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Mineralogical Magazine; August 2004; v. 68; no. 4; p. 699-708; DOI: 10.1180/0026461046840214
© 2004 Mineralogical Society of Great Britain and Ireland
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Letter

Suprasubduction-zone peridotite in the northern USA Appalachians: evidence from mineral composition

R. A. Coish* and P. Gardner

Geology Department, Bicentennial Hall, Middlebury College, Middlebury, Vermont 05753, USA

* E-mail: coish{at}middlebury.edu

ABSTRACT

Mineral compositions of small peridotite bodies in an Ordovician collision zone of the Vermont Appalachians provide important clues to their tectonic environment of origin. The bodies have been deformed and partially serpentinized under greenschist- to lower amphibolite-facies conditions during the Ordovician and Devonian. Before serpentinization, the peridotite bodies were dunite as shown by their mineral assemblage and by their high MgO, and low Ti and Al whole-rock contents. Despite deformation and metamorphism, remnant olivine and spinel grains occur; their compositions are taken to represent conditions prior to regional metamorphic events. High Mg/(Mg+Fe) in olivine and very high Cr/(Cr+Al) in spinel indicate that the peridotites formed as highly-depleted mantle residues. The compositions are similar to those in harzburgite and dunite from some ophiolites and from fore-arc regions of subduction zones. Accordingly, the southern Vermont peridotites probably formed in a fore-arc, supra-subduction zone during the Early Palaeozoic. They were subsequently emplaced by obduction of the upper plate of an east-facing subduction complex.

KEYWORDS: peridotite, subduction zone, Appalachians, Vermont, USA






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