Geochronology of Adirondack Pegmatites by LA-MC-ICP-MS

To constrain the timing of major economic and fabric forming events in the Adirondack Highlands and Lowlands zircon samples were analyzed for U-Th-Pb isotopes by LA-ICP-MS. Samples ranged from large zircons (up to 2 cm in size) to populations of several smaller grains (2-5 mm). Uranium-Th-Pb concentrations varied widely from highly Uranium enriched, dark metamict grains, to large, clear to pink, low-U grains. Between 20-40 individual 35 micron spots located either on a single grain or distributed among several grains were analyzed. Three samples were too metamict and discordant to provide useful information. Eight other samples provided geologically reasonable ages consistent with known constraints. In the Adirondack Lowlands a pegmatite intruding marble from the Rossie “loxoclase” location yielded an age of 1212+/-17 Ma. The McLear pegmatite (ksp-plg-qtz-diop-phlog-titan-fluoro edenite-fluorian tremolite) from DeKalb yielded an age of 1170+31/-7 Ma and an inherited component with an age of ca. 1213 Ma. The pegmatite at Scott's Farm yielded an age of 1063+/-9 Ma. These results confirm the age of the metasedimentary sequence in the Lowlands (>1212 Ma), and the presence of both Shawinigan and Ottawan pegmatite events. The oldest age in the Adirondack Highlands comes from a small pegmatite body near Hull's Falls in the eastern Highlands. Ottawan ages associated with iron deposits of the Lyon Mountain Granite include: 1028+20/-46 Ma from small (2-5mm), pink zircon crystals found with successive rims of magnetite and titanite, included in scapolite at the Barton Hills Mine, Mineville; 1040+/-9 Ma from a magnetite-apatite ore (immiscible nelsonite-like body and dioritic phase of the Lyon Mountain Granite?) at the Old Bed Mine, Mineville; 1042+/-21 Ma from the Rose Spar bed pegmatite, Crown Point. The youngest age, 995+/-17 Ma, comes from pegmatite cutting the wollastonite-hedenbergite skarn at the Wollastonite Quarry in Lewis.