Protolith Determination of Mylonitic Rocks adjacent to the Madison Mylonite Zone of the

Antelope Basin, Henry’s Lake Mountains, Madison County, southwestern Montana

Jesse DavenportJesse Davenport

What is Keck?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: LaurentiaTectonic 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 ProvinceTectonic 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

ObjectivesObjectives• 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 AreaStudy Area

Study AreaStudy Area

(Erslev 1983)

Study AreaStudy Area

MylonitesMylonites

• 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 ZoneMylonites: Madison Mylonite Zone(Erslev and Sutter 1990)

Mylonites

(Vernon 2004)

Mylonites

MethodsMethods

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 ConsortiumThe Keck Consortium• Dr. Tekla Harms, Dr. Jack Cheney, Kristina Dr. Tekla Harms, Dr. Jack Cheney, Kristina

Doyle (Amherst College), Ali Walker and Dr. Doyle (Amherst College), Ali Walker and Dr. John Brady (Smith College), Danielle Lerner John Brady (Smith College), Danielle Lerner (Mt. Holyoke), Caleb Lucy (Williams College), (Mt. Holyoke), Caleb Lucy (Williams College), and Parker Haynes (University of North and Parker Haynes (University of North Carolina)Carolina)

• Dr. Shelley Judge and Dr. Meagen PollockDr. Shelley Judge and Dr. Meagen Pollock

AcknowledgementsAcknowledgements

ReferencesReferences• 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. 169-179.

• 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. 1681-1694.

• 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. 543-603.

• 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:http://www.choralfestival.org/pdf/Northwest.pdf. Accessed October 4, 2010.

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