Syn‐thinning pluton emplacement during Mesozoic extension in eastern Mongolia

[1] This paper documents relationships between deformation and magmatic activity that occurred in the central part of eastern Mongolia during late Mesozoic continental-scale NW-SE extension. Two coarse-grained, biotite-bearing, syn-tectonic intrusions are described. The Nartyn granite that extends over an area greater than 30 by 10 km was emplaced within low-grade metasediments and shows a weak pervasive, magmatic fabric reworked by solid-state deformation along its margins. The northwestern roof of the granite is marked by a normal shear zone, the Choyr Shear Zone, characterized by top-to-the-NW motions. The shear zone is overlain by the Choyr Basin, which is filled with unmetamorphosed continental sedimentary rocks of early Cretaceous ages. From structural and geochronological data, we propose that the Nartyn massif was emplaced as a flat laccolith-type intrusion at ca. 136–130 Ma during crustal thinning. The Altanshiree granite, located ∼140 km east of the Nartyn granite, is a syn-kinematic pluton of similar age (134–128 Ma), also emplaced during crustal thinning. In the Nartyn and Altanshiree areas, extension implies pervasive crustal thinning, combined with limited exhumation. These areas are different from classical metamorphic core complexes, where strong strain localization along detachments induces exhumation of hot middle to lower crust. Results also suggest that early Cretaceous syn-extensional intrusions are an important feature of the tectonic history of eastern Mongolia.