ZONED ACCESSORY CR-SPINEL IN THE STATEN ISLAND SERPENTINITE FROM A NEW RICHMOND ROAD OUTCROP IN STATEN ISLAND, NY

The Staten Island Serpentinite was mapped by Lyttle and Epstein (1987) as a lens shaped body whose long axis trends NE-SW. This is one of the many discontinuous ultramafic bodies that extend from Quebec, Canada into the southern Appalachians. Samples for this study were collected from a recent excavation for a shopping center in the Staten Island Serpentinite (N 40.5813° and W 74.1123°). The average composition of the serpentinite at this location is SiO₂ 34.96, Al₂O₃ 0.21, CaO 0.04, MgO 41.41, Na₂O<0.01, K₂O 0.03, Fe₂O₃ 7.66, MnO 0.01, TiO₂ 0.02, Cr₂O₃ 0.44, and LOI 15.57 sum 100.36 wt %. The Cr-spinel grains are disseminated in the serpentinite; the grains are fractured and some are zoned with a Cr-rich core and Fe-rich rim. The Fe-rich rims of the zoned grains of Cr-spinel would be classified as “ferritchromit” as described by Spangenberg (1943). Electron microprobe analyses were used to compute the following empirical formulas for core (Fe⁺² _0.70 Mg _0.28 Mn _0.02) _Σ=1.00 (Cr _1.47 Al _0.39 Fe⁺³_0.14 ) _Σ=2.00 O₄ and rim (Fe⁺²_0.88Mg _0.11Mn_0.01) _Σ=1.00 (Cr _0.7028 Al _0.01 Fe⁺³ _1.28) _Σ=1.99 O₄ respectively. Cr numbers range from 94-99 in the rim to 79-87 in the core and the Mg numbers from 9-11 in the rim to 25-30 in the core indicating a probable dunite-harzburgite derivation. The core of the spinels analyzed in this study plot at the margin of the fore-arc peridotite region of Coish and Gardner (2004) in the (Cr/Cr+Al)_sp vs. ( Mg/Mg+Fe)_sp diagram. The data suggest that this serpentinite body probably formed in the same way as that described by Coish and Gardner (2004) for the serpentinite in the Vermont Appalachians namely in a fore-arc suprasubduction zone.