Integrating Thermochronology with Thermobarometry to Resolve a Contradictory Metamorphic Gradient

One dimensional thermal modeling of rocks from the southern Connecticut Valley zone in southwestern Connecticut resolves an apparent contradiction between a regionally flat temperature gradient and a north-south, Devonian-Permian cooling-age gradient.

Rim compositions of garnet-biotite pairs yield regionally uniform temperatures of 600-650°C. Pressures calculated using GASP barometry show a progressive decrease in metamorphic pressure from 9-10 kbar in the north to 5-6 kbar in the south. Index minerals, however, suggest that metamorphic grade increases to the south. Northern idioblastic staurolite and kyanite grade into fibrolite mats and subidioblastic staurolite and kyanite to fabric-forming sillimanite with xenoblastic kyanite in the south. Along the same gradient, quartz veins in the north are joined by plagioclase and finally become migmatitic in the south. Northern garnets show prograde decompression zoning, but garnets are nearly unzoned in the south suggesting that southern garnets were homogenized during higher grade metamorphism.

Late Devonian and Carboniferous K/Ar and ⁴⁰Ar/³⁹Ar ages of amphibole, muscovite, and biotite from the north have been interpreted as cooling from Acadian metamorphism, while Carboniferous to Permian ages in the south have been interpreted as cooling from an overprinting Alleghenian orogeny. One dimensional thermal modeling of the Devonian P-T data and cooling ages in the north shows that the metamorphic gradient in the south can also be explained by Acadian metamorphism. The younger cooling ages and higher metamorphic grade in the south could result from Permian tilting toward the north of rocks that cooled in the Devonian.