Abstract

Xenoliths of granulite are relatively common in the Stockdale kimberlite pipe, Kansas. Two rock types in particular, a pyroxenite and a metagabbro, display mineral reactions which appear to have frozen-in by the incorporation of the xenolith during the rapid ascent of the kimberlite to the surface. Most striking is the replacement of green aluminous spinel by sapphirine, (Mg2.95Fe0.46Al4.58)VI(A14.67Si1.33ІVО20. The replacement of plagioclase (An53–An60) by a symplectic intergrowth of fassaite (Di83Cats17) and sillimanite appears to be unique. A third reaction includes the formation of garnet (Py45Al33Gr22) from earlier fassaite and plagioclase. Pressure and temperature for these reactions have been deduced as being in the range 10–14 kbar and 800–1000°C based on phase chemistry and Fe/Mg partitioning between coexisting garnet and clinopyroxene. These mineral reactions reflect the attempt of basic igneous rocks to reequilibrate under extreme conditions of metamorphism at depths in the region of 30–40 km beneath the continental crust in Kansas.

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