Quartzite transformation as a consequence of late tectonic events in the evolution of the Chuya and Gargan blocks

We consider the evolution of the tectonic setting and geochemical features of rocks in ultrapure quartz areas of the Baikal region. These are the Chuya uplift of the northern Baikal region, with the Tyya quartz deposit, as well as the Bural-Sar’dag and other deposits in the Gargan block of East Sayan. Similarities are observed in the sedimentation and tectonic evolution of the regions. The conditions of the deposition of quartzites (forearc basins within siliceous-carbonate rocks with slight tholeiitic volcanism and a small amount of terrigenous material) ensure their relative initial purity. Low-grade quartzite-bearing strata are bedded or thrust by collision over an older high-grade basement consisting of a tonalite–trondhjemite–granodiorite association (Gargan block) or active-continental-margin metavolcanics (Chuya uplift). Migmatization of the basement and the intrusion of granitoids therein cause its uplifting and fault displacements and erosion of the overlying rocks. If the thrusting is associated with retrograde metamorphism of the basement and progressive metamorphism of the cover, backward motion can cause only quartz recrystallization with its purification and the grinding of the other minerals. This leads to the formation of lenses of “superquartzites,” mylonites, cataclasites, and breccias in the autochthon–allochthon suture and within the allochthon.