GEOCHRONOLOGY OF IGNEOUS AND METAMORPHIC EVENTS IN THE GNEISSIC BASEMENT ROCKS OF WESTERN CONNECTICUT

Geologic mapping and U-Pb geochronology by ion microprobe (SHRIMP) on zircon, titanite, and monazite in the New Milford quadrangle, western Connecticut indicate Mesoproterozoic events at c. 1.3, 1.05, and 0.99 Ga in the Laurentian basement rocks. Pink, peraluminous calc-alkaline granite gneiss with a volcanic arc signature intruded a paragneiss at 1311 ± 7 Ma, perhaps during the early stages of the Elzevirian Orogeny . During the Ottawan Orogeny, syn-tectonic anatexis of paragneiss produced a belt of stromatic migmatite at 1057 ± 10 Ma. Ottawan igneous activity also included syn-tectonic intrusion of abundant sills of biotite granite gneiss into the paragneiss at 1050 ± 14 and 1048 ± 11 Ma, and intrusion of the Danbury augen granite at 1045 ± 8 Ma. Overgrowths on igneous zircon in the granitic gneisses and cores and rims of metamorphic zircon in hornblende amphibolite gneiss indicate that terminal Grenvillian metamorphism occurred at approximately 993 ± 8 Ma.

Late Ordovician syn-tectonic events included synchronous intrusion of a leucogranite dike into the 453 Ma Brookfield Gneiss at 453 ± 6 Ma, and intrusion of the Candlewood Granite into basement gneisses and Lower Paleozoic cover rocks at 443 ± 7 Ma. A monazite age from the Candlewood Granite of 445 ± 9 Ma agrees with the zircon age. A second phase of migmatization in the basement rocks is associated with the injection of numerous granitic sills at 444 ± 6 Ma along the margin of the Candlewood Granite. Titanite ages from 431 - 406 Ma in all dated rocks indicate several growth events from the Silurian to the Early Devonian. The lack of Grenville-age titanite in the basement suggests that Paleozoic heating was sufficient to completely reset old titanite in the massif.