U-PB GEOCHRONOLOGY STUDIES IN VERMONT

Conventional (TIMS) and ion microprobe (SHRIMP) U-Pb zircon geochronology of Mesoproterozoic and Paleozoic metaigneous and igneous rocks were conducted from 1987 to 2009, in support of new geologic mapping and compilation for the Bedrock Geologic Map of Vermont. Rocks were carefully studied prior to collection for geochronology in order to better understand the protoliths of metaigneous rocks and to aid in the interpretation of U-Pb results. Fourteen samples of tonalite and trondhjemite from the Mt. Holy Complex (Green Mountain massif), dated by TIMS and SHRIMP, confirm their emplacement between 1.39-1.32 Ga. These rocks are correlative with arc-related rocks to the west (Adirondacks) and in Grenville inliers to the south (western Connecticut; Hudson Highlands, NY; New Jersey Highlands). The College Hill Granite and other granitic gneisses were emplaced at about 1.22-1.25 Ga; a suite of augen gneisses and megacrystic granites formed at about 1.12-1.15 Ga; one deformed pegmatite is about 1.04 Ga; and rapakivi granites of the Cardinal Brook Intrusive Suite formed at about 0.95-0.97 Ga. Thus, the protoliths of these rocks crystallized during magmatic events that correlate, respectively, with the Elzivirian, Shawinigan, Ottawan, and Rigolet regional orogenic events.

Four episodes of Neoproterozoic to Paleozoic magmatism have been documented using both TIMS and SHRIMP ages, including: (1) 571 ± 5 Ma (Pinney Hollow Formation felsic volcanic), (2) about 485-500 Ma (North River Igneous Suite), (3) about 420 Ma (Braintree complex), and (4) about 370-390 Ma (syn- to late Acadian granitic rocks, such as the Barre Granite). Biotite-hornblende monzodiorite from the interior of a 60 m-thick dike of diorite of the Braintree Complex near Randolph, VT provided subhedral to euhedral zircons that yielded a concordant TIMS age of 419.3 ± 0.4 Ma. The well-preserved character of these zircons (no inheritance, minimal Pb-loss), coupled with the high Zr content of the monzodiorite, make this sample ideal for use as an ion microprobe standard (R33). All U-Pb geochronology research conducted at the USGS/Stanford SHRIMP lab utilizes this standard, as do many other labs throughout the world.