RECOGNITION OF TECTONIC TERRANES WITH DISTINCT TECTONIC HISTORIES IN THE EASTERN ADIRONDACK HIGHLANDS

The Adirondack Mountains have traditionally been divided into two major tectonic domains, the Adirondack Highlands (AH) and Adirondack Lowlands (AL) with different tectonic histories. New structural, petrologic, and in-situ monazite geochronology suggest the eastern Highlands may also include distinct sub-terranes with distinct histories. Monazite-bearing (generally garnet-rich) gneisses have been analyzed along a broad transect from Fort Ann (South) to Ticonderoga (North). All grains were located in-situ within full section WDS maps, and then individual grains were mapped to place monazite domains within the context of the reaction and deformation history. Regionally, four thermo-tectonic events have been recognized in the AH: (1) the 1.17Ga Shawinigan orogeny, (2) the ca. 1.15 emplacement of AMCG magmas, (3) the 1.09-1.03 Ottawan orogeny, and (4) post-Ottawan orogenic collapse. Rocks from our sample transect can be divided into two subdomains or blocks. Rocks from the northern block, in particular Al-rich “khondalites” (Grt-Sil-Ksp-Pla-Qtz rocks interpreted to be restites), contain monazite with three main domains: ca. 1.18 Ga cores (Shawinigan), 1.15 Ga outer cores (AMCG), and rims that are younger than 1.05 Ga (Post-Ottawan). Y and Th compositions suggests that garnet growth, partial melting, and deformation occurred before or during 1.15 Ga. Rocks in the southern block contain monazite with small and rather uncommon core domains. Most yield ca. 1.15 (AMCG) dates, but some 1.18 (Shawinigan) core domains are present. Most grains have large 1.08-1.05 (Ottawan) outer core domains and 1.05-1.0 (Post-Ottawan) rim domains. Compositions suggest little Shawinigan or AMCG garnet growth but significant Ottawan garnet growth. Rim compositions and textures indicate syn-tectonic monazite growth during the early stages of garnet break-down. Thus, monazite data suggest that the northern block experienced high-grade Shawinigan and AMCG tectonism with little Ottawan overprint; the southern block preserves sub-garnet-grade Shawinigan and AMCG events and high-grade Ottawan tectonism with strong evidence for syn-tectonic post-Ottawan collapse. The two blocks apparently existed at different levels or different parts of the orogen until juxtaposition after the Ottawan orogeny.