IN-SITU MONAZITE GEOCHRONOLOGY OF THE CARTHAGE-COLTON SHEAR ZONE: NEW CONSTRAINTS ON MAGMATISM, METAMORPHISM, AND DEFORMATION ALONG A MAJOR TECTONIC BOUNDARY IN THE ADIRONDACKS

The Adirondack Mountains form the southern end of the contiguous Grenville Province where voluminous poly-deformed mid-lower crustal rocks record collisional and extensional tectonism during assembly of Rodinia in the Mesoproterozoic. The Carthage-Colton Shear Zone (CCSZ) forms a major tectonic boundary between the Adirondack Highlands and Lowlands, where granulite-facies meta-igneous rocks of the Highland lithologies are juxtaposed against mainly amphibolite-facies metasedimentary rocks of the Lowlands along SW-NE oriented ductile shear zone. Although magmatism, metamorphism, and deformation related to early phases of the Grenville Orogeny (Shawinigan (ca. 1190-1140 Ma)) were recorded by both domains, the late phases (Ottawan (ca. 1090-1020 Ma) and Rigolet (ca. 1010-980 Ma)) affected only the Highlands as the Lowlands lacks such U-Pb zircon growth ages. We present in-situ electron microprobe U-Pb-Th monazite geochronology from two mylonitic samples in order to directly date deformation, metamorphism, and magmatism in the CCSZ. Both samples are mylonitic orthogneiss consist of quartz, K-feldspar, plagioclase, biotite, chlorite, opaque and accessory minerals (zircon, monazite, apatite). Sample S-6 contains several monazite populations with core-rim relations with diverse compositional variations. Dates vary between ca. 1177-1125 Ma. The oldest domains (ca 1177-1166 Ma) have variable Yttrium content. Monazite dates of ca. 1150 Ma are similar to the weighted mean U-Pb zircon age (1150±5 Ma) and likely indicate anatectic fluids near the end of the Shawinigan Orogeny. Several monazite grains with high-Y domains yielding late-Shawinigan ages (ca 1141-1126 Ma) may be related to collapse and exhumation soon after the Shawinigan Orogeny. Sample S-9 yields a distinct texture with patchy curvilinear domains with sharp compositional boundaries, a typical manifestation of fluid-related dissolution-reprecipitation texture. Here Yttrium concentration defines the main domains: High-Y domains form ca. 1130 Ma cores whereas low-Y rims yield ages between ca 1054-1023 Ma. These Ottawan ages are synchronous with normal sense of shear during the orogenic collapse and indicate deformation and pervasive fluid percolation along the CCSZ. Overall, our results confirm the CCSZ is a long-lived tectonic boundary which was acted as the locus of ductile deformation and fluid flow during Shawinigan and Ottawan Orogenies.