Abstract

Peridotite inclusions in basanite at Dish Hill and Siberia craters near Ludlow, California, contain abundant amphibole, smaller quantities of phlogopite and apatite, and rare plagioclase. These minerals occur as veins transecting the peridotite and as interstitial components. Veins of undeformed amphibole cut across wehrlite-lherzolite layering and metamorphic foliation in lherzolite and offset kink bands in olivine, indicating that the amphibole postdates both anhydrous mineral layering and plastic deformation of the peridotite. Gradations in textural behavior of amphibole from veins to interstitial modes of occurrence suggest that all of the amphibole and associated minerals are post-consolidation additions to the peridotite, unrelated to its original crystallization. This conclusion is supported by the presence of amphibole with the same textural relations in all types of peridotite, irrespective of their modal composition or degree of deformation and recrystallization.

The source of the secondary hydrous minerals is not known. They may predate the basanite and have contributed to its formation, or they may have been derived from the basanite or related magma. The absence of amphibole phenocrysts in the basanite, as well as similar occurrences of amphibole in alpine-type peridotites that were not carried into the crust by mafic alkaline magma, support the former view. Both alternatives are currently under investigation.

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