U-PB ZIRCON GEOCHRONOLOGY OF CORONITIC METAGABBROS IN THE ADIRONDACK HIGHLANDS

Garnetiferous amphibolites (GA) are common in thin (<1 km) E-W-trending bands in the Central Adirondack region, where many of them contain cores of preserved coronitic metagabbro (CMG). A nearly continuous transect (~1 km long) near Newcomb, NY, exposes rocks exhibiting transition from CMG to GA. We present petrological, geochemical, and U-Pb geochronological analyses of these units to better understand the timing and conditions of magmatism and metamorphism. Although samples display various mineralogical assemblages and deformation textures, major and trace element analyses do not show any significant variability, similar to other locations in the Highlands. Zircons separated from the CMG yielded three populations of grains: 1) large (200-300 µm), elongate prismatic dipyramids (1143±3 Ma; U = 318±192; U/Th = 1.3±0.3; ²⁰⁶Pb/²⁰⁴Pb = 56529); 2) polygonal zircon clusters (1037±6 Ma; U = 53±19; U/Th = 2.6±1.1; ²⁰⁶Pb/²⁰⁴Pb = 7960) and 3) individual polygonal grains (1033±3 Ma; U = 50±28; U/Th = 3.0±1.1; ²⁰⁶Pb/²⁰⁴Pb = 8098). The coarsest-grained GA has large (150-300 µm), rounded zircons and can be grouped based on U concentration (Low U: 1023±22 Ma; U = 50±10; U/Th = 13.0±1.6; ²⁰⁶Pb/²⁰⁴Pb = 2985; High U: 1057±13 Ma; U = 226±209; U/Th = 9.5±5.9; ²⁰⁶Pb/²⁰⁴Pb = 13257). These zircons display remnant cores, sector zoning, tortoiseshell textures, and partially to completely recrystallized and isotopically reset, yielding ages consistent with metamorphic ages in the CMG. In contrast, elongate euhedral igneous zircons display a higher and greater range of U contents, significantly higher ²⁰⁶Pb/²⁰⁴Pb ratios, older U-Pb ages consistent with crystallization of the suite across the Highlands, lower and more consistent U/Th ratios, and a more restricted range in concordancy compared to metamorphic zircons. Overall our data indicate that the transition from CG to GA is isochemical with complete recrystallization of igneous zircon enhanced by deformational processes.