11-5 Shonkinite Magma Petrogenesis in the Adel Hills Volcanic Field, Montana

Session: Applications of Igneous Petrology, Geochemistry, and Geochronology to Understanding Earth and Other Planetary Systems (Posters)

Poster Booth No.: 79

Presenting Author:

Claire McLeod

Authors:

McLeod, Claire L.¹, Lytle, Marion², Cracas, Thomas Joseph³, Brown, Kenneth⁴, Shaulis, Barry ⁵, Loocke, Matthew Phillip⁶

(1) Miami University, Dept of Geology & Environmental Earth Sciences, Oxford, OH, , (2) Miami University, Dept. Geology and Environmental Earth Science, Oxford, OH, USA, (3) Miami University, Dept of Geology & Environmental Earth Sciences, Oxford, OH, USA, (4) DePauw University, Department of Geology & Env. Geosciences, Greencastle, IN, USA, (5) University of Arkansas, Trace Element and Radiogenic Isotope Laboratory, Fayetteville, AR, USA, (6) Louisiana State University, Geology and Geophysics, Baton Rouge, LA, USA,

Abstract:

The Adel Hills Volcanic Field (AHVF) in west-central Montana is a Late Cretaceous (c.76-73Ma), alkali-rich magmatic system located within the Great Falls Tectonic Zone which itself extends along the northern edge of the Wyoming Craton. The AHVF is characterized by extensive sequences of lava flows, volcaniclastics, dikes, and sills which at present day are exposed across ~900km². This study focuses on the shonkinitic (high-alkali), diopside-phyric, dikes that variably cross cut interbedded volcaniclastics. The aim of this work is to provide new bulk rock and crystal-scale insights into the petrogenesis of alkali-rich magmas in the AHVF during contractional deformation and the development of the Sevier fold and thrust belt. 

Sampled dikes are porphyritic and characterized by fresh, euhedral to subhedral, diopsides (~25-35%) up to ~1.5cm with euhedral to anhedral magnetite, feldspar, and apatite inclusions. Diopsides are often sector-zoned and/or oscillatory-zoned. The surrounding aphanitic groundmass (~65-75%) is feldspathic with vugs filled in by zeolites (analcime), plagioclase, and calcite. Subhedral to anhedral augite and amphibole are also present but rarer (<5%). 

Major and minor mineralogies determined via bulk powder XRD are consistent with observations via microscopy. Bulk wt. %  SiO₂ vs. wt. % K₂O systematics are consistent with shoshonitic signatures, while MgO-FeO*-Na₂O-K₂O systematics are consistent with calc-alkaline compositions. Bulk primitive-mantle normalized signatures reveal LILE enrichment and Nb-Ta depletion, consistent with a subduction-related signature. Chondrite-normalized REE signatures from bulk samples, diopsides, and fluorapatites exhibit limited Eu anomalies (0.92 to 1.33) while fluorapatite Ce/Ce* vs. Eu/Eu* systematics indicate formation in a hydrous, moderately oxidizing magmatic environment, in which the crystallization of plagioclase was suppressed. Si and Mg-enrichment in sector-zoned diopsides is accompanied by Al, Ti, Na, and Cr-depletion on {−111} faces. With the symmetrically equivalent prism sectors displaying enrichment in Al, Ti, Na, and Cr, Si, and Mg depletion, this is interpreted as reflecting low degrees of undercooling (ΔT =13-25^oC). No dendritic growth at the Al sector boundaries was observed. 

Subduction of the Farallon Plate during the late Cretaceous facilitated mantle partial melting and the generation of alkali-rich magmas. However, previous metasomatic events throughout the history of the western US over c. 2Gyrs likely established a geochemically heterogeneous, hydrous, and fertile mantle lithosphere which ultimately produced the highly-alkalic shonkinitic magmas of the AHVF.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

doi: 10.1130/abs/2026SE-12756

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