HR: 17:30h
AN: T14B-07    [Abstracts]
TI: Evolution of a long-lived magmatic center in Idaho
AU: * Gaschnig, R M
EM: gaschnig@wsu.edu
AF: School of Earth and Environmental Sciences, Washington State University, PO Box 642812, Pullman, WA 99164-2812, United States
AU: Vervoort, J D
EM: vervoort@wsu.edu
AF: School of Earth and Environmental Sciences, Washington State University, PO Box 642812, Pullman, WA 99164-2812, United States
AU: Lewis, R S
EM: reedl@uidaho.edu
AF: Idaho Geological Survey, U of Idaho, PO Box 443014, Moscow, ID 83844, United States
AU: Dufrane, A
EM: dufrane@wsu.edu
AF: School of Earth and Environmental Sciences, Washington State University, PO Box 642812, Pullman, WA 99164-2812, United States
AB: The Idaho batholith/Challis magmatic system is characterized by a history of magmatism roughly 75 million years in length, marked by major shifts in the locus and composition of magma in an evolving tectonic setting, making it a key but poorly understood element in the Cordilleran batholith system of western North America. Integration of new in situ U-Pb geochronology and Hf isotopic analysis of zircon with existing data is beginning to elucidate the story told by this long-lived magmatic center. Magmatism in this region began around 125 Ma, during the terminal stages of the accretion of the Wallowa- Seven Devils terrane to North America. Small, deformed tonalite, trondhjemite, and quartz diorite plutons range in age from 125 Ma to about 100 Ma and intrude the suture zone formed by this collision. These early plutons are characterized by primitive epsilon Hf values (~ +10) indicative of a dominantly mantle source. These were followed around 90 Ma by a more voluminous pulse of tonalitic magmatism just east of the suture zone and the emplacement of a discontinuous belt of metaluminous hornblende-bearing plutons approximately 70 km to the east, all with more unradiogenic Hf and Nd (~ -5 to -10) isotopic signatures. The main pulse of peraluminous magmatism which produced the biotite granodiorite and muscovite-biotite granites of the Atlanta lobe of the Idaho batholith lasted roughly from 85 to 70 Ma. This was distinct from a later episode of magmatism that formed the Bitterroot lobe to the north, which was focused between 62 and 54 Ma. While similar to the Atlanta lobe in terms of petrography and major and trace element geochemistry, the Bitterroot lobe shows a noticeably less radiogenic Hf isotopic signature with epsilon values ranging from -15 to -21, compared to a range of -8 to -12 for the Atlanta lobe. These isotopic signatures, coupled with the ubiquitous presence of inherited Paleoproterozoic zircon cores with epsilon Hf values ranging from -30 to -50, suggest that the bulk of the batholith is largely the product crustal recycling with a relatively small mantle component. Space, time, and compositional patterns suggest that the mass contribution of primitive arc magmas never was comparable to Cordilleran batholiths, and decreased through time as the crust was progressively thickened. Only during the Eocene relaxation of the lithosphere, corresponding to the bimodal Challis magmatic flare-up and a regional southward sweep in magmatism, did mantle melts reestablish a significant direct role in the modification of the lithosphere in this sector of the Cordillera.
DE: 1040 Radiogenic isotope geochemistry
DE: 1115 Radioisotope geochronology
DE: 8104 Continental margins: convergent
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting