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.     

Geochemistry of Calcareous–Silicate Rocks of the Kharagol Formation,Southern Baikal Region

Complete geochemical characteristics have been found for calcareous–silicate rocks of the Kharagol Formation that serves as a reference unit within the Khamardaban polymetamorphic composite terrain (southern folded framing of the Siberian Platform, Baikal–Khubsugul region). We studied the high-grade (granulite) domain known as Slyudyanka crystalline complex of the southern Baikal region. The Kharagol Formation is composed of different diopside, scapolite–diopside, and wollastonite–scapolite–diopside gneisses and crystalline schists with a variable calcite content. Petrogeochemical data suggest that their protoliths were tuffoids with different amounts of calcareous material. The volcanoclastic component of tuffoids is closest to the Andean-type calc-alkaline andesites. The Kharagol paleobasin was probably located at the active continental margin. Within the studied metamorphic sequence, the Kharagol Formation marks a sharp change in sedimentation conditions and may be considered a boundary of subterrains with protoliths formed in different paleogeodynamic settings. Geochemical indicators of calcareous–silicate rocks of the Kharagol Formation in different metamorphic zones may be high Cr, Ni, and Ba contents.     

Geochemistry, petrogenesis, and geodynamic typification of metavolcanics of the Tunka terrane (Baikal–Hövsgöl region)

The geochemical composition of metavolcanics (metabasalts and metaandesites) of the Tunka terrane is considered. They are differently enriched in incompatible elements relative to N-MORB. The geochemical features of the basalts (LILE and LREE enrichment, Zr and Hf negative anomalies) point to the suprasubductional nature of synsedimentary volcanism; at the same time, the rocks lack negative Nb, Ta, and Ti anomalies. Comparison of the Tunka metavolcanics with those of modern geodynamic settings shows their analogy to back-arc basin basalts.