Thermochronology of early Mesozoic shear in the East Gobi Fault Zone, Mongolia

Tectonic studies of the East Gobi Fault Zone in SE Mongolia reveal distinct intracontinental deformation events postdating Late Paleozoic arc accretion and continental amalgamation. Significantly, metamorphic tectonites of the Tsagan Subarga and Tavan Har blocks, previously mapped as Precambrian basement, comprise a shear zone dominated by steeply-dipping foliations and shallowly-plunging stretching lineations. Kinematic indicators such as sigma and delta-type objects, asymmetric boudinage, and S-C fabrics indicate sinistral shear sense. 40Ar/39Ar step-heating analyses of minerals from pre-, syn-, and late- to post-kinematic lithologies bracket the timing of ductile deformation as Middle to Late Triassic. The NE-trending shear zone has been documented over 250 km along strike and has been reactivated by at least three subsequent brittle deformation events in the Mesozoic and Cenozoic. The oldest 40Ar/39Ar age from the metamorphic rocks, 240.3 +/- 2.2 Ma (plateau ages and 2 sigma errors are reported), was obtained on amphibole from a migmatite and provides a minimum age for a partial melting event prior to activation of the shear zone. The timing of sinistral shear at amphibolite-facies conditions is constrained at ca. 225 Ma by 40Ar/39Ar dating of amphibole that defines the stretching lineation in amphibolite-facies gneiss (224.5 +/- 1.5 Ma), syn-kinematic white mica in garnet-grade metapelite (226.5 +/- 1.4 Ma), and amphibole from a syn-kinematic intrusion (223.3 +/- 1.7 Ma). Waning of ductile deformation, at least locally, is constrained by a 211.8 +/- 1.1 Ma 40Ar/39Ar age obtained on biotite from a dike that cross-cuts the foliation. While the dike appears undeformed in outcrop, quartz and feldspar microstructures indicate minor deformation at the microscopic scale. Cooling below ~300 deg C was achieved by the Early Jurassic. All apparent age spectra are complicated by argon loss profiles that, in most cases, based on geologic context and microstructural evidence can be related to Mesozoic and Cenozoic brittle deformation; minimum ages range from 191 to 47 Ma.