GEOCHRONOLOGY AND GEOCHEMISTRY OF MULTIPLE GENERATIONS OF MONAZITE FROM THE WEPAWAUG SCHIST, CONNECTICUT, USA - IMPLICATIONS FOR MONAZITE STABILITY IN METAMORPHIC ROCKS

U-Pb isotope analyses, rare earth and trace element analyses, and petr ographic observations are presented for monazites from the Wepawaug Sc hist in southern Connecticut, USA. Two samples of kyanite zone schist were collected less than a meter apart. Each sample contains a differe nt variety of monazite with distinct morphology, chemistry, and Pb iso topic composition. One sample preserves a largely amphibolite facies m ineralogy, including kyanite, staurolite, garnet, biotite, and chlorit e, with little textural evidence of later shearing. Monazites from thi s sample are xenoblastic with about 1 wt% ThO2, 0.3 wt% CaO, and a mor e LREE enriched pattern than monazites from the second sample. These x enoblastic monazites preserve textural evidence of a retrograde reacti on to apatite which involves chlorite, indicating that these monazites became unstable during retrograde chloritization of biotite. These mo nazites give strongly discordant U-Pb ages which fit a chord with an u pper intercept age of 411 +/- 18 Ma, interpreted as the minimum growth age of these xenoblastic monazites, perhaps during amphibolite facies metamorphism. The second sample contains S-C banding, evidence of dyn amic recrystallization, and abundant retrograde chlorite. This sample contains idioblastic monazites with about 3 wt% ThO2, 0.8 wt% CaO, and with less fractionated REE patterns. These monazites give close to co ncordant U-Pb ages with a mean Pb-207/Pb-206* age of 388 +/- 2 Ma. Th is age is interpreted as probably representing the time of retrogressi on of the sample from an amphibolite to greenschist facies mineralogy.