AGE ESTIMATION FOR GARGINSKY GNEISS-GRANITE BLOCK OF

BARGUZIN- VITIM SUPERTERRAIN

Rytsk E.Y., Makeev A.F., Salnikova E.B.,Fedoseenko A.M..

Institute of Precambrian Geology and Geochronlogy RAS, S-Petersburg, Russia, ERytsk@geogem.spb.ru

Angara-Vitim batholite of upper Paleozoic (Hercynian) age (Yarmolyuk et al., 1997; Neimark et al., 1993; Zygankov et al., 2007) occupies the larger part of Barguzin-Vitim super terrain. Earlier gneiss-granite complexes of local occurrence are not dated accurately – their age estimations vary from Paleozoic to early Proterozoic. At the same time, information on the age of “pre-batholite” gneiss-granite complexes has essential significance for of tectonic evolution of the super terrain.

“Pre-batholite” gneiss-granites occur in the area between two rivers – the Garga and Ikata, where Garginsky block of early Precambrian basement had been marked out (Salop, 1964). According geological survey-50 (Pervukhin et al., 1980) Garginsky block is represented by a cupola structure formed by early Paleozoic (?) sin-metamorphic gneiss-granites. The latter are represented by homogeneous fine grained biotite granite-gneisses and gneiss-like granites with xenolithes of migmatizated chlorite-sericite-quartz schists of possibly Ikat epoch of Cambrian period and are cut by pegmatoid granites. For isotope investigations within the cupola structure there were taken two samples of sin-metamorphic granite-gneisses on the right bank (№ 2091) and on the left bank (№ 1069) of the Garga river valley in 4 km up from the mouth of the Ikata river (collection of Fishev N.A. and Shelgachev K.N., FSUE “Buryatgeocentre”).

U-Pb studies have been carried out for five weights of magmatic zircon of different size fractions and micro-weight of monazite from granite-gneiss (sample N2091). Three signatures of zircon isotope composition form the line of regression and are close to its lower intersection with concordia which corresponds to the age of 493±6 Ma, the upper intersection corresponds to 1273±34 , MSWD=0.97. Isotope signatures for zircons of larger size are a little further from discordia, this can be explained by different age of inherited component of radiogenic lead in the studied zircon. Monazite is characterized by concordant age 425±2 Ma (MSWD=1.3, probability=0.26) and reflects the overprinted thermal event. Taking into consideration magmatic origin of zircon, it is possible to interpret the obtained age estimation 493±6 Ma as the age of its crystallization and correspondently formation of gneiss-granites.

U-Pb isotope data obtained for sample N1069 from granite-gneiss show linear age discordance t(²⁰⁶Pb/²³⁸U)<t(²⁰⁷Pb/²³⁵U)< t(²⁰⁷Pb/²⁰⁶Pb) for four zircon size fractions and on the diagram with concordia the signatures of their isotope composition are approximated by regression line with upper intersection with concordia corresponding to the age 681±18 Ma and the lower corresponds to the age estimation of 248±84 Ma (MSWD=1.3).

So, in the course of Garginsky cupola structure formation there have been not less than three episodes of granite formation – at 700-660, 490 and 440-420 Ma. Estimated early Ordovician age of gneiss-granites of Garginsky structure precisely corresponds to the main collision stage in the evolution of Caledonian structures of CAFB and Barguzin-Vitim super terrain as its part. Age estimations of post collision thermal events in the interval 440-420 Ma coincide with the age of large granite-gneiss massifs of “pre-batholite” Ogemi-Davansky complex of Kater zone (Rytsk et al., 1999). According Koshkin V. (FSUE “Buryatgeocentre”) such “pre-batholite” (“pre-vitimkansky”) granite-diorite massifs and gneiss-granite cupola structures are widely spread in the area between the Garga, Ikata and Vitimkan rivers and gravitate to the wide zone of hidden tectonic contact between Barguzin and Vitimkan-Zipinsky terrains. Original data evidence that among gigantic granite blocks of the Easter Baikal region which are traditionally attached to Angara-Vitim batholite considerable number of them could belong to sin- and post collision granites of Caledonian stage and to Precambrian formations.

The Work was carried out under financial support of RFFI, grant 05-05-65316 and Foundation for development of geology.

References

Neimark L.A., Rytsk E.Yu., Rizvanova N.G. (1993). The Hercynian age and Pre-Cambrian crust protolith of the Angara-Vitim batholith Barguzin granites: U-Pb and Sm-Nd isotope evidence // Doklady of RAS. V.331. N6. p.726-729.

Rytsk E.Yu., Neimark L.A., Amelin Yu.V. (1998) Age and geodynamic settings of the Paleozoic granitoid formation in the northern Baikal Fold Area // Geotectonics. N5. p.46-60.

Salop L.I. (1967) Geology of Baikal mountain area. M. Nedra. V.2. 700p.

Tsygankov A.A., Matukov D.I., Berezhnaya N.G. et al. (2007) Sources of magmas and stages of the West Transbaikalian Late Paleozoic granitoid formation // Geology and Geophysics. V.48. N1. p.156-180.

Yarmolyuk V.V., Kovalenko V.I., Kotov A.B., Salnikova E.B. (1997) The Angara-Vitim batholith: to problem of batholith formation geodynamics in Central Asian Fold Belt //Geotectonics. N5. p.18-32.