Upper Mantle and Crustal Fragments in the Ithaca Kimberlites

Abstract

Xenocrysts and xenoliths from three Ithaca kimberlite localities are consistent with derivation from mantle depths of less than 150 km. Mantle mineral assemblages reconstructed from xenocrysts and rare 2-4 mineral fragments include spinel and garnet peridotites, and garnet pyroxenites. The Cr-rich garnets, pyroxenes, and spinels and the subcalcic diopsides commonly found in mantle xenoliths from other kimberlites are apparently absent. Secondary pargasitic amphibole attributed to mantle metasomatism is found associated with pyrope garnet. Al-rich pyroxenes and spinels suggest that Al-spinel lherzolites of the type commonly found in alkali basalts, but rarely found in kimberlites, occur. Some of the spinels are the most aluminous ever reported from kimberlites. These Al-spinel lherzolites suggest either a fossil high geothermal gradient or a very undepleted mantle in the Ithaca region. Fragments of presumed crustal origin include a suite of Mg-rich minerals which may belong to a granulite carbonate assemblage, small mafic syenite xenoliths, and clinopyroxene and garnet xenocrysts similar to minerals found in basic to intermediate composition granulites. The Ithaca basement is probably an extension of the Grenville metamorphic terrain, and the lower crust may be in the granulite facies. The Ithaca kimberlites are kimberlites on the basis of their whole rock compositions and mineral components, yet they lack the deep mantle fragments found in other kimberlites. The Ithaca kimberlites may have been finally emplaced by explosive eruptions from depths of less than 150 km.