U-PB ZIRCON CONSTRAINTS ON DEFORMATION AND INTRUSION, NORTHWEST ADIRONDACK HIGHLANDS AND ADJACENT ADIRONDACK LOWLANDS, NEW YORK

U-Pb zircon geochronology of the northern Adirondack Highlands documents intrusive and deformational history of a critical portion of the Grenville Orogen. Undeformed intrusive rocks of the ca. 1045 Ma Lyon Mountain Granite (LMG) suite cross-cut deformed metaplutonic rocks in the northwestern Adirondack Highlands of New York. Southwest of the Carthage-Colton Shear Zone (CCSZ), near Croghan, NY, LMG dikes intrude deformed mafic syenite of the Diana Complex, which was emplaced ca. 1155 Ma. Near Cranberry Lake, NY, slightly deformed LMG granite engulfs and cross-cuts foliated amphibolite, and slightly younger pegmatite cross-cuts both.

Within the CCSZ, ca. 1045 Ma LMG granite was intruded during extensional collapse (Selleck, et al 2005), followed by uplift and rapid cooling of the region (Bonamici, et al 2011). LMG pegmatite intrudes ultramylonite, and is itself mylonitized, documenting syndeformational intrusion. Within the immediately adjacent Adirondack Lowlands terrane, ca. 1035 Ma calcite-pyroxene vein-dikes were emplaced within older, deformed (Shawinigan – ca. 1180-1160 Ma) plutonic and metasedimentary rock. These vein-dikes, fluorite-bearing pegmatite and associated Cl-amphibole-scapolite veins, represent distal, fluid-rich derivatives of LMG plutons emplaced at depth beneath cooler, extending, upper-plate rocks of the Lowlands terrane.

Geochemical analyses of zircon from the rock suites in the vicinity of the CCSZ show that U/Th ratios vary widely. Demonstrably metamorphic zircons within metagabbros have U/Th ratios that overlap with U/Th of demonstrably igneous zircons in nearby granites. These data suggest that U/Th ratios are unreliable indicators of magmatic vs. metamorphic zircon growth, particularly in highly evolved crustal rock systems such as leucogranites and associated pegmatites.