"ADIRONDACK" LAURENTIAN BASEMENT OF THE NORTHERN APPALACHIANS: STRATIGRAPHY, PLUTONISM, GEOCHEMISTRY, AND TECTONICS

Laurentian basement (LB) is ca.1.4 to 0.950 Ga polycyclic crust nonconformably overlain by a Neoproterozoic-Cambrian rift and drift cover sequence along the Appalachian orogen. The last Mesoproterozoic tectono-thermal effects are at about 1 Ga. in the northern Appalachians. LB is confidently traced from the Adirondacks to Vermont and to the NY- NJ highlands and Manhattan prong, based on continuity of Mesoproterozoic stratigraphy, ages and geochemistry of plutonic suites, and tectonics based on ages of Mesoproterozoic and Neoproterozoic dike events. LB of the external massifs-- Green Mountain(GM),Berkshire(BM),Housatonic(HM),New Milford(NM), Hudson Highlands(HH) and the internal massifs of the Chester-Athens domes(CA), and Manhattan prong (MP)-- contains a 1.39-1.3Ga trondhjemitic to granitic core. Subsequent intrusive-deformational events occur at ca.1.2, 1.17, and 1.1Ga. Geochemistry of dated igneous suites aids correlation. Paragneiss sequences including felsic metavolcanics represent approximately 25% by area and are associated with quartzites, aluminous gneiss and calc-silicate gneisses consistent with a 1.4-1.3 Ga continental arc sequence; other metasedimentary gneisses may be as young as 1.17Ga. Gneissosity older than 1.2Ga, 1.17Ga and 1.13 Ga is evidence for polydeformation prior to the Ottawan. Post-Ottawan rapakivi plutons (ca. 950Ma) of the Cardinal Brook Intrusive Suite (GM, AC, BM) are alkalic within plate granites comparable to late Pinwarian plutons that suture the Ottawan Grenville belt of Labrador. The Pound Ridge Granite (0.562± 5Ga.) truncates Meosproterozoic structures in the Fordham Gneiss and autocthonous cover older than the Lowerre Quartzite is not required. An E-W structural grain predates Ottawan deformation and bimodal plutonism at about 1.13 Ga. Nd/Sm TDM ages range from 1.58-1.36 Ga for dated suites (1.37 – 1.13 Ga ). For the oldest 1.37-1.3Ga Londonderry Intrusive Suite (GM, AC), Reservoir Gneiss(HH), and part of the Fordham Gneiss(MP) the Nd-Sm TDM of 12 samples is 1.44 and with positive Epsilon of 3.8 to 3. These data suggest juvenile generation from 1.45 Ga. for the core of the LB from Vermont to New York, as do comparable data from the Adirondacks.