Abstract

In western Montana, a 15-km-thick sequence of allochthonous Belt Series metasedimentary rocks is thought to have moved eastward off the Northern, or Bitterroot, lobe of the emergent Idaho batholith. The zone of detachment south of Missoula, defined by mylonitic rocks ∼500 m thick, is largely in the batholith but traverses the granite-metasediment boundary. Later chloritic breccias, characterized by intense transgranular fracturing, overlie and locally transect the mylonites.

In granite and pegmatite mylonites, quartz (δ18O = 10.4 to 11.1) and coarse muscovite (8.4‰) yield isotopic temperatures of 550 ± 50 °C, interpreted as the temperature of ductile deformation, and would have involved fluids of 9.5 ± 0.5‰, which is close to the magmatic or high-temperature metamorphic range. K-feldspar (7.4 to 1.3‰), fine muscovite (8.3 to 6.8‰), and biotite (2.0 ± 0.2‰) have been shifted by variable magnitudes to low δ18O, which reflects post-mylonite, meteoric-water incursion resetting, less retentive minerals, an effect also observed in undeformed precursors to mylonites. Mylonitic metasedimentary rocks 25 km from the Bitterroot Lobe have not been isotopically disturbed by exchange with meteoric water but retain isotopic concordancy for coexisting quartz, k-feldspar, and muscovite, corresponding to estimated temperatures of 450 to 500 °C the ambient conditions of ductile deformation.

In chloritic breccias, quartz and feldspar have undergone shifts of about −10‰ relative to their values in mylonitic granites and exhibit disequilibrium fractions (Δ = 5.3 to 9.6‰), due to preferential exchange of feldspar down to lower temperatures. Albite and chlorite yield temperatures of 370 °C (Δ = 4.5‰) to 250 °C (Δ = 6.3‰), and calculated δ18O of fluids in equilibrium with albite is −7 (370 °C) to −11.8 (250 °C). This temperature range is corroborated by fluid-inclusion data.

High deduced temperatures and fluid isotopic compositions in the mylonites are commensurate with a pluton roof-zone environment at near magmatic conditions, providing enhanced ductility in mylonites. The chloritic breccia is regarded as a structural domain that accommodated late movement of the overlying rocks subsequent to removal of the main cover and provided conduits for incursion of low-temperature meteoric waters.

The structural sequence reflects the change from high-temperature ductile deformation of mylonites, under conditions of crust-equilibrated fluids at low water/rock ratio, to a regime of brittle fracturing at diminished temperatures and lower confining stress. In the latter structural environment, hydrological communication to the surface promoted elevated water/rock ratios in the domains of fracturing.

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