TIMING OF ANATEXIS WITHIN METAPELITES OF THE ADIRONDACK HIGHLANDS AND LOWLANDS, NEW YORK

Migmatitic metapelites occur within the Major Paragneiss of the Adirondack Lowlands and the Peck Lake Gneiss of the southern and eastern Adirondack Highlands. Both gneisses consist of dark garnet-biotite-quartz-oligoclase melanosome interlayered with garnet-quartz-feldspar leucosome. Locally the leucosomes are crosscutting, coarse granite or pegmatite, but more commonly are sub-parallel, disrupted layers of porcellanous mylonite that, together with the melanosome, form straight gneisses. At least two phases of isoclinal folding and intense penetrative deformation affected the migmatites. The primary age of the host metapelites is uncertain, but they are crosscut by the ca. 1.2 Ga Antwerp Granite. Although the anatexis has been considered an Ottawan event, textural and fabric features suggested earlier formation. In order to ascertain the age of anatexis samples were collected from both leucosome and melanosome in the Lowlands and the Highlands. Euhedral, igneous zircon grains from both leucosome and melanosome samples were dated by U/Pb SHRIMP techniques at the GSC facility in Ottawa. The results are as follows: Lowlands – south of Balmat, 1173 + 19 Ma; Trout Lake, 1162 + 12 Ma; Devil’s Elbow, 1160 + 30 Ma; Highlands – Peck Lake, 1163 + 10 Ma; south of Wells, 1165 + 13 Ma; south of Speculator, 1174 + 14 Ma; Conklingville Dam, 1125 + 21 Ma; Treadway Mt, 1060 + 9 Ma. Some of these zircons contain inherited cores with ages of 1250-1350 Ma, but Treadway Mt zircons contain resorbed cores of ca. 1140-1175 Ma surrounded by ca. 1060 Ma mantles exhibiting oscillatory zoning. Zircons from the melanosome fractions are commonly rounded and yield ages in the 1250-1350 Ma range, but ca. 1165 Ma igneous zircons are also present. Highland zircons have ca. 1060 Ma overgrowths, but these are lacking in zircons from the Lowlands, suggesting that this area did not experience Ottawan (ca. 1050 Ma) temperatures sufficiently high to grow zircon. These results document major anatexis across the Adirondacks during the 1170 + 10 Ma Shawinigan Orogeny, consistent with juxtaposition of the Highlands and Lowlands at this time. Absence of remelting during the Ottawan is attributed to earlier Shawinigan dehydration. Treadway Mt. leucosome represents an exception due, perhaps, to local influxes of H2O at ca 1060 Ma.