29-1 Intruded intrusions of the Flint Creek Range, western Montana: implications for the Sevier-Laramide Orogeny

Session: Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories (Posters)

Poster Booth No.: 1

Presenting Author:

Megan Stuart

Authors:

Stuart, Megan E.¹, Mannino, Isabella², Pinder, Kyra R.³, Webber, Jeffrey R.⁴, Regan, Sean P.⁵, Roberts, Nicolas M.⁶, Peterman, Emily⁷

(1) Geology Program, Stockton University, Galloway, , (2) Geology Program, Stockton University, Galloway, , (3) Geology Program, Stockton University, Galloway, , (4) Geology Program, Stockton University, Galloway, , (5) Department of Geosciences, University of Alaska, Fairbanks, , (6) Geosciences Program, Hamilton College, Clinton, , (7) Department of Earth and Oceanographic Science, Bowdoin College, BRUNSWICK, ,

Abstract:

The Flint Creek Range of western Montana hosts intrusions that spatially and temporally coincide with the culmination and collapse of the Sevier-Laramide Orogen. Foliated Late Cretaceous mafic intrusions host the Paleocene two-mica Mount Powell Batholith (MPB), which defines the easternmost extent of the Anaconda Metamorphic Core Complex. As such, this location occupies an important transition between contractional orogenesis and crustal extension, where intrusions may play active and passive roles through evolving structural regimes. Yet, few constraints are established here for the timing of pluton emplacement, structural control of intrusions, and the role of magmatism in the evolution of the Cordillera. Our investigation combines field observations with Anisotropy of Magnetic Susceptibility (AMS), electron backscatter diffraction (EBSD), and U-Pb zircon geochronology to better define the timing and significance of Cretaceous--Paleogene magmatism within western Montana. Magnetic susceptibility data collected at Hamilton College from samples of mafic rocks (k_m ~ 3x10^-2 SI) and two-mica granite (k_m~2x10^-3 SI) indicate that mafic intrusions contain up to 1% magnetite by volume, whereas granitoid intrusions contain less than 0.06% magnetite by volume. The abundance of ferrimagnetic minerals such as magnetite within mafic units suggest that the magnetic fabric obtained through AMS may represent a deformational fabric. Preliminary data obtained from the mafic host of the MPB indicates a magnetic foliation of 214°, 28°; which is concordant to the orientation of a contact between the mafic unit and a pegmatite dike of two-mica granite (211°, 36°). This suggests that subsolidus deformation of the mafic intrusion occurred prior to, or during emplacement of the MPB. EBSD analyses collected at Bowdoin College will provide insight to aspects of magmatism and associated subsolidus fabrics by defining compositional variations through the quantification of mineralogical modes and may resolve deformation temperatures of metasedimentary host units. Preliminary results from opening-angle thermometry based on quartzite host rock, suggest deformation temperatures of ~700℃. Preliminary U-Pb zircon geochronology obtained from the Arizona LaserChron Center includes one age for the MPB (64.78 +/- 0.52 Ma) and one age for the mafic host (75.27 +/- 0.39). These results suggest the mafic intrusion was emplaced during contractional orogenesis and subsequently deformed, whereas Paleocene intrusion of the MPB may represent the initiation of orogenic collapse.

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