ALLEGHANIAN OVERPRINTING OF ACADIAN METAPELITES BY DISSOLUTION-PRECIPITATION CREEP IN THE RETROGRADE EAST DERBY SHEAR ZONE

The Wepawaug Schist (WS) in southcentral Connecticut (Fritts, 1965) was metamorphosed to amphibolite facies conditions (Ague 1994; 2002) during the Acadian orogeny (Lanzirotti and Hanson 1996). However, retrograde overprinting of the schist during dextral transpression resulted in the ductile shortening of the block by >75%, the telescoping of Acadian isograds, and an apparent geobaric field gradient of >50 MPa/km (Growdon et al, 2013). ⁴⁰Ar/³⁹Ar ages of bulk muscovite (Ms) samples within the WS are partially reset by recrystallization and range in age from 355 to 280 Ma, indicating final isograd telescoping occurred during the Alleghanian orogeny (Growdon et al., 2013).

The WS is bounded on the west by the East Derby shear zone (EDSZ), a >30 km long NE-striking belt of phyllonites containing the greenschist facies assemblage: Ms + chlorite +quartz + albite. The partial to complete replacement of garnet, staurolite, and kyanite on the shoulders of the zone, and the presence of flakes of paragonite (pa)-rich Ms (20-35% pa) sandwiched between folia of Na-poor Ms (5-15% pa) ± chlorite in the phyllonites indicate a high-grade pelitic protolith. Flakes of Na-rich Ms and of earlier Ms + chlorite folia truncated by later folia containing almost pure Na-poor Ms provide evidence for a dissolution-precipitation creep as a critical deformation mechanism producing the phyllonites. Evidence for synchronous grain boundary sliding comes from (1) the flexural slip folding of Ms -rich folia in which hinges - opened by folding - are filled with crescent-shaped chlorite chevrons reminiscent of saddle reefs, and (2) by disharmonic folding of Ms folia where the base of the fold train is marked by either straight Ms folia, or by quartz ribbons. Thus, strain in the phyllonites in the EDSZ was accommodated by dissolution-precipitation creep and by grain boundary sliding. Evidence against the operation of dislocation creep in Ms comes from the micro-boudinage or kinking of Na-rich Ms. New targeted ⁴⁰Ar/³⁹Ar dating of Ms from selected folia or from bulk masses of fine-grained Ms replacing staurolite yield ages of ~300, ~270, and <255 Ma, documenting Ms crystallization and phyllonitic overprinting of Acadian metamorphic rocks during the assembly of Pangea in the Alleghanian orogeny.