An ancient depleted mantle sample from a 42-Ma dike in Montana;constraints on persistence of the lithosphere during Eocene magmatism

Abstract

Recent models for the Cenozoic tectonic evolution of the westernmargin of North America propose that delamination of ancientlithosphere accompanied asthenospheric upwelling, magmatism, anduplift subsequent to Laramide deformation. On the basis of the age ofan alkaline dike in south-central Montana, thermometry of mantlexenoliths from the dike, and Sr, Nd, and Pb isotopic compositions ofthe dike and a xenocryst, we show that refractory lithosphere, derivedfrom ancient, depleted mantle, remained in place under the WyomingCraton as late as 42 Ma. The Haymond School Dike, a camptonite, yieldsa (super 40) Ar/ (super 39) Ar plateau date of 41.97 + or - 0.19 Ma(2sigma ). Paleomagnetic data are consistent with this date andindicate intrusion during chron C19r. The dike has Sr, Nd, and Pbisotopic compositions similar to those of other Eocene alkaline rocksfrom central Montana. A clinopyroxene megacryst from the dike hasepsilon (sub 42) = 17, and (super 87) Sr/ (super 86) Sr = 0.70288,indicating that it derives from ancient, depleted mantle isotopicallydistinct from the source of the host camptonite. Thermometry ofxenoliths from the dike shows pyroxene populations that formed at 880degrees and 1200 degrees C. Combining thermometry with previousestimates of the regional Eocene geotherm inferred from xenoliths inkimberlites, and with the Al-in-orthopyroxene barometer, we infer thatlithospheric mantle remained intact to depths of 110-150 km as late as42 Ma. Eocene magmatism was not accompanied by complete removal ofancient lithosphere.

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