ZIRCON SHRIMP U-PB GEOCHRONOLOGY OF ORTHOGNEISSES FROM THE WATERBURY DOME, WEST-CENTRAL CONNECTICUT: EVIDENCE FOR LATE ORDOVICIAN/EARLY SILURIAN MIGMATIZATION IN RESPONSE TO TACONIC COLLISION
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.