Retrograde metamorphism of eclogite in the southern Appalachian Mountains,U.S.A.–A case involving seamount subduction?

This work supports a growing body of evidence that the Ashe Metamorphic Suite (AMS) of the eastern Blue Ridge province in North Carolina has an ensimatic origin and is part of a subduction‐related accretionary mélange, marking the Taconic suture between the North American craton and the Inner Piedmont. In a palinspastic reconstruction, the thrust fault at the base of the AMS appears to have intercepted the greatest depths (i.e. highest‐P metamorphic rocks) beneath parts of the AMS now exposed adjacent to the Grandfather Mountain window. The greatest volume of mafic rock is found in these same areas. We suggest that the nascent, subduction‐related, basal thrust fault was deflected downward by an obstacle in the form of an isolated, mafic volcanic edifice on the oceanic crust–a sea mount. Pelitic and mafic rocks dominate the AMS. North of the Grandfather Mountain window, retrograded eclogite occurs in the amphibolite near the base of the AMS. Textures and mineralogy indicate that an original eclogite assemblage was subjected to the following sequence of parageneses: (a) Eclogite(I) facies: omphacite+garnet+quartz, (b) Eclogite(II) facies: omphacite+garnet+epidote+quartz, (c) Symplectic (diopside+plagioclase)+garnet+epidote+quartz, (d) Amphibolite facies: (diopside+plagioclase)+garnet+epidote+hornblende+quartz, (e) Amphibolite facies: plagioclase+garnet+epidote+hornblende+quartz. P–T conditions, estimated from geothermobarometry applied to relevant mineral compositions, are c.720 °C and c.16 kbar for (b) eclogite(II) facies; c.655 °C and c.8.5 kbar for (e) amphibolite facies.