|Title:||Taconian retrograde eclogite from northwest Connecticut, USA, and its petrotectonic implications|
|Authors:||X. Chu, J. J. Ague, J. A. Axler, and M. Tian|
|Publication:||Lithos, v. 240–243, p. 276‐294.|
|Publish Date:||January 2016|
Mafic lenses hosted by felsic paragneiss in a Taconic thrust slice (Canaan Mountain Formation) from northwest Connecticut (New England, USA) contain relict mineral assemblages and decompression textures indicative of high pressure (HP) precursors. Symplectic intergrowths consisting mostly of diopside + plagioclase or biotite + plagioclase are pseudomorphous after omphacite and phengite, respectively. Pseudosection analysis and thermobarometry demonstrate that the inferred peak assemblage of garnet + clinopyroxene + phengite formed at N14 kbar and 710 °C in the eclogite facies. Bulk‐rock geochemistry and field relations indicate that the protoliths of the mafic gneisses were likely rifting related mafic intrusions. Zircon U‐Pb dating by ion probe yields a 456 ± 4.6 Ma (2σ) metamorphic age for the mafic gneiss. The zircons from the felsic host rocks have an identical 456 ± 11 Ma metamorphic rim age and Grenvillian detrital cores. The HP metamorphism and the coeval arc magmatism reflect the collision between the Laurentian passive margin and a Taconic arc complex over an east‐dipping subduction zone that was active until 456 Ma in Connecticut (southern New England). The P‐T path is characterized by post‐peak‐T compression, suggesting that the eclogite‐facies metamorphism was associated with the deformation of the collision zone after the initial continent‐arc collision. After the culmination of collision the subduction polarity switched; metamorphic ages decrease southward along the orogen suggesting that this reversal occurred 10Myr later in New England than in Newfoundland. Nearly all mafic rocks in the study area crop out as fairly ordinary‐looking amphibolites, so it is reasonable to speculate that HP metamorphism was more extensive than currently recognized in New England but has been obscured by thermal overprinting and retrogression.