73-5 Cenozoic Basaltic Volcanism in the Idaho Basin and Range: Insights from North of the Yellowstone Hotspot Track

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes (Posters)

Poster Booth No.: 275

Presenting Author:

Megan Melgren

Authors:

Melgren, Megan¹, Brueseke, Matthew E.², Hindall, Andrew G.³

(1) Department of Geology, Kansas State University, Manhattan, KS, USA, (2) Department of Geology, Kansas State University, Manhattan, KS, USA, (3) Department of Geology, Kansas State University, Manhattan, KS, USA,

Abstract:

Hotspot magmatism is recognized by identification of linear chains of volcanoes that young in one direction, but these age-progressions are unclear when volcanism along a hotspot track does not temporally fit in the age progression and is therefore, out of sequence, and at times off-axis. The Snake River plain-Yellowstone (SRPY) volcanic province is the archetypal example of continental hotspot magmatism and major SRPY caldera-sourced silicic “supervolcano” eruptions are well documented. Here, we present constraints on a different, but equally significant defining characteristic of the SRPY province: small-volume mafic magmatism that erupted adjacent to the hotspot track. Our focus areas occur ~60 km north of the East Snake River Plain (ESRP) proper, in the Idaho Basin and Range province. One location is a previously mapped (Janecke, 1992) Plio-Pleistocene monogenetic scoria cone located northwest of Hawley Mountain, in the Little Lost River valley (ID). Here, the cone formed adjacent to a mapped normal fault, which also cuts the western side of the cone.  Initially, an ~5-m-thick package of lavas erupted and flowed east for ~1 km. This effusive package transitioned to a scoriaceous, cone-building stage, which was characterized by primarily Hawaiian-style eruptions. The 170-m-high cone is characterized by pervasively oxidized pyroclasts and welded agglutinate, where larger bombs are concentrated along the northwest volcano rim. Spindle and breadcrust bombs are abundant, especially along the northern and western crater rim. Some of the bombs are up to 2-m-wide, just adjacent to the inferred location of the final eruptions from the cone. The second location is an ~75-m-thick package of lavas in the Lemhi Valley, near Lone Pine (ID). At least seven distinct compound pahoehoe lava lobes crop out, some of which display brecciated basal crusts and flow-top breccias (i.e., rubbly pahoehoe), typical of ESRP Snake River olivine tholeiites. It is unclear if this lava package was sourced from vents on the ESRP, but the mapped extent of the package is consistent with flow down a paleovalley. Ongoing work is focusing on the mineralogy and bulk rock geochemical analyses of representative samples from both locations to better decipher if the off-axis basalts are related to the Yellowstone hotspot or represent a different melt generation process and mantle source, such as alkali basalt volcanism in the Nevada Basin and Range.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8020

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