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Layered mafic sill complex beneath the eastern Snake River Plain: Evidence from cyclic geochemical variations in basalt

¹ Department of Geology, Utah State University, Logan, Utah 84322-4505, USA
² Department of Geology, Centenary College, Shreveport, Louisiana 71104-1188, USA
³ Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA

The eastern Snake River Plain in southern Idaho, western United States, is characterized by 1–2 km of Pleistocene to late Pliocene basalt overlying rhyolite caldera complexes. Cyclic variations in the chemical composition of basalts from 1136 m of scientific drill core show that the parent magmas of these lavas evolved by crystal fractionation at shallow to intermediate crustal depths, punctuated by episodic recharge with more primitive compositions and assimilation of adjacent wall rock. We have identified 10 upward fractionation cycles and four reversed cycles; assimilation of sialic crust was limited and mainly affects the oldest basalts, which directly overlie rhyolites. We infer that the crystal fractionation and/or recharge cycles took place in a series of sill-like intrusions at intermediate crustal depths that now form a layered mafic intrusion that underlies the eastern Snake River Plain at depth. This layered sill complex is represented by the 10-km-thick "basaltic sill" that has been imaged seismically at 12–22 km depth. The association of this mid-crustal sill complex with geochemical fractionation cycles in basalt supports the concept that exposed layered mafic intrusions may be linked to overlying basalt provinces that have since been removed by erosion.

Key Words: basaltic volcanism • basalt • layered intrusions • crustal structure

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