NEW EVIDENCE FOR LATE TO POST-SHAWINIGAN INTRUSION OF THE HAWKEYE GRANITE GNEISS AND LYON MOUNTAIN GRANITE GNEISS, ADIRONDACK HIGHLANDS, NY

Aleinikoff and Walsh (2015) presented SHRIMP U-Pb data from complexly zoned zircon that led to a new interpretation of ages of ca. 1160 and 1145 Ma for the Hawkeye Granite Gneiss (HG) and Lyon Mountain Granite Gneiss (LMG), respectively. The conclusion for the age of LMG was met with great skepticism; previous interpretations stressed that because LMG lacks penetrative deformation it must be younger than the Ottawan phase (~1080-1030 Ma) of the Grenville Orogeny, and younger than the previous age of ca. 1100 Ma for deformed HG. According to this interpretation, oscillatory-zoned zircon cores (OZZC) in LMG were inherited from 1.18-1.15 Ga AMCG rocks, and rims represented the igneous portions of the grains. In contrast, we interpret the age of OZZC to record the time of crystallization of LMG, and multiple weakly-zoned to unzoned overgrowths, revealed by high-resolution cathodoluminescence imaging, date several episodes of post-emplacement metamorphism. This conclusion implies that the Ottawa was an event of high heat flow but limited deformation in the Adirondack Highlands.

New analyses address this controversy. OZZC from strongly deformed HG (from the base of Lyon Mountain) are 1155±10 Ma; OZZC from moderately deformed HG (at Stillwater Reservoir spillway) are 1138±10 Ma; OZZC from moderately deformed HG (at Hawkeye hamlet) are 1144±10 Ma (plus inheritance at about 1.18 Ga). All zircon samples of HG have multiple rims that formed at about 1040 and 1000 Ma. Near Sevey Corners, foliated amphibolite intruded by LMG contains zircon with two morphologies: (1) elongate with oscillatory zoning, and (2) equant with sector zoning. These are interpreted as igneous and metamorphic, respectively, in origin. Cores of Type 1 grains are 1159±6 Ma; rims on Type 1 grains and all Type 2 grains are 1036±4 Ma. OZZC from cross-cutting LMG formed at 1146±9 Ma. Dark inner rims with sector zoning formed at 1070±8 Ma; black, narrow outer rims grew at about 1040 Ma. We conclude that the protolith of the amphibolite crystallized as a syn-tectonic intrusion at about 1160 Ma, and was intruded by LMG at about 1145 Ma. These ages tightly bracket the end of Shawinigan deformation and support the interpretation that the Ottawan was a major thermal event, but not a regionally penetrative deformation event in the Adirondack Highlands.