The Cambrian Volcanosedimentary Strata of the Systighem Terrane (Central Tuva): Results of Isotopic-Geochronology and Geochemical Studies

The article presents the first results of isotope-geochemical (Sm‒Nd, Rb‒Sr) and geochronological (U‒Pb LA-ICP-MS) studies of Cambrian volcanosedimentary strata (Terek and Ezhim formations) of the Systighem terrane (Central Tuva). It was established that the origin of the primary basaltic magmas of the Terek and Ezhim formations is associated with partial melting of a mantle source (OIB type), metasomatically worked through by processes associated with contamination during melting of a dehydrated slab. Their formation occurred in rift basins marking extension processes in the rear part of the accretionary complex with transitional or continental crust against subduction of Paleo-Asian Ocean crust underneath it. The formation of the volcanosedimentary strata of the Terek and Ezhim formations occurred nearly simultaneously in the interval 512‒510 Ma. Similar isotope-geochemical characteristics of the composition of volcanic rocks of the studied formations (ε_Nd(510) = +3.9 to –0.45 and ε_Nd(510) = +4.5 to –0.23) and the time of their formation indicate a single sequence, which undergoing folding or other tectonic processes was divided into disconnected fragments, deformed, and displaced relative to each other by horizontal movements during accretion–collision events. On the modern erosional section, they are represented by a series of combined tectonic plates, altered to varying degrees, thrust upon each other and, on an existing and buried continental block.