Geochronology of the northern Idaho Batholith and the Bitterrootmetamorphic core complex; magmatism preceding and contemporaneous withextension

Abstract

Granitic plutonism and extension are broadly contemporaneous in manymetamorphic core complexes. However, the relationship betweenmagmatism and extension is rarely unambiguous. The northern Idahobatholith (Idaho-Bitterroot batholith), Montana and Idaho, composesthe footwall for most of the Bitterroot metamorphic core complex andthus is an ideal area for assessing the relationships betweenmagmatism and extension. We analyzed zircon from six samples ofgranitic rock from the Idaho-Bitterroot batholith using the SHRIMP(II) ion microprobe. Three samples of mylonitic granite from the BearCreek pluton, Lost Horse Canyon, give a weighted mean (super 206) Pb/(super 238) U age of 54.3+ or -0.7 Ma. A protomylonitic granite fromthe central part of the Bitterroot core complex (also Bear Creekpluton) gives a similar (super 206) Pb/ (super 238) U age of 54.6+ or-0.8 Ma. Mylonitic megacrystic granite from Sweathouse Canyon yieldsan age of 63.6+ or -0.6 Ma. A granite sample from the Lochsa Canyon,in the central Idaho-Bitterroot batholith, gives an age of 56.7+ or-1.0 Ma. Inherited zircon from the granitoids ranges in age from 800to 1820 Ma, but the majority of grains have formation ages of1750-1800 Ma. This suggests that Paleoproterozoic crust dominates thesource region of the Idaho-Bitterroot batholith. Hornblende (super 40)Ar- (super 39) Ar age spectra for mafic dikes intruded duringlate-stage crystallization of main-phase granite in the centralIdaho-Bitterroot batholith suggest crystallization of the main-phaseplutons in this area at ca. 57 Ma. New and previously published (super40) Ar- (super 39) Ar and K-Ar apparent ages of biotite and muscovitefrom the Lochsa River area and the western and central Bitterroot corecomplex are 50 to 47 Ma. Younger mica ages (46-43 Ma) are restrictedto the vicinity of the Bitterroot mylonite zone. These resultsindicate that the cessation of main-phase magmatism within theBitterroot metamorphic core complex migrated east with time, and thatmost of the plutons in the core complex were intruded during thePaleocene and early Eocene. When the regional tectonic setting changedfrom compression to extension at ca. 50 Ma, the late stages ofmid-crustal, peraluminous plutonism appear to have been localizedwithin the Bitterroot core complex. The presence of the youngestmid-crustal plutons in this area may have focused extensionaldeformation leading to the thick mylonite zone, as a consequence ofrheological contrasts with cooler areas to the east and west. Aprogression of K-Ar and (super 40) Ar- (super 39) Ar cooling ages fromwest to east within the core complex part of the batholith isconsistent with top-to-the-east shear indicators in the mylonite zone.Thermochronology indicates that the western part of the Bitterrootmetamorphic core complex was below nearly equal 350 degrees C at thesame time as the last stage of granite emplacement and metamorphism inthe east. Therefore, the transition from mylonitization to brittledeformation to inactivity of the shear zone was progressive from westto east across the core complex from ca. 50 to 44 Ma. These featuresoffer an explanation for the previously enigmatic occurrence ofamphibolite facies ductile deformation in the eastern part of the corecomplex coincident with emplacement of epizonal, alkali-feldspargranite plutons along the western side of the complex.

10 samples from this reference