CONSTRAINTS FROM MONAZITE GEOCHEMISTRY AND LA-ICP-MS ZIRCON ANALYSES ON THE NATURE AND TIMING OF METAMORPHISM IN THE DRESDEN STATION AREA, EASTERN ADIRONDACKS

EPMA U-Th-Pb monazite analyses from the cores of monazite in garnet-sillimanite gneisses from Dresden Station in the eastern Adirondacks yield ages between ~1200-1170 and have relatively elevated concentrations of REE’s such as Y, Dy, and Gd, while the rims or inner rims of monazite yield ages ~ 1160-1140 and show a marked decrease in heavier REE’s. These ages correlate with the Shawinigan Orogeny and the subsequent intrusion of the AMCG plutonic suite. The decrease in REE’s in the 1160-1140 Ma monazite are consistent with a significant period of garnet growth during this time and preferential portioning of the REE’s into garnet. The gneisses are garnet rich with little to no biotite and are interpreted to reflect garnet growth via biotite dehydration melting. The outmost rims of monazite yield ages between 1030-970 Ma with elevated concentration of REE’s compared to AMCG monazite. The increase in REE’s in the younger monazite is attributed to the breakdown of garnet during post orogenic collapse. The distribution of LA-ICP-MS U-Pb zircon ages from the garnet-sillimanite gneisses show prominent peaks at ~1180 Ma and ~1150-60 Ma with widely varying U/Th ratios. Importantly, there is no record of the (1090-1050) Ottawan orogeny in either monazite or zircon from these rocks. The garnet-sillimanite gneisses are cut by a body of weakly to undeformed coronitic metagabbro that truncates the foliation in the gneiss. Previously reported U-Pb zircon ages from this body range from 1144 Ma to 1106 Ma and are interpreted as crystallization ages. Our in-situ, LA-ICP-MS U-Pb zircon data from the metagabbro are consistent with an AMCG age, but also include a population of zircon with 1070-1040 Ma ages suggesting an Ottawan overprint. Coronitic textures include amphibole and biotite and thus require infiltration H2O-bearing fluid. The zircon and monazite data support the interpretation of metamorphism and anataxis synchronous with the intrusion of the AMCG suite (ca. 1150 Ma). Metamorphism may have been driven by the heat associated with AMCG intrusive bodies. The lack of evidence of the Ottawan orogeny in these gneisses may be the result of the limited fluid infiltration, limited deformation, or alternatively these rocks may have been in a more distal location during the Ottawan orogeny and then juxtaposed during post orogenic tectonism.