OROGENIC EVOLUTION OF THE NEW-ENGLAND APPALACHIANS OF SOUTHWESTERN CONNECTICUT

New U-Pb ages are presented that place constraints on the Paleozoic or ogenic evolution of the New England Appalachians in southwestern Conne cticut. Beardsley and Pumpkin Ground orthogneisses yield concordant U- Pb zircon ages of 446 +/- 2 Ma and 428 +/- 2 Ma, respectively. These o rthogneisses were considered conformable metavolcanic members of the H arrison Gneiss but are now recognized as structurally juxtaposed metap lutonic units. Pumpkin Ground orthogneiss contains young inherited zir cons and has K-feldspars with elevated Pb-207/Pb-204, a characteristic of Avalonian crust. The Ansonia leucogranite (406 +/- 13 Ma; discorda nt zircon and monazite) marks the inception of abundant leucogranite m agmatism that heralds the onset of the Acadian orogeny. The Shelton mu scovite granite, previously interpreted as a metamorphosed siliceous t uff (Shelton white gneiss), yields a concordant U-Pb garnet age of 380 +/- 3 Ma, in agreement with U-Pb monazite ages. Middle Devonian grani tes have K-feldspars with elevated Pb-207/Pb-204 and zircons with poss ible Avalonian inheritance. Morphologically distinct monazites from st aurolite-kyanite-grade pelitic schists of the Trap Falls Formation dis play complicated U-Pb systematics that indicate Acadian metamorphism w as a prolonged event, lasting from ca. 395 to 376 Ma. A U-Pb sphene ag e of 362 +/- 1.5 Ma from the Pumpkin Ground orthogneiss suggests that cooling through the approximately 550-degrees-C isotherm was attained by 362 Ma. Based on an assumed temperature of 700-degrees-C at 375 Ma, the cooling rate immediately following the Acadian orogeny was approx imately 12-degrees-C/Ma. Constraints on deformation are provided by fo liated syn-metamorphic granites and cross-cutting post-tectonic intrus ions. Deformed intrusions include the Ansonia leucogranite (406 +/- 13 Ma), a transposed dike of muscovite granite (391 +/- 3 Ma; concordant garnets), and the Shelton muscovite granite (380 +/- 3 Ma). Deformati on accompanied or post-dated emplacement. Late-kinematic pegmatite and post-kinematic granite yield U-Pb monazite ages of 375 +/- 1 Ma and 3 76 +/- 2 Ma, respectively, and show that kinematic deformation pre-dat ed 375 Ma.The Alleghenian orogeny in southwestern Connecticut was asso ciated with the growth of morphologically distinct metamorphic monazit e in the Ansonia leucogranite at 296 +/- 2 Ma and emplacement of the P inewood Adamellite at 291 +/- 4 Ma. During this orogeny, temperatures approached but did not exceed approximately 550-degrees-C at the prese nt erosional surface. Combined field and laboratory studies provide a geologic link to the U-Pb geochronology and highlight necessary revisi ons to current tectonic models for the New England Appalachians of sou thwestern Connecticut. Plutonic rocks constitute a large proportion of exposed units and record nearly continuous magmatism from 446 Ma to 3 75 Ma. This constrasts sharply with models advocating little to no mag matism in southwestern Connecticut. Crustal-scale imbrication of basem ent rocks with Grenvillian and Avalonian Pb isotopic signatures occurr ed by Early Silurian time. The Acadian was a prolonged (ca. 395 to 376 Ma) middle-crustal-level orogenic event that generated considerable v olumes of leucogranite.