Title

Geochronology of the House Mountain Gneiss Complex in the Atlanta Lobe of the Idaho Batholith

Publication Date

4-2007

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology, Geochronology

Department

Geosciences

Major Advisor

Mark D. Schmitz

Abstract

The House Mountain gneiss complex is a ~100 km2 exposure of middle to upper amphibolite facies ortho- and paragneisses within the Atlanta lobe of the Idaho Batholith, south central Idaho. The complex comprises a paragneiss suite juxtaposed between and intruded by a tonalitic gneiss (structurally above) and a granitic gneisses (structurally below). The entire complex contains a pervasive NW-striking, E-dipping foliation, and is intruded by deformed concordant and nondeformed discordant granitic pegmatite sills and dikes, and nondeformed granite of the Idaho Batholith. As remnants of the pre-batholith crust, these gneisses provide rare and valuable insights into the pre-Cretaceous geologic history of the region. U/Pb zircon data for six lithologies within this gneiss complex illustrate a tectonomagmatic evolution spanning the Neoproterozoic to Latest Cretaceous. Single zircons from the granitic Lower Orthogneiss, imaged with cathodoluminescence to reveal magmatic growth zones and probable metamorphic rims, define a chord with an upper intercept of 726 ± 5 Ma (igneous crystallization) and lower intercept of 82 ± 15 Ma (age of younger metamorphic recrystallization and overgrowth). These data confirm the presence of Precambrian crust in southwestern Idaho, specifically within the region of the central Atlanta lobe. Detrital zircons extracted from a quartzite unit within the paragneiss sequence yielded ages ranging from 2948 to 587 Ma. However, the 587 Ma age for the youngest grain is suspected of being the result of mixing between two domains within the grain, one very young and one older than 726 Ma. Both the Lower Orthogneiss and the paragneiss sequence are likely related to the rifting of the supercontinent Rodinia. Based on lithological similarities and approximate age synchrony, correlations can be drawn between the paragneiss sequence and the Windermere Supergroup of central Idaho, specifically the Gospel peaks sequence A of Lund (2003). Zircons from the tonalitic Upper Orthogneiss, displaying simple oscillatory zoning in CL, yield an igneous crystallization age of ca.157 Ma, potentially linking Late Jurassic plutonism in Northern Nevada and the Sierra Nevada Batholith with NW Idaho. Multifaceted, low-U zircons, most likely of metamorphic origin, from an ~10m thick amphibolite sill within the paragneiss stack give concordant ages of ca.122 to 125 Ma. These ages are interpreted as a first constraint on the age of amphibolite-facies metamorphism of the House Mountain gneisses. Petrography reveals that penetrative deformation was synchronous with and outlasted peak metamorphism. Four zircons from one of the many deformed P1 pegmatite sills within the Lower Orthogneiss define an intrusive age of 82 Ma. This age coincides with the age of magmatic overgrowth recorded in the Lower Orthogneiss zircons and, due to the geometry of the sills, provides a date for at least a portion of the kinematic history of the gneisses. The P1 pegmatites are inferred as being related to the early stages of Idaho Batholith magmatism. The fifth sample analyzed is one of numerous nondeformed P2 pegmatitic dikes intruding the Lower Orthogneiss. Zircons from this sample produce near concordant crystallization ages of ca. 66 Ma, and help to constrain the end of kinematic deformation in the gneisses. The P2 pegmatites are inferred as being related to decompressional melting following the initiation of exhumation of the House Mountain area, which was completed by the time of the Eocene Challis Volcanic event.

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