ARE SERPENTINITES FROM THE GREATER NYC AREA ALL OPHIOLITES? INSIGHT FROM B ISOTOPES AND SPINEL-GROUP MINERALS (Invited Presentation)

Mafic/ultramafic complexes occur throughout the Appalachians (Eastern USA) and are commonly interpreted as ophiolites, i.e., a piece of oceanic lithosphere thrust onto continents during continental collision. However, the assumption that these ultramafic rocks represent the lowermost part of an ophiolite predates development of analytical techniques capable of discriminating the origin(s) of metasomatized mantellic rocks.

In the present contribution, serpentinite samples from Massachusetts, Connecticut, NYC area, and Maryland were studied using B isotopic composition (to track the origin of fluid responsible for serpentinization) and spinel-group minerals (to decipher primary signatures). Based on B isotopes (δ¹¹B mostly straddling 0 ‰), only samples from Maryland seem to unequivocally represent a Supra Subduction Zone Ophiolite, supporting a previous hypotheses based on bulk-rock chemistry (Guice et al., 2021). Serpentinites from NYC area and Connecticut have strongly positive δ¹¹B, which indicate serpentinization by seawater, meaning that these samples likely represent the fossil subducting slab. Primary spinel-group minerals from the NYC area strongly support a mantellic origin (abstract ID# 393217), but show variable Cr-numbers that could reflect variation in the degree of partial melting and/or fluid history. Serpentinites from Massachusetts display an unexpected feature: although all four samples are from a single location, two of them have strongly negative δ¹¹B —typically attributed to deep subducted crust metamorphic-related fluids — while two samples have δ¹¹B in the range 0 — 10 ‰, which may indicate Supra Subduction Zone Ophiolite serpentinites.

Thus, coupling B isotopes and spinel-group minerals seems promising in reconstructing the entire history of serpentinites encountered in the East Coast of the US, from their magmatic stage to the metasomatic alteration.