Protolith Determination of Mylonitic Rocks adjacent to the Madison Mylonite Zone of the Antelope Basin, Henry’s Lake Mountains, Madison County, southwestern Montana Jesse Davenport

What is Keck? Dr. Tekla Harms, Dr. Jack Cheney, and Kristina Doyle – Amherst College Dr. John Brady and Ali Walker – Smith College Danielle Lerner – Mt. Holyoke Caleb Lucy - Williams College Parker Haynes - University of North Carolina

Tectonic Setting: Laurentia Laurentia  craton, stable Seven Archean Provinces – Wyoming, Burwell, Rae, Nain, Slave, Superior, and Hearne Laurentia is the result of numerous collisions and tectonic events (Hoffman 1988)

Tectonic Setting: Wyoming Province Accretion and collision along a SW- NE trend at ~1.8 Ga Hearne, Medicine Hat, and Wyoming collide while simultaneously moving toward the Superior (Burger et al. 2005)

Big Sky Orogeny Two events characterize the orogeny (along a P-T path) – M1, a high pressure/high temperature phase Widespread throughout all rock types – M2, lower temperatures and pressures leading to isothermal decompression Widespread throughout all rock types

Objectives Petrologic analysis of samples via hand sample and thin section analysis Determine protoliths of collected mylonitic rocks via thin section, and geochemical analyses – Characterize alteration, metamorphic assemblages, and other pertinent areas (not yet decided upon Compare results to geochemical results found by fellow Keck colleagues and by Erslev and Sutter (1990)

Study Area

(Erslev 1983)

Study Area

Mylonites According to Vernon (2004), mylonites are strongly foliated and lineated rocks that: – Occur in discrete zones (mylonite zones or ductile shear zones) – Show higher strain accumulation and finer grain size than the adjacent rocks.

Mylonites: Madison Mylonite Zone (Erslev and Sutter 1990)

Mylonites (Vernon 2004)

Mylonites

Methods

Future Work Finish powdering and send out samples for analysis Receive thin sections and analyze them Continue writing I.S. Attend January workshop to be held at Amherst College

The Keck Consortium The Keck Consortium Dr. Tekla Harms, Dr. Jack Cheney, Kristina Doyle (Amherst College), Ali Walker and Dr. John Brady (Smith College), Danielle Lerner (Mt. Holyoke), Caleb Lucy (Williams College), and Parker Haynes (University of North Carolina) Dr. Tekla Harms, Dr. Jack Cheney, Kristina Doyle (Amherst College), Ali Walker and Dr. John Brady (Smith College), Danielle Lerner (Mt. Holyoke), Caleb Lucy (Williams College), and Parker Haynes (University of North Carolina) Dr. Shelley Judge and Dr. Meagen Pollock Dr. Shelley Judge and Dr. Meagen Pollock Acknowledgements

References Burger, H.R. et al., 2005, Paleoproterozoic metamorphism in the northern Wyoming Province; Implications for the assembly of Laurentia: Journal of Geology, v. 113, no. 2, p Erslev, E.A., 1983, Pre-Beltian geology of the southern Madison Range, southwestern, Montana: Montana Bureau of Mines and Geology Memoir 55, 26 pp. Erslev, E.A., and Sutter, J.F., 1990, Evidence for Proterozoic mylonitization in the northwestern Wyoming province: Geological Society of America Bulletin, v. 102, p Hoffman, P.F., 1988, United plates of America, the birth of a craton; early Proterozoic assembly and growth of Laurentia: Annual Review of Earth and Planetary Sciences, v. 16, p Vernon, R.H., 2004, A Practical guide to Rock Microstructure: Cambridge University Press, Cambridge, U.K., 594 pp. Visitor Information, 2009, International Choral Festival Homepage (WWW document). URL: Accessed October 4, 2010.

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