Abstract
New U-Pb ages are presented that place constraints on the Paleozoic orogenic evolution of the New England Appalachians in southwestern Connecticut. Beardsley and Pumpkin Ground orthogneisses yield concordant U-Pb zircon ages of 446 ± 2 Ma and 428 ± 2 Ma, respectively. These orthogneisses were considered conformable metavolcanic members of the Harrison Gneiss but are now recognized as structurally juxtaposed metaplutonic units. Pumpkin Ground orthogneiss contains young inherited zircons and has K-feldspars with elevated 207Pb/204Pb, a characteristic of Avalonian crust. The Ansonia leucogranite (406 ± 13 Ma; discordant zircon and monazite) marks the inception of abundant leucogranite magmatism that heralds the onset of the Acadian orogeny. The Shelton muscovite granite, previously interpreted as a metamorphosed siliceous tuff (Shelton white gneiss), yields a concordant U-Pb garnet age of 380 ± 3 Ma, in agreement with U-Pb monazite ages. Middle Devonian granites have K-feldspars with elevated 207Pb/204Pb and zircons with possible Avalonian inheritance. Morphologically distinct monazites from staurolite-kyanite-grade pelitic schists of the Trap Falls Formation display complicated U-Pb systematics that indicate Acadian metamorphism was a prolonged event, lasting from ca. 395 to 376 Ma. A U-Pb sphene age of 362 ± 1.5 Ma from the Pumpkin Ground orthogneiss suggests that cooling through the ∼550 °C isotherm was attained by 362 Ma. Based on an assumed temperature of 700 °C at 375 Ma, the cooling rate immediately following the Acadian orogeny was ∼12 °C/Ma.
Constraints on deformation are provided by foliated syn-metamorphic granites and cross-cutting post-tectonic intrusions. 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). Deformation accompanied or post-dated emplacement. Late-kinematic pegmatite and post-kinematic granite yield U-Pb monazite ages of 375 ± 1 Ma and 376 ± 2 Ma, respectively, and show that kinematic deformation pre-dated 375 Ma.
The Alleghenian orogeny in southwestern Connecticut was associated with the growth of morphologically distinct metamorphic monazite in the Ansonia leucogranite at 296 ± 2 Ma and emplacement of the Pinewood Adamellite at 291 ± 4 Ma. During this orogeny, temperatures approached but did not exceed ∼550 °C at the present erosional surface.
Combined field and laboratory studies provide a geologic link to the U-Pb geochronology and highlight necessary revisions to current tectonic models for the New England Appalachians of southwestern Connecticut. Plutonic rocks constitute a large proportion of exposed units and record nearly continuous magmatism from 446 Ma to 375 Ma. This constrasts sharply with models advocating little to no magmatism in southwestern Connecticut. Crustal-scale imbrication of basement rocks with Grenvillian and Avalonian Pb isotopic signatures occurred by Early Silurian time. The Acadian was a prolonged (ca. 395 to 376 Ma) middle- crustal-level orogenic event that generated considerable volumes of leucogranite.
