Figure 1. Simplified geologic map of the Idaho batholith with U-Pb zircon ages determined as part of this project. Figure is modified from Gaschnig et al. (2010). The suites were defined on the basis of age range and bulk chemistry/lithology.
Figure 2. Cathodoluminescence (CL) image of zircons from the Idaho batholith with spots (30 micron diameter) dated by LA-ICP-MS, modified from Gaschnig et al. (2010). CL imaging reveals subtle chemical differences in zircons that reflect a kind of growth zoning and can be used to identify xenocrystic cores, such as in the right-hand zircon.
This work started as my Ph.D. and a short postdoc at Washington State University with Jeff Vervoort. It was part of a larger NSF-funded EarthScope project known as IDOR, which integrated seismic imaging with structural geology, geochemistry, and geochronology. The PIs (in addition to Jeff) are Basil Tikoff (U of Wisconsin-Madison), John Hole (Virginia Tech), and Ray Russo (U of Florida). (http://www.geophys.geos.vt.edu/hole/idor/ and http://serc.carleton.edu/earthscope_chronicles/idor_project.html)
I worked to develop a comprehensive geochronological portrait of the Idaho batholith and link the age progression to the isotopic evolution (using Sr, Nd, Hf, and Pb isotope tracers). Figure 1 shows the results of the zircon LA-ICP-MS U-Pb geochronology campaign. Ages for the Idaho batholith range from ~100 to 53 Ma and isotopic tracers (Figure 3) indicate a shift from mixed crust-mantle sources to largely crustal melting with time, a change that can be linked to regional crustal thickening. Subsequent lithospheric relaxation in the Eocene led to renewed mantle input leading to the formation of the Challis magmatic province. Superimposed on the trend of waning and waxing mantle input is the influence of different crustal terranes, which are readily distinguishable through differences in common Pb isotopes and xenocrystic zircon ages (see Figure 2). Overall, the history of the Idaho bathlith differs significantly from the Sierra Nevada and Peninsular Ranges batholiths in California and Baja California, and although the ages ranges bear a closer resemblance to the Coast Mountains batholith in British Columbia, magma compositions are fundamentally different, being predominantly peraluminous and isotopically evolved.
In addition to working on the Idaho batholith, I have more recently been working on the geochronology and geochemistry of older plutonic and meta-plutonic rocks in western Idaho, eastern Oregon, and the Idaho panhandle, and have been involved in research on Precambrian basement in the region.
The major papers on the Idaho batholith work that have so far been published are listed below:
Gaschnig, R. M., Vervoort, J. D., Tikoff, B., and Lewis, R. S., 2016, Construction and preservation of batholiths in the northern U.S. Cordillera: Lithosphere. pdf
Gaschnig, R. M., Vervoort, J. D., Lewis, R. S., and Tikoff, B., 2013, Probing for Proterozoic and Archean crust in the northern U.S. Cordillera with inherited zircon from the Idaho batholith: Geological Society of America Bulletin, v. 125, no. 1-2, p. 73-88. pdf
Gaschnig, R. M., Vervoort, J. D., Lewis, R. S., and Tikoff, B., 2011, Isotopic evolution of the Idaho batholith and Challis intrusive province, northern US Cordillera: Journal of Petrology, v. 52, no. 12, p. 2397-2429. pdf
Gaschnig, R. M., Vervoort, J. D., Lewis, R. S., and McClelland, W. C., 2010, Migrating magmatism in the northern US Cordillera: in situ U–Pb geochronology of the Idaho batholith: Contributions to Mineralogy and Petrology, v. 159, no. 6, p. 863-883. pdf
Additional papers dealing with this work have just been published in a special volume on the IDOR project in the journal Lithosphere. See my publications page.
Figure 3. Neodymium isotopic composition vs age for rocks of the Idaho batholith and Challis intrusive province. Nd isotopes can be used to distinguish between mantle-derived magmas (high values) and magmas derived from old continental crust (low values). In this case, mantle input into the system waned with time and gave way to pure crustal melting in response to crustal thickening until the establishment of an extensional tectonic regime in the Eocene allowed renewed mantle input.