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Full text release has been delayed at the author's request until May 10, 2026
ETD Abstract Container
Abstract Header
Insights Into Shonkinite Petrogenesis from the Adel Hills Volcanic Field, Montana
Author Info
Cracas, Thomas Joseph
ORCID® Identifier
http://orcid.org/0009-0001-3997-0309
Permalink:
http://rave.ohiolink.edu/etdc/view?acc_num=miami1745416271229098
Abstract Details
Year and Degree
2025, Master of Science, Miami University, Geology and Environmental Earth Science.
Abstract
Shonkinites are rare alkali-rich igneous rocks found in various tectonic settings. This study investigates the Upper Cretaceous shonkinites from the Adel Hills Volcanic Field (AHVF) in central Montana. The AHVF shonkinites exhibit porphyritic textures, featuring large pyroxenes with a mineralogical composition defined by clinopyroxene (diopside), plagioclase, K-feldspar, and secondary zeolite. SEM-EDS aided in identifying magnetite, rare ilmenite, rare pyrite, and calcite. Bulk rock wt. % SiO2 ranges from 47 to 49 at wt. % Na2O + K2O from 5.50 and 7.34 wt. %. Normalized bulk trace elements indicate LILE enrichment and Nb-Ta depletion, consistent with a volcanic arc setting. Bulk rock and mineral chondrite-normalized REE signatures lack an Eu-anomaly. For diopside, Si- and Mg-enrichment is accompanied by Al, Ti, Na, and Cr-depletion on {-111}. This is interpreted as resulting from low degrees of undercooling during crystallization (ΔT = 13-25 degrees C). Apatite Ce/Ce* vs. Eu/Eu* systematics are consistent with a hydrous, oxidizing magmatic environment in which plagioclase crystallization was suppressed. The AHVF shonkinites are interpreted to have formed in a contractional environment where a metasomatized arc mantle wedge underwent low-degree partial melting. Future research should employ bulk rock and in situ isotopic analysis to assess contributions from crustal and mantle sources.
Committee
Claire McLeod (Advisor)
Pages
112 p.
Subject Headings
Geology
;
Petrology
Keywords
igneous petrology
;
alkaline igneous rocks
;
shonkinite
;
Montana
;
Adel Hills
;
alkali igneous rocks, alkaline igneous systems
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Refworks
EndNote
RIS
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Citations
Cracas, T. J. (2025).
Insights Into Shonkinite Petrogenesis from the Adel Hills Volcanic Field, Montana
[Master's thesis, Miami University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=miami1745416271229098
APA Style (7th edition)
Cracas, Thomas.
Insights Into Shonkinite Petrogenesis from the Adel Hills Volcanic Field, Montana.
2025. Miami University, Master's thesis.
OhioLINK Electronic Theses and Dissertations Center
, http://rave.ohiolink.edu/etdc/view?acc_num=miami1745416271229098.
MLA Style (8th edition)
Cracas, Thomas. "Insights Into Shonkinite Petrogenesis from the Adel Hills Volcanic Field, Montana." Master's thesis, Miami University, 2025. http://rave.ohiolink.edu/etdc/view?acc_num=miami1745416271229098
Chicago Manual of Style (17th edition)
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Document number:
miami1745416271229098
Copyright Info
© 2025, all rights reserved.
This open access ETD is published by Miami University and OhioLINK.