MANTLE SLIVERS FOUND IN NEW YORK CITY AND NEW JERSEY: EVIDENCE FROM SPINEL-GROUP MINERAL CHEMISTRY

The Appalachian–Caledonian orogen, which records the closure of the Iapetus ocean and collision of Laurentia, Avalonia and Baltica 500-400 million years ago, contains a plethora of ultramafic-mafic complexes throughout its 6000 km strike length. Many of the occurrences in the Caledonian and northern Appalachian portions are well-constrained as ophiolites, but those in the central and southern Appalachians have more ambiguous origins.

In this study, we compare four ultramafic samples from Leka, Norway — a well constrained ophiolite at the northern end of the Appalachian–Caledonian system — to 4 ultramafic samples from the New York City area (Hoboken and Staten Island) in the central Appalachians, whose tectonic settings are ambiguous. We present the results of optical microscopy and major and minor element analyses on spinel-group minerals (conducted using an electron microprobe), aiming to understand the primary origin of the ultramafic rocks. Backscattered electron images were taken of every spinel grain analyzed to distinguish chemical zoning and refine interpretations made on the basis of the point analyses.

The Leka samples contain olivine (25–40%), pyroxene (5–10%), and serpentine (40–65%), the Hoboken samples comprise serpentine (60–85%) and olivine (5–15%), and the Staten Island samples contain serpentine (70–85%), olivine (10–15%), and pyroxene (5–10%). Samples from all localities contain accessory spinel-group minerals.

Primary spinel compositions from Leka contain <0.077% TiO2, with Cr-numbers (molar Cr/[Cr+Al]) ranging from 51.96–52.97, Fe2+-numbers (molar Fe2+/[Fe2++Mg]) ranging from 0.48–0.49, and Fe3+-numbers (molar Fe3+/[Fe3++Cr+Al]) ranging from 0.05–0.06. In comparison, primary Hoboken compositions contain <0.065% TiO2, with Cr-numbers of 78-84, Fe2+-number ranging from 0.57–0.75, and Fe3+-numbers of 0.02–0.11. Primary Staten Island compositions contain <0.045% TiO2, with Cr-numbers ranging from 62–70, Fe2+-numbers ranging from 0.59–0.66, and Fe3+-numbers ranging from 0.01–0.05. The chemical similarity of the Hoboken and Staten Island spinel-group minerals to the Leka data (including the extremely low TiO2 abundances), strongly suggest that there are pieces of the mantle preserved in Hoboken (NJ) and Staten Island (NY).