TIMING OF ALLEGHANIAN OROGENESIS AND DOME FORMATION IN SOUTHERN CONNECTICUT

Geologic mapping, structural analysis, and geochronology in the area of the Lyme dome, constrains the origin of the dome rocks, the timing of the tectonic events attributed to Alleghanian orogenesis, and the processes of dome formation. Detrital zircon geochronology and the ages of intrusive rocks bracket the deposition of quartzite in the core of the dome to between ca. 925 and 620 Ma. Granite and granodiorite intruded the metasedimentary rocks in the core of the dome at ca. 620 to 610 Ma. Four early Permian tectonic events associated with the Alleghanian orogeny affected the rocks. Syn-tectonic migmatization, penetrative deformation (D1, ca. 300 to 290 Ma), and emplacement of alaskite at 290 ± 4 Ma occurred during foliation development and aluminosilicate-orthoclase metamorphic conditions. Rocks of the Avalon terrane may have wedged between Gander cover rocks and Gander basement in the core of the dome during D1. Limited structural evidence indicates that diapiric uplift started late in D1 and was completed by D2 (ca. 290 to 280 Ma) when horizontal WNW contractional stresses dominated over vertical stresses. Second sillimanite metamorphism continued during D2 emplacement of granite pegmatite (288 ± 4 Ma) and the Joshua Rock Granite Gneiss (284 ± 3 Ma). North-northwest extension during D3 (ca. 280 to 275 Ma) led to granitic pegmatite intrusion along S3 cleavage and in boudin necks during hydraulic failure and decompression melting. Intrusion of a Westerly Granite dike at 275 ± 4 Ma suggests that D3 extension was still active at ca. 275 Ma. Late gently dipping pegmatite dikes record a change from NNW extension to vertical unloading and exhumation during D4 (ca. 275 to 260 Ma). The ages of monazite and metamorphic rims on zircon record this event at ca. 259 Ma. Late regional uplift, extension, and normal faulting at higher crustal levels may have been caused by diapiric rise of the lower crust beneath the level of Lyme dome. The rocks record no evidence of Acadian metamorphism or deformation, suggesting that high-grade metamorphism of these Gander zone rocks was delayed until tectonic wedging of the Avalon terrane during the Alleghanian orogeny.