TACONIC ECLOGITE FROM NORTHWEST CONNECTICUT: IMPLICATIONS FOR THE TIMING OF SUBDUCTION POLARITY REVERSAL

The structure and collision process of the Taconic orogen are highly controversial. The tectonic implications of switched subduction polarity hinge on the age of high-pressure (HP) metamorphism of the Laurentian margin in the collision zone, but such records are rare. Mafic lenses in the Canaan Mountain Formation of Northwest Connecticut contain relict mineral assemblages and decompression textures that demonstrate HP eclogite precursors. Symplectic intergrowths of diopside + plagioclase and biotite + plagioclase form pseudomorphs after omphacite and phengite, respectively. Pseudosection analysis and thermobarometry indicate that the rocks were first heated to >750 ⁰C at ~8 kbar, followed by compression to 14-15 kbar at ~700 ⁰C in the eclogite facies. Based on bulk-rock geochemistry and field relations, the protoliths were likely rifting-related mafic dikes. SIMS U–Pb zircon dating yields a 456 ± 4.6 (2σ) Ma metamorphic age and Early Cambrian igneous 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 metamorphic ages show that the east-dipping subduction of the Laurentian margin lasted until 456 Ma in southern New England. This new result, considered together with the Brookfield plutons (≤ 454 Ma) which intrude the accretionary prism and cut the Taconic lineations (Sevigny & Hanson 1995, Geol Soc Am Bull), provides a tight time constraint for the subduction polarity reversal. The culmination of Taconic metamorphism and the following polarity reversal took place ~10 Myr later than in Newfoundland, consistent with the southward-zipping model proposed for the orogen (Bradley 1989, Tectonics; van Staal et al. 1998, Geol Soc Spec Publ; Waldron et al. 2014, Geology)