Date of Award:

5-2016

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

John Shervais

Abstract

The Snake River Plain (SRP) in southern Idaho is one of the best preserved examples of continental hotspot tracks. A layered mafic intrusion has been seismically imaged in the mid-crust, however, few geochemical studies have investigated the midcrustal sill complex’s importance in the evolution of SRP magmas. This thesis investigates the geochemistry of 3 xenoliths from a mid-to-lower crustal sill complex sampled from core taken from Kimama, ID, and 3 xenoliths from a shallow sub-volcanic magma chamber beneath the SRP sampled from Sid Butte, ID. Forward modeling and partition coefficient calculations estimate the parent melt of the Sid Butte xenoliths was a Snake River Plain olivine tholeiite magma and the parent melt of the Kimama xenoliths was an evolved Craters of the Moon (COM) type S magma. Geobarometic estimates constrain the Sid Butte xenoliths origin to a shallow magma chamber at ~3.3 km and the Kimama xenoliths in a mid-to-lower crustal sill complex at ~22 km. Kimama xenoliths have average δ18O of 5.42, indicating mantle origin and little-to-no assimilation of felsic country rock. The presence of COM-type magmas in a mid-to-lower crustal sill complex that have assimilated little-to-no crustal material indicate that fractionation processes alone can produce evolved basaltic magmas in the subsurface of the SRP. This is in contrast with theories that require a substantial amount of assimilation of felsic material for the generation of evolved COM-type magmas in the SRP. This thesis not only provides a better understanding of the crustal architecture of the SRP, it provides evidence for the importance of crustal sill complexes and shallow subvolcanic magma chambers in the evolution of SRP basalts.

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