history. A Dioritic gneiss sample yields non-complex zircon grains with a near unimodal age of 1156±5 Ma (Shawinigan). Similarly, a granitic gneiss sample yields large euhedral zircons with a slightly discordant age of 1151±10 Ma (Shawinigan). Zircons from a well-foliated and lineated granitic mylonite have two age populations of 1204±11 Ma and 1157±13 Ma (Shawinigan). A boudinaged diopside-rich layer within CCSZ has a single age zircon population of 1057±9 Ma. (Ottawan). An undeformed pegmatitic vein cross-cutting the mylonitic rocks of the CCSZ has a complex zircon population where core ages range from 1332±9 Ma (pre-Grenville) and 1162±3 Ma (Shawinigan), and rim ages are ca. 1050 Ma (Ottawan). Apatite ages from three samples range between ca. 972 Ma to 924 Ma (post-Ottawan). Overall results of this study highlight the polyphase nature of the CCMZ, further constrain the timing of mylonitization (post-1155 Ma), and indicate incorporation of older crustal units during the Grenville orogeny.