MAFIC ROCKS IN THE ASHE METAMORPHIC SUITE ADJACENT TO THE GRANDFATHER MOUNTAIN WINDOW, NORTHWESTERN NORTH CAROLINA–SUBDUCTION OF A SEAMOUNT?

A growing body of evidence supports the view that the Ashe Metamorphic Suite (AMS) 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 (GMW) (Abbott and Raymond, 1997; Abbott and Greenwood, 2000). The greatest volume of mafic rock is found in these same areas. Abbott and co-workers proposed that the nascent, subduction-related, basal thrust fault was deflected downward by an obstacle in the form of an isolated, mafic volcanic edifice on oceanic crust, i.e., a seamount.

A cross-strike section through the AMS just north of the GMW consists of up to 75% essentially uninterrupted amphibolite. Rankin (1970) indicated only that the original thickness of the AMS "must...be measured in miles." Presumably, the thickness of the amphibolite component of the AMS would also be measured in "miles," or kilometers. Trace-element and REE geochemistry of the amphibolites in the AMS suggest two important environments for the protolithic basalt (Misra and Conte, 1991), ocean ridge (N-MORB) and ocean plume (T-MORB, seamount). Thus, the AMS adjacent to the GMW may represent the deformed, sheared, multiply metamorphosed, partially dismembered, but still largely contiguous, parts of a seamount and associated ocean-floor basalt.