GEOCHEMISTRY AND TECTONIC IMPLICATIONS OF SERPENTINIZED ULTRAMAFIC ROCKS IN VERMONT

A narrow, north-south trending belt of Early Paleozoic, serpentinized ultramafic bodies crops out along the entire length of the Vermont Appalachians. The belt is in or close to the Rowe Thrust Zone, interpreted as the Taconic collision region. The bodies range in size from a few meters up to 5 km long. Most are completely serpentinized; however, some of the larger ones in southern and northern Vermont have as little as 20% serpentinization, and thus preserve remnant igneous mineralogy. One of the best preserved is the East Dover body in southern Vermont. The mineralogy consists of serpentine surrounding "islands" of olivine, with accessory chrome spinel, minor pyroxene and secondary magnetite and calcite. Optical continuity of many deformed olivine "islands" suggests grains were originally 1 cm or more across. Textures are reminiscent of those found in ultramafic tectonic units from ophiolites.

Low whole rock Al₂O₃ (<1 wt %) and TiO₂ (<0.05 wt %) contents indicate the body was a dunite before serpentinization. Compositions of non-recrystallized olivine grains range from Fo₈₉ to Fo₉₅. Spinel ranges in composition from chromite to magnetite, which occurs in secondary rims around chromite cores. Cr numbers (Cr/(Cr+Al) from the cores of chrome spinel are greater than 0.60.  With such mineral compositions, the body can be categorized as a Type III alpine peridotite. The narrow range of Fo contents indicates it is not cumulate in origin. Rather it is probably an ultramafic tectonite formed either as a residue from multiple partial melting events or by a replacive mechanism whereby melt infiltrates already depleted harzburgite and dissolves pyroxene to leave dunite.

Based on petrology and geochemistry, the East Dover ultramafic body is correlated with mantle tectonite portions of ophiolites and fragments thereof in northern Vermont, Quebec and Newfoundland. Furthermore, the very high Fo content of olivine and high Cr numbers in spinel are tentatively taken to indicate that the parent body originated in the fore-arc region of an Early Paleozoic subduction zone.