CONSTRAINTS ON ARC ACCRETION BASED ON ELECTRON MICROPROBE AGE MAPPING AND CHEMICAL DATING OF MONAZITE FROM ARENIG OR OLDER, HIGH-GRADE METASEDIMENTS FROM THE WATERBURY DOME, WESTERN CONNECTICUT

The core of the Waterbury dome (WD), the southernmost in the series of Acadian domes of the Gneiss Dome Belt (GDB) of southwestern New England, is composed largely of Ky+Kfs+Bt+Grt+Ms+Qtz+Pl+Rt schist and paragneiss interlayered with andestitic gneiss. Migmatitic layering is deformed by a crenulation cleavage axial planar to interfering folds related to doming. A thrust sheet composed largely of quartz-rich gniess and granofels, and in part, of a mélange-like assembly of calc-silicate, amphibolite, quartzite, and coticule blocks and boudins in migmatitic sulfidic schist, intruded by 452 Ma tonalite, rests on top of the WD core rocks along the Waterbury Thrust. On top of this thrust sheet lies Taconic arc-related rocks: the Arenig or older Taine Mountain Formation (forearc sediments) and the Collinsville Formation (metabasites of boninitic, arc-like, and MORB composition).

High-resolution composition maps of Y, Th, and U show complex, patchy zoning patterns and in some grains, define cores and rims. We did not find distinct core and rim age domains. A typical quantitative analysis yielded the following wt. % concentrations and (1 sigma wt. %s): Y 1.8532 (0.004554), Th 4.7844 (0.01437), Pb 0.1535 (0.004282), and U 0.8393 (0.004563). In the core of the WD, 7 single-point ages from the core of a monazite inclusion in pre-crenulation garnet from migmatitic pelitic schist yielded a mean age of 437 Ma [and a standard error x 2 of 12 Ma]. Rounded, xenoblastic grains from the migmatitic schist above the Waterbury Thrust yield sets of ages that are older and more scattered: for example, 528 [20] Ma, n=12. Xenoblastic monazite from this same rock analyzed by TIMS yielded a concordant U-Pb age of 432±2 Ma and discordant fractions as old as 495±10 Ma (Lanzirotti, unpub. data).

These ages of monazite are consistent with Late Ordovician collision and overthrusting of Taconic arc rocks in the GDB. The suggestion of older ages of rounded, xenoblastic monazites from above the Waterbury Thrust can be interpreted as relic detrital grains. If so, their ages indicate Avalon-related source rocks.