Geochemistry of Plutonic Spinels from the North Kamchatka Arc: Comparisons with Spinels from Other Tectonic Settings

Pavel K. Kepezhinskas*, Rex N. Taylor and Hisao Tanaka
Institute of Lithosphere, Russian Academy of Sciences, Staromonetny per., 22, Moscow 109180, Russia
Department of Geology, University of Southampton, Southampton, U.K.
Department of Earth Sciences, Yamagata University, Yamagata, Japan
*Now at: Department of Geology, University of South Florida, Tampa, Florida 33620, U.S.A.

Abstract: Ultramafic to mafic plutons in the Olyutor Range, North Kamchatka, represent the magmatic roots of a late Eocene arc, related to the westward subduction of the Komandorsky Basin beneath the Asian continental margin. Olyutor Range plutons are concentrically zoned with cumulate dunite cores mantled by a wehrlite-pyroxenite transitional zone and, in turn, by a narrow gabbroic rim.

Spinel is a common accessory mineral in these arc plutonics, and we present analyses of spinels from a range of lithologies. A continuous compositional trend is observed from Cr-spinel in the ultramafics to Cr-rich magnetite in marginal gabbros. Complex chemical zoning patterns within individual spinel grains suggest an interplay between fO2, fractionation, volatile content and subsequent sub-solidus re-equilibration of spinel with co-existing silicates (mainly olivine).

In general, the spinels from magmatic arc environments are characterised by high total Fe and high Fe3+ contents compared to MORB and boninitic spinels and higher Cr-values relative to oceanic basin spinels. These differences imply a high oxygen fugacity during arc petrogenesis. Differences are also observed between plutonic spinels from arcs and low-Ti supra-subduction zone ophiolites. Low-Ti ophiolitic spinels are generally poorer in iron and richer in Cr, and hence are similar in composition and perhaps tectonic setting to fore-arc boninitic spinels.

Keywords: spinel • ultramafic • mafic • pluton • Kamchatka • geochemistry

Mineralogical Magazine; December 1993 v. 57; no. 389; p. 575-589; DOI: 10.1180/minmag.1993.057.389.02
© 1993, The Mineralogical Society
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