THE BREN GRANITOID COMPLEX AND SAILYG VOLCANOGENIC DEPOSITION AS THE LATE ORDOVICIAN VOLCANO-PLUTONIC ASSOCIATION OF EAST TUVA

Mongush A.A., Sugorakova A.M.

Tuvinian Institute for Exploration of Natural Resources SB RAS, Kyzyl, Russia

amongush@inbox.ru, samina51@inbox.ru

From all diversity of the Paleozoic volcano-plutonic associations of the Altai-Sayan region, it is the granitoidal massifs and associating with them acid volcanogenic rock mass that provoke the most doubts about age and geodynamic position. Earlier, the association was supposed to be related to the Devonian stage of tectonomagmatic activity. With geological, paleontological, and geochronologic knowledge accumulated, the hypothesis on the Devonian age becomes increasingly unsupported (Khomichev, 1988; etc).

In the report, we give intermediate results of such a volcano-plutonic association study. For convenience, we will call it as the Bren volcano-plutonic association. It is situated in East Tuva, the Buren River basin, in conjunction of the Mongol -Tuvinian massif with Ondum subzone of the Tannuola-Khamsara zone. The intrusive part of the association is composed by granitoid pluton (~1500 km²) from the Bren complex. The reliable Late Ordovician age of the complex (450 million years) has been recently established (Kozakov et al., 2003; Rudnev et al., 2004). (Parenthetically, the Sarkhoi complex is marked by rather unsuccessful researchers). The volcanic component is presented by the Sailyg rock mass. Previously, this rock mass was regarded as the Late Devonian. At present, stratigraphic and faunal data have supported its Pre-Late Devonian level (Sugorakova, 2007). Land survey and petrographic study have shown it is considerably of ignimbrite composition. The rock mass remains as some kind of spots on the modern erosional truncation. These spots are confined to the positive topographic form (the vertical stripping up to 700 m) and down spatially towards the western and northern borders of Bren pluton places. Areas of the largest spots of the Belbei and Balgazyn places are ~350 and ~100 km², respectively.

Peculiarities of geological setting, petrography, and petrochemistry of the formations were studied minutely in the Belbei place (Teleshev, 1981). One of the main conclusions of A.E. Teleshev is as follows: volcanites of Sailyg rock mass were formed considerably later then granitoids of the Bren pluton. In Teleshev’s opinion, the main argument for separate time of volcanic activity is the presence of high-titaniferous basite dikes in granitoids and their absence in volcanites. However, in the early 1990s, dikes of basite composition were revealed in volcanites of the Sailyg rock mass, in the process of thematic studies, carried out by A.M. Sugorakova within the frame of the Tuvinian geological prospecting expedition. Although the dikes have not been studied minutely, there are some reasons to consider them as belonging to the dike complex that cuts granitoids of the Bren pluton. Probably, the basite dikes serve as incurrent canals for basalt effusive rocks, occurring on the faunally dated Late Devonian red rock mass. In its turn, this rock mass occurs on the crust of weathering of the Bren pluton granitoids and Sailyg rock mass volcanites, that form the basement of the Belbei trough (Sugorakova, 2007).

In the Balgazyn place, B.N. Lapin mapped high-titaniferous basites of diatreme facies, which break through granitoids and vulcanites of the Sailyg rock mass. In the central part of the neck, they have abundant fragments (up to 90 %) of host granitoids and volcanites, and effusive analogues of basites, occurring on granite-porphyries of the Bren pluton and Sailyg rock mass (Lapin, 1969). In 2008, we have carried out additional land survey which has confirmed B.N. Lapin’s conclusions. Moreover, on the left bank of the Soi River, in 5 km to the east from the village of Balgazyn, we have established a gradual transition in series: granites- granite-porphyries – rhyolite-porphyries with fragments of granites – clastic volcanic rocks (ignimbrites). The association is broken through by isolated basite dikes.

Petrochemical analysis of granitoids of the Bren pluton (87 an.) and volcanites of Sailyg rock mass (26 an.) in Harker’s diagrams has showed their absolute identity. SiO₂ variations in the volcano-plutonic association are 65-77 mass.%, Na₂O +K₂O - 6-9 mass.%. By many petrochemical parameters, granitoids of the Bren pluton are identical to ones of the Derzig pluton (D_1-2) (Teleshev et al., 1976), the elevated alkalinity level of Derzig pluton is notable. Basite subvolcanic bodies, which cut the Bren volcano-plutonic association are similar to Early Devonian basites of the Tuva riftogenic depression.

In conclusion, we point to the fact that the forming the Sailyg rock mass could not be considerably separate from time of the Bren pluton formation. It is supposed that the overall granite-formation, which hardened completely the accretional-collisional structure of the Altai-Sayan region in the Ordovician, was accompanied, in some cases, by the overall volcanic activity with ignimbrite eruption in the distributional area of the Pre-Cambrian metamorphosed rocks. As for the Devonian magma-formation, to which the Bren volcano-plutonic association members were incorrectly related, it is known that it has riftogenic nature and largely occurs in the Tuva depression and its branches (with Derzig pluton). Evidently, out of the rift, they occurred insignificantly. In particular, the formation of basite subvolcanic bodies that break out the Bren volcano-plutonic association, was related to these processes.

The work has been carried out with financial support of Russian Foundation for Basic Research (grants 06-05-64235, 07-05-00601) and an integration project of SB of the RAS № 65.

References

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