Zircons from three orthogneisses from the Waterbury dome (WD), previously dated by TIMS, have been re-dated by SHRIMP microanalysis, clarifying our understanding of their primary ages and the timing of migmatization of rocks of the Gneiss Dome Belt of western New England. Tectonic windows in the cores of this set of domes expose orthogneisses and migmatitic ky-kfs paragneisses that form the deepest structural levels of the orogen between massifs of Mesoproterozoic basement to the west and Neoproterozoic basement to the southeast.

There are two primary map units of the WD: the structurally lower Waterbury Gneiss and lying above it along the Waterbury thrust, a heterogeneous assemblage of qtz-rich paragneiss, graphitic and sulfidic schist, and amphibolite, with minor amounts of quartzite, calc-silicate, and ultramafic rock. Both units are migmatitic and are intruded by orthogneiss. The three dated orthogneisses are from the structurally higher unit. A foliated grt-ms-bt leucogranite that grades into migmatite gneiss contains elongated, euhedral zircon with resorbed igneous cores and high- and low-U overgrowths. Ages from both cores and overgrowths are 437±4 Ma. Stubby zircons in this rock yield ages in the range 380-360 Ma — similar to hornblende argon ages from the same structural level (Dietsch and Sutter, 1987). A foliated meta-tonalite that cross-cuts migmatitic S1 in paragneiss contains elongated, euhedral, zoned zircon. SHRIMP ages from these grains also are 437±4 Ma, which we interpret as the crystallization age. Another meta-tonalite engulfed by migmatitic gneiss contains igneous zircon with very high U and exsolved thorite that yields 434±4 Ma. Two previously determined single grain TIMS monazite U-Pb ages from migmatitic paragneiss above the Waterbury thrust are 434±13 and 432±4 (Dietsch, et al., 1998).

Migmatization at conditions of at least 9.0 kb and 775ºC, accompanied by intrusion of intermediate to felsic magmas, occurred in response to crustal thickening. Our new ages show this high-grade metamorphism took place in latest Ordovician or earliest Silurian time, in response to — that is, after —the Bronson Hill Arc collided with Laurentia during the latest Caradoc.