High-magnesium picrite–basalt associations of the Tunka terrane (Baikal–Hövsgöl region) as an indicator of the back-arc basin spreading

High-Mg metabasalts and metapicrites discovered within the Urtagol Formation in the central zone of the Tunka bald mountains (East Sayan) are studied. In geochemistry the high-Mg metavolcanics are similar to subductional rocks. We have established that the Nb-rich recycled material of oceanic crust (RSC) was a source of elements for high-Ti metabasalts, and subductional fluid rich in LREE and Th relative to Nb was the source of these elements for high-Ti metapicrites. The enrichment of low-Ti metavolcanics, formed, probably, at the early stages of the basin opening, was due to the contamination of melt with continental-crust material. A comparison of the metavolcanics with nonmetamorphosed analogs is made, and some genesis aspects are considered. The results obtained led to the conclusion that the metavolcanics mark the paleospreading of the back-arc basin.