Layered mafic sill complex beneath the eastern Snake River plain;evidence from cyclic geochemical variations in basalt

Abstract

The eastern Snake River plain in southern Idaho, Western UnitedStates, is characterized by 1-2 km of Pleistocene to late Pliocenebasalt overlying rhyolite caldera complexes. Cyclic variations in thechemical composition of basalts from 1136 m of scientific drill coreshow that the parent magmas of these lavas evolved by crystalfractionation at shallow to intermediate crustal depths, punctuated byepisodic recharge with more primitive compositions and assimilation ofadjacent wall rock. We have identified 10 upward fractionation cyclesand four reversed cycles; assimilation of sialic crust was limited andmainly affects the oldest basalts, which directly overlie rhyolites.We infer that the crystal fractionation and/or recharge cycles tookplace in a series of sill-like intrusions at intermediate crustaldepths that now form a layered mafic intrusion that underlies theeastern Snake River plain at depth. This layered sill complex isrepresented by the approximately 10-km-thick ""basaltic sill"" that hasbeen imaged seismically at approximately 12-22 km depth. Theassociation of this mid-crustal sill complex with geochemicalfractionation cycles in basalt supports the concept that exposedlayered mafic intrusions may be linked to overlying basalt provincesthat have since been removed by erosion.

58 samples from this reference