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Abstract:
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The texture and geochemistry of ultramafic rocks exposed at the Earth’s surface can be used to distinguish their origin as either magmatic, replacive or tectonic. Tectonite ultramafics can be indisputably linked to ophiolite sequences and are interpreted as uplifted sections of the upper mantle, whereas cumulate and replacive ultramafics are interpreted as either part of an ophiolite sequence or as the lower “strata” of a layered intrusion (Kubo, 2002). Preliminary investigation of the petrology and geochemistry of the East Dover ultramafic unit in south central Vermont indicates that the serpentinized dunitic body formed as residual mantle emplaced tectonically, rather than ultramafic cumulate formed by a magmatic intrusion, or replacive dunite formed by magma infiltration. The East Dover ultramafic body consists of serpentine surrounding islands of olivine, with accessory chrome spinel, minor pyroxene and secondary calcite. Plots of Al2O3 vs. TiO2 for the East Dover ultramafic body classify it within the dunite field. Because chrome spinels from throughout the unit contain Cr# [(Cr/(Cr+Al)] >0.60, the rock can be classified according to Dick et. al. (1984), as a Type III Alpine-Type peridotite. A mantle origin is widely accepted for Type III Alpine-Type peridotites (Dick et. al., 1984). Additional support for the interpretation of the East Dover ultramafic unit as a fragment of uplifted residual mantle is provided by compositional data from olivine and chrome spinel, using Ni (~2200-2700 ppm) vs forsterite (Fo) content (~88-96%) in olivine grains (Leblanc et. al., 1984), and Cr-Al-Fe+3 in chrome spinels (Barnes et. al., 2001). The interpretation of the East Dover ultramafic unit as the mantle tectonite section of an ophiolite sequence suggests that it can be correlated with the ophiolites and fragments thereof in northern Vermont, Quebec and Newfoundland. |
