Accretionary and collisional structure of West Transbaikalia that forms the north-eastern segment of Central Asian Fold Belt represents terrane collage of various geodynamic origin (Fig.1, page 5). Terranes of the Patom Fold Belt form the northern part of the Baikal Fold Belt. From the south, ophiolite and island arc terranes of the Late Riphean Baikal-Muya island arc that is accreted to passive margin of the Siberian continent at Pre-Vendian time are adjacent to them. Further to the south, the Barguzin-Vitim block is located that represents, by some data (Zonenshine et al., 1990) the Early Pre-Cambrian microcontinent, and by other data, - Riphean turbidite basin that is attached by the Eravna (Uda-Vitim) Early Cambrian island arc (island arc terrane) (Gordienko 2006).

The flysch and metamorphic blocks are the basis of the South West Transbaikalia tectonic structure.

The tectonic structure formation of the studied region was completed in the late Caledonian stage, subsequently the main tectono-magmatic processes shifted to the south and were associated with evolution of the Mongolo-Okhotsk oceanic basin.

In the Late Paleozoic and Mesozoic, West Transbaikalia represented an area of interpolate magmatism that resulted in granitoid province formation being one of the largest in the world, where granites make up more than 80% of the whole area. Among various type and age (from Proterozoic to Cretaceous inclusively) granitoids of West Transbaikalia, the Angara-Vitim batholith makes up the greatest part (more than 130 thous. km²) (Fig. 2, page 6), where B.A.Litvinovsky et al. (1992) distinguished granitoids of higher basicity (monzonitoids, quartz syenites) composing ~10% of the batholith area, and moderately acid granites with predominance of allochthonous varieties at relatively subordinate role of autochthonous ones. Granitoids of batholith cut deeply metamorphosed formations of the Pre-Cambtian blocks, Early and Middle Paleozoic (Ruzhentsev et al., 2005; Minina 2003) terrigenous- carbonaceous stratas of marginal continental type, volcanogenic and plutonic formations of the Riphean (Antipin et al., 2006; Tsygankov, 2005) and Early Paleozoic (Gordienko, 2006; Parfenov et al., 1995) island arcs. All this testifies to extreme heterogeneity of basement, on which the Late Paleozoic and Mesozoic magmatism of the studied region formed.

L.I. Salop (1967) who was the first to recognize the Angara-Vitim batholith (AVB) considered the granitoids that composed it as the Late Proterozoic formations. Later, a conclusion was made about its Early Paleozoic (O-S) age based on cutting the batholith of Cambrian sediments by granitoids (Litvinovsky et al., 1992). By present, reliable data of the Late Paleozoic (Late Carboniferous – Early Permian) age of batholith granitoids (Fig.3, page 7). Moreover, it was clarified (Yarmolyuk et al., 1997; Litvinovsky et al., 1999, Litvinovsky et al., 2002; Tsygankov et al., 2007) that many types of granites that were distinguished as complexes of various ages formed at about the same time, i.e. in the interval 340-270 Ma. In particular, high potassium calc-alkaline and subalkaline granites and quartz syenites of the Zaza (Late Zhida) complex, its massifs belonging to the central, southern and south-western parts of the Angara-Vitim batholith are of the age from 303 to 278 Ma (Tsygankov et al., 2007 and bibliography). Geological ratios show that the next impulse of magmatism, by time, is marked by quartz syenite and monzonite formation with slight amount of high potassium gabbroids in the Low Selenga complex. Massifs of the complex in West Transbaikalia extend for more than 700 km, but real area of their aureole may be significantly larger. A numerous big plutons are included in the complex. Some of them, for instance, the Usty-Kholok pluton occupies an area of about 1000 km². U-Pb age of syenites from this pluton equals 279-280 Ma (Litvinovsky et al., 1999; Litvonovsky et al., in press).The Khorinsky and Bryansky plutons are nearly of the same age (Litvinovsky et al., 2002; Litvinovsky et al., in press); they are composed by alkaline granites and alkaline-feldspar syenites, being similar to the Mesozoic granitoids of A-type.

Bimodal trachybasalt-trachite-trachyrhyolite dike belts (Shadaev et al., 2005) that have been recognized in the central part of West Transbaikalia cut the granitoids of the Angara-Vitim batholith, but are of very close age 305-285 Ma (Rb-Sr) that can be considered as direct evidence of the Earth's crust extension, at least, in the AVB final stages.

The formation of the Mongolo-Transbaikalian alkaline-granitoid belt (Fig.2, page 6) of the Late Triassic (220-210 Ma, Litvinovsky et al., in press) became the next large magmatic event in the region by time (Fig,2,) (Zanvilevich et al., 1985). Magmatic activity in this stage was controlled by rift zones of north-east trending, their formation having been continued through the whole Mesozoic.

Thus, the geochronological data obtained by present allow to distinguish two big epochs of granite formation in West Transbaikalia, i.e. Late Paleozoic and Mesozoic, each of them in its turn is subdivided into some stages or time intervals differed by geological position and geochemical peculiarities of the developing granitoids that is likely associated with evolution of geodynamic conditions.

The moderately acid auto- and allochthonous biotite granites of normal alkalinity that compose a larger part of the Angara-Vitim batholith (AVB) formed in the early- and late post-collisional stage of the Late Paleozoic magmatism (340-320 Ma) as a result of anatectic melting the metaterrigene protoliths. The late stage (310 – 270 Ma), associated with transition from collisional compression to post-orogenic extension is the most complicated one. At that time, the formation of the Angara-Vitim batholith granitoids continued. In addition, most massifs of monzonitoids and quartz syenites (Tsygankov et al., 2007) that were associated with the early stage of AVB development by B.A.Litvinovsky and co-authors (1992) formed along with normal calc-alkaline granites. At the same time, several geochemical types of granitoids and associated rocks formed: (1) shoshonite gabbros, monzonites and syenites; (2) alkaline and peraluminous alkaline-feldspar syenites and granites; (3) high potassium calc-alkaline granites and quartz syenites; 4) leucocratic syenogranites of A-type.

It can be said without any exaggeration that we have a new petrological problem: what was the reason and what were the conditions of forming several, simultaneously conjugated in space, but very different by composition granitoid associations within rather not large area.

The geological excursions included in the Program of the Conference comprise two objects (Fig.4, page 8): (1) the Shaluty quartz syenite-leucogranite massif of the Zaza intrusive complex with bodies of syn-plutonic gabbroids and composite dikes; (2) Ermakovka F-Be deposit (quarry) associated with intrusion of alkaline granites.