PRELIMINARY PETROLOGICAL INSIGHTS INTO SHONKINITE MAGMATISM FROM THE ADEL HILLS VOLCANIC FIELD, MONTANA

Shonkinites are a rare type of alkali-rich igneous rock found throughout the geological record. They occur across tectonic settings, are often associated with REE enrichments, and provide an opportunity to investigate alkali magma petrogenesis. This study focuses on the Upper Cretaceous shonkinites found in the Adel Hills Volcanic Field (AHVF) of central Montana and seeks to understand shonkinite petrogenesis within the context of the tectonomagmatic evolution of the Cordilleran fold and thrust belt. Previous 40Ar/39Ar and K-Ar geochronology constrains volcanic activity in the AHVF to a 2-3 Myr time period ca. 76-73 Ma with the shonkinites representing shallow-level intrusives that cross cut volcaniclastics. Samples which form the basis of this study originate from the north-central region of the AHVF. Hand samples are porphyritic and contain large glossy pyroxenes (up to 1.1 cm in maximum width) present up to 30-35 vol. %. In thin section, euhedral clinopyroxenes are sometimes visibly sector zoned and contain inclusions of apatite, oxides, and K-feldspar; minor plagioclase occurs throughout the groundmass. Initial in-situ analysis of clinopyroxenes (n=100) via EPMA from one shonkinite thin section reveal them to be consistently diopside: En35-42, Fs11-17, Wo44-49. Corresponding Mg#s range from 68-79. High resolution elemental mapping via EPMA of one diopside crystal reveals sector zonation in addition to chemical zonation which is oscillatory at the micron scale. In situ analysis of plagioclase (n=24) reveals a range of compositions from anorthite (An96) to albite (An3) while all analyzed oxides (n=12) are shown to be magnetites with Mg#s from 13-17 and wt. % TiO2 from 3.9-5.2. Future work will involve bulk-rock characterization utilizing XRD for major and minor mineralogy, followed by XRF and ICP-MS. This will be aided by the in-situ study of additional thin sections via SEM-EDS and in-situ analysis of major and minor mineral components via EPMA and laser-ablation (LA)-ICP-MS). If possible, U-Pb analysis of apatites will be pursued. Collectively, the data of this study seeks to provide new insights into shonkinite magma petrogenesis and the potentially dynamic processes occurring during magma fractionation, ascent, and eruption.