Home – Home – Jornals – Russian Geology and Geophysics 2008 number 11

The recognition of pre-Riphean metamorphosed residual crust of deep chemical weathering on the Chuya-Nechera (Chuya-Kodar) granites on the basis of found gravel-like quartz-sericite rocks (apparently, residual products of feldspar hydrolysis) is still debatable. Study of this crust in relation with deep-level hypergenesis shows that it consists of not only quartz pseudogravelstones of dispersed paleozone but also pseudoquartzite schists, pseudoquartzites of lithomarge paleozone, and physically weathered granites of clastic and fissure zones in its bottom. Inequigranular (pisolitic, fine- and thin-grained) fissured quartz differentiated with depth is the main indicator of this crust. This quartz and associated sericite are the lithified and metamorphosed direct (hydrolysate alumina), side (hydrolysate silica), and residual (large-ingrowth quartz) products of the chemical decomposition of feldspar. Some of them accumulated in situ - in lateritic crust, and others were redeposited - first, in the lower layers of the weathering profile and then, in the residual crust. The early quartz and sericite accumulations in the weathering profile date the formation of peneplain, and the late ones, marking the residual crust, date the beginning of the marginal-sea transgression onto the passive continental margin, the peneplain erosion, and the origin of the Bodaibo inner trough.