Abstract

Border-zone rocks of the northeastern Idaho batholith have been subjected to multiphase penetrative deformation, regional metamorphism up to sillimanite-orthoclase grade, and multiple intrusion. Intrusion was accompanied by diapiric uprise of the plutonic-metamorphic complex, with extensive cataclasis and flow folding along the eastern margin of the rising gneiss dome. Normal faulting followed, with mylonitization and greenschist-grade metamorphism in local shear zones.

Ages of some thermal events are estimated as follows: 85 ± 35 m.y. for a metamorphic event that affected quartzofeldspathic gneiss (Rb/Sr whole-rock line of best fit); 82 ± 10 m.y. for the emplacement of a quartz-diorite orthogneiss (estimate based on U/Pb isotopic ratios from a single zircon fraction); 66 ± 10 m.y. for the main stage of batholithic emplacement (U-Pb Concordia–lower intercept age); 46 ± 5 m.y., 42 ± 8 m.y., and 39 ± 2 m.y. for Rb/Sr isotopic equilibration late in the thermal evolution of the plutons (Rb/Sr mineral isochrons).

The older dates conform generally to the range of intrusive ages assigned to the Flint Creek plutons and the Boulder batholith to the east. The younger dates reflect the igneous-hydrothermal-tectonic event of Eocene-Oligocene time which has reset K-Ar and fission-track (apatite) dates within the general Idaho batholith igneous complex.

Magmas that formed the northeastern Idaho batholith were derived from, or contaminated with, older crustal material. This statement is supported by a 1,900- to 2,250-m.y. upper intercept age for batholithic zircons, some of which show euhedral overgrowths on anhedral cores, and by relatively high whole-rock Sr87/Sr86 ratios from samples with low Rb87/Sr86 ratios.

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