ORIGIN OF THE META-ULTRAMAFIC ROCKS FROM THE APPALACHIAN MOUNTAINS (EAST COAST OF THE US)

Upper mantle is often transformed into serpentinite by the action of water. Most commonly, serpentinization occurs within the seafloor, when seawater percolates through fractures to alter underlying mantle. To a lesser extent, serpentinization occurs in subduction zones, when aqueous fluids coming from the dehydration of the slab ascend through the mantle wedge during the entire downgoing path. In suture zones, serpentinites from these different origins, as well as serpentinites representing the sole of an obducted ophiolite, are often found together and distinguishing between them is difficult. Recently, studies conducted on serpentinites from well-known tectonic settings highlighted that their boron isotopic signatures are distinct in function of their tectonic origin. Serpentinites from the deep mantle wedge have a strong negative δ¹¹B (down to -14‰), serpentinites from the forearc and from obducted ophiolite have a δ¹¹B in the range -5 to +10‰, and serpentinites from subducted slab have a strong positive δ¹¹B (up to +30‰).

Metaultramafic rocks are encountered all across the Appalachian Mountains, from Newfoundland (Canada) to Tennessee (USA), but they are so deformed that identifying their tectonic origin(s) is difficult. The literature of metaultramafic bodies in the Appalachian is quite limited, and gives contradictory conclusions. For example, serpentinites in the Blue Ridge Mountains, North Carolina are either interpreted as a tectonic mélange, i.e., from the mantle wedge, or a dismembered obducted ophiolite, whereas ultramafic bodies in Newfoundland are interpreted as an obducted ophiolite.

In the present study, we selected samples from Massachusetts, Staten Island, and Virginia to identify their tectonic origin, and ultimately to provide a better overview of the geometry of the Taconian collision. Preliminary results indicate at least two different origins for the meta-ultramafic bodies. Serpentinites from Massachusetts have δ¹¹B ranging from +0.12±1.52 to +7.07±1.00‰, suggesting they represent part of an ophiolite, and samples from Staten Island have δ¹¹B ranging from +7.45±0.45 to +27.30±1.30‰, arguing for subducted slab serpentinites. Thus, some of the serpentinite bodies found in the Appalachian Mountains are true obducted ophiolites, but some others come from Laurentian slab.