MAGMATIC AND TECTONIC EVOLUTION OF THE ADIRONDACK MOUNTAINS AND THEIR RELATIONSHIPS TO OTHER LATE-PROTEROZOIC TERRAINS

The Adirondacks contain juvenile, arc-related calcalkaline plutons (1.35-1.25 Ga) similar to those in the Green Mts. as well as Canada’s Elzevir Terrane and Mauricie area, and recording arc accretion along an Andean (?) margin (Elzevirian Orogeny). Similar records from Canada to Texas and Australia reflect a large-scale magmatic arc. The Elzevirian Orogeny culminated (1.2-1.17 Ga) when the Highland-Green Mt. block collided with Laurentia (including the Lowlands) producing the Hyde School-Rockport granitic suite. Collision was followed by delamination-related magmatism (AMCG suite) that swept east from Frontenac Terrane to the Marcy anorthosite massif during the interval ca. 1.17-1.13 Ga. Rocks of similar age and type have been recognized in the Hudson Highlands and Virginia (Aleinikoff, pers. comm.). Elsewhere, continued compression is recorded by the Shawinigan orogenic pulse. At ca 1.1-1.09 Ga granitic rocks interpreted as far-field echoes of the Midcontinent rift plume invaded the Adirondacks, and granites of comparable age occur in Virginia. Associated temperature rise increased Adirondack ductility prior to loading during the collisional Ottawan Orogeny (ca 1.09-1.03 Ga). Ductile deformation in the Adirondacks gave way NW to thrust-dominated tectonics in the more rigid Canadian crust. Late in the Ottawan Orogeny the Adirondacks were intruded by leucogranites belonging to the late- to post-tectonic Lyon Mt. Granitic Gneiss (LMG) accompanied by hydrothermal sodic alteration and deposition of Kiruna-type iron-oxide ores. Aleinikoff reports rocks of similar age and composition, but of synorogenic character, in Virginia. In west Texas the Hazel Orogeny has been dated at ca. 1.08-1.05 Ga corresponding in age to the Ottawan but occurring at low grade, and granite-rhyolite conglomeratic boulders dated at ca 1.12 Ga suggest earlier orogenesis. In the Llano uplift of central Texas a major collision at ca 1.15-1.12 Ga appears to represent a younger variant of the Ottawan Orogeny complete with post-tectonic granitic magmatism. Finally, Mazatzal, 1.45 Ga, and perhaps Yavapai rocks, have counterparts in the northeastern Grenville Province and reflect the existence of a long-lived magmatic-collisional margin that extended across North America and probably into Australia.