Abstract

The Needle Falls Shear Zone is a major Paleoproterozoic discontinuity separating an Archean craton from accreted Paleoproterozoic terranes in the western Trans-Hudson Orogen. It is a 1 km-wide subvertical mylonite zone forming the southwestern sector of a northeast-trending shear system extending for more than 400 km. Kinematic data indicate oblique, east-side-up dextral strike-slip displacement throughout its evolution from ductile through late brittle deformation. For most of its exposed length, it marks the northwestern margin of the Wathaman Batholith. The oldest Pb-evaporation ages of zircons from mylonite average 1857 Ma, consistent with published zircon ages of the Wathaman Batholith, confirming the batholith is protolith for much of the shear zone. The synkinematic metamorphic assemblage of amphibole–biotite–quartz–feldspar signifies deformation under amphibolite-facies conditions (500–630°C). Oxygen isotope fractionations between quartz–hornblende and quartz–biotite pairs in textural equilibrium give temperatures from 430°C to 610°C, with a mode at 550°C. For hornblende–biotite pairs, 40Ar/39Ar plateau ages of hornblende cluster at 1780 Ma and of biotite at 1757 Ma. Given textural and oxygen isotope equilibrium between hornblende, biotite, and quartz, these ages are interpreted as cooling ages; consequently, this sector of the Trans-Hudson Orogen cooled from 500°C to 300–350°C over about 23 million years following ductile deformation. The timing of ductile deformation cannot be well constrained, but occurred after emplacement of the Wathaman Batholith at 1857 Ma and prior to cooling of amphibole below its Ar blocking temperature at 1780 Ma.

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