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Paper No. 12
Presentation Time: 10:55 AM


LINK, Paul K.1, CAMERON, Angela1, AUTENRIETH, Kathleen2 and FANNING, C. Mark3, (1)Department of Geosciences, Idaho State University, Pocatello, ID 83209, (2)Department of Geosciences, Idaho State University, Mail Stop 8072, Pocatello, ID 83209, (3)Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia,

In the Pioneer Mountains core complex of south-central Idaho, the structurally lowest metamorphic rocks of the Wildhorse Gneiss Complex (lower gneiss) include metaquartzite and calc-silicate. Three quartzite samples were tested for detrital zircons. Two have solely Archean detrital grains, with ages from 2.4 to 3.3 Ga, One has a sharp detrital peak at 1.7-1.8 Ga.

The samples lacking Paleoproterozoic grains and containing a diversity of Archean grains are unique. Similar detrital-zircon spectra have been sampled in the quartzite of Yost at Raft River Narrows, Utah. Further, the Clarks Basin Quartzite on the northern flank of the Albion Range, resembles one sample from the lower Wildhorse gneiss in having a strong 1.7-1.8 Ga peak and no younger grains.

The middle unit of the Wildhorse complex is a quartzo-feldspathic Neoarchean orthogneiss, from which four SHRIMP Concordia ages have been obtained, all between 2.6 and 2.5 Ga. This unit is similar in age to Archean orthogneiss of the Grouse Creek block south of the Snake River Plain, but also may be related to 2.45 Ga basement of the Beaverhead Mountains to the east.

The upper gneiss of the Wildhorse complex overlies the Archean orthogneiss across a ductile extensional shear zone. It contains calc-silicate and metaquartzite paragneiss with detrital grains from 1.45 to 1.85 Ga, the same age-range as the Mesoproterozoic Lemhi Group in the Beaverhead and Salmon River Mountains. The upper gneiss is intruded by 695 Ma orthogneiss (four samples have been dated) which is likely related to Rodinian rifting, and to coeval volcanic rocks near Edwardsburg, central Idaho.

Thus, the Wildhorse Gneiss Complex contains a basal package of Paleoproterozoic or younger quartzites with affinity with the Albion Range south of the Snake River Plain. Overlying this is Neoarchean orthogneiss, itself overlain by metasedimentary Mesoproterozoic Lemhi Group. Thrusting of Archean orthogneiss over Proterozoic quartzite is the simplest way to achieve these relations. This basement thrust may represent the first episode of crustal thickening in what was to become the Pioneer Mountains core complex, and it may even be a product of the Antler orogeny.

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