Abstract
Basalts erupted in the Snake River Plain of central Idaho and sampled in the Kimama drill core link eruptive processes to the construction of mafic intrusions over 5.5 Ma. Cyclic variations in basalt composition reveal temporal chemical heterogeneity related to fractional crystallization and the assimilation of previously-intruded mafic sills. A range of compositional types are identified within 1912 m of continuous drill core: Snake River olivine tholeiite (SROT), low K SROT, high Fe-Ti, and evolved and high K-Fe lavas similar to those erupted at Craters of the Moon National Monument. Detailed lithologic and geophysical logs document 432 flow units comprising 183 distinct lava flows and 78 flow groups. Each lava flow represents a single eruptive episode, while flow groups document chemically and temporally related flows that formed over extended periods of time. Temporal chemical variation demonstrates the importance of source heterogeneity and magma processing in basalt petrogenesis. Low-K SROT and high Fe-Ti basalts are genetically related to SROT as, respectively, hydrothermally-altered and fractionated daughters. Cyclic variations in the chemical composition of Kimama flow groups are apparent as 21 upward fractionation cycles, six recharge cycles, eight recharge-fractionation cycles, and five fractionation-recharge cycles. We propose that most Kimama basalt flows represent typical fractionation and recharge patterns, consistent with the repeated influx of primitive SROT parental magmas and extensive fractional crystallization coupled with varying degrees of assimilation of gabbroic to ferrodioritic sills at shallow to intermediate depths over short durations. Trace element models show that parental SROT basalts were generated by 5-10% partial melting of enriched mantle at shallow depths above the garnet-spinel lherzolite transition. The distinctive evolved and high K-Fe lavas are rare. Found at four depths, 319 m, 1045 m, 1078 m, and 1189 m, evolved and high K-Fe flows are compositionally unrelated to SROT magmas and represent highly fractionated basalt, probably accompanied by crustal assimilation. These evolved lavas may be sourced from the Craters of the Moon/Great Rift system to the northeast. The Kimama drill core is the longest record of geochemical variation in the central Snake River Plain and reinforces the concept of magma processing in a layered complex.
Highlights
The relationship between volcanic rocks erupted on Earth’s surface and their putative plutonic roots is a long-standing geologic problem (e.g., Bachmann et al, 2007; Annen et al, 2015; Cashman et al, 2017)
Major elements were analyzed by fused bead X-ray fluorescence spectrometry (XRF), and trace elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS)
The Kimama area includes late Neogene to Quaternary basalts that were erupted from low-relief shield volcanoes, most of which are normal magnetic polarity lavas erupted during the (>781 ka) C1n Brunhes Normal Polarity Chron (Shervais et al, 2005). 21 paleomagnetic Chrons and subchrons were identified in the Kimama core (Potter, 2014)
Summary
The relationship between volcanic rocks erupted on Earth’s surface and their putative plutonic roots is a long-standing geologic problem (e.g., Bachmann et al, 2007; Annen et al, 2015; Cashman et al, 2017). The petrologic and chemical evolution of volcanic provinces and the formation of continental crustal plutonic complexes are typically studied in isolation from one another, yet each of these topics has implications for the other related to magmatic flux, intrusion geometries, fractionation mechanisms, and the effects of partial melting and assimilation (Annen et al, 2015). Large mafic layered intrusions have been studied extensively (e.g., Irvine, 1970) and the effect of basalt intrusions on crustal heat flux has been modeled (Annen and Sparks, 2002), there are few studies that infer mafic plutonic evolution from basalt compositions in long-lived mafic volcanic provinces (e.g., Shervais et al, 2006). In this paper we use the petrologic and geochemical evolution of basalt recovered in drill core to infer crustal magmatic processes over 5.5 Ma of mafic volcanism in continental crust.
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