MAGMATISM OF THE CENTRAL PART OF THE BAIKAL MOUNTAIN REGION

Kremenetsky I.G.*, Khrustaljov V.K.**

*«GeoCenter», Ulan-Ude, Russia

**Geological Institute SB RAS, Ulan-Ude, Russia, smetanina@gin.bscnet.ru

Magmatic rocks of the Baikal mountainous region (BMR) represent diverse volcanic and plutonic rocks of different age and a wide range of compositions varying from ultrabasic and basic to medium, acid and alkaline ones.

Magrmatic rocks of the central part of BMR were formed through two stages. Stage I (Riphean-Ordovician) proceeded in the geodynamic setting of continuously developing basin of Central-Asian paleo-ocean. Stage II (Silurian-Triassic) developed in the within-continental (collisional) foldbelt.

The lowest (Riphean) part of oceanic cross-section (ophiolite formation) is composed of meta-rocks derived largely from basaltoid subwater magmatism occurring as effusives. Of minor importance are the intrusive rocks, e.g. ultrabasite, gabbro, plagiogranites. Basic and ultrabasic rocks occur as small volumes.

Plagiogranites of the ophiolite formation make up small (to 5 km²) NW-striking bodies involved in folding jointly with stratified formations of Ripjhean age. Similar rocks were described by Rukavets to lie in the basement of Caledonides referred to "Turkinsky" complex. In the northern parts of BMR they are known as the Muisky intrusive complex.

The Vendian-Lower Cambrian tectogenesis in the Central part of BMR, irrespective of the inherited basic Riphean structures, is very much different from the previous one by the pattern of magmatism. Volcanic arcs (Kydzhimit River, Eravna lakes) emplaced in the setting of the active continental margin, where thick sequences of the Early Paleozoic settings were formed.

Volcanic units are located nearby zones of NE-striking faults. Thе crates of volcanoes are composed o quartz porphyries, eruptive and automagatic breccias. The same rocks often produce bedded formations, rarely stocks and dykes among effusive sequences. It is undoubtful they represent their later intrusive phase or the channels transporting lava eruptions (Oldynda subvolcanic complex).

The collisional (orogenic) stage of central BMR is linked with the period of ascending movements, origination of intracontinental fold belt and covers continuous time from Ordovician to Triassic.

At the initial stage of collisional fold belt the intrusions of the Atarkhansky and Barguzin complexes (S-granitoids) were formed in the central BMR. The basic rocks of the former make up small (up to 40 km²) massifs among Vendian-Paleozoic sediments, most often xenoliths in the fields of the Barguzin complex development.

The Atarkhansky complex is dominated by the presence of gabbroids varying to diorites.

Available geological data on the age of gabbroids of the Atarkhansky complex indicate the time of their formation in Silurian. The lower age boundary is based on numerous facts of disrupting faunitsic sediments of Cambrian (Atarkhan River, Turka), and on the left bank of Kydzhimit River, 800 m above the Epidotovy creek the trenches disclosed the eruptive contact between the massif of gabbro diorites and Ordovician sediments. The upper age boundary of the rocks of the Atarkhansky complex is defined by their disrupture by the Barguzin granitoids.

The main part of central BMR is composed by intrusions of the Barguzin complex producing the huge Angara-Vitim batholith. The most characteristic varieties of the complex are medium-, coarse-grained and porphyry-like granites (90% of volume), granodiorites, granosyenites, quartz diorites connected by gradual transitions.

During continuous time the age of granitoids of the Barguzin complex has been the subject of discussion. Some researchers relate emplacement of the complex with the final stage of Baikal folding (Upper Proterozoic), whereas some others accept the Paleozoic age.

Disruption of the Vendian-Cambrian sediments in the central BMR by the Barguzin granitoids is marked by numerous investigators. We have made such observations in the basins of the Kurba, Abaga, Turka, Kydzhmit, Ina and Barguzin Rivers.

The lower age boundary of the Barguzin complex is defined by the disrupture of the Ordovician-Early Silurian sediments. These relationships were observed in the following sites. 1. On the right bank of the Kydzhimit River (watershed of its tributaries Khakusy-Ara-Zaza), where biotite porphyry-like granites hosted xenoliths of meta-rocks, in which the Ordovician fauna of cephalopods was found.

2. On the left side of Kydzhimit River the Barguzin granites break into meta-rocks with the fauna of Ordovician pearlweed between the two creeks Khortyak and Alexandrovsky.

3. In the basin of Middle Myldylgen river (mouth of Dovatka creek) the biotite, biotite-amphibole medium-grained granites of the Barguzin complex cut sandstones and limestones containing Ordovician filiform weed.

The upper age boundary of the Barguzin complex was established on the NW slope of the Ikat ridge (Chertov Golets and Akul River valley). Over distance 20 km the weathered crust of the Barguzin granites is overlain by poorly metamorphosed conglomerate-sand-siltstone sequence with Devonian microfossils. The basement of the sequence hosts horizon of basal conglomerates containing boulders and pebbles of the low-lying granites and carbonate rocks of the Ukhshikan formation of the Lower Cambrian.

Thus, geological observations defined the time of granitoids of the Barguzin complex to be Silurian as the absolute age.

The intrusive activity was controlled by deep-seated splitting along which the plutons of the Kydzhimit (Zazinsky) granites intruded. The Kydzhimit intrusions proceeded at the moderate or small depth, which is displayed by a wide development of granite-porphyres and miarole cavities in the marginal and apical parts.

The rocks of the complex show sameness of chemical composition. The figurative points of model composition on petrochemical diagrams produce local fields, which correspond to normal, rarely subalkaline granites. Specific feature is abnormal silica contents (74%) and low contents of Mg, Ti and Fe. Potassium is dominant in the balance of alkalies. The age of the complex is believed to be Middle- Upper Paleozoic.

The age can be defined more precisely by geological methods. The lower boundary is determined in the Ikatsky range, where they cut granitoids of the Barguzin complex and conglomerate sandy and volcanogenic sediments of the Devonian age. The upper boundary is defined by superposition of Permian-Triassic effusive rocks of the Tsagan-Khentey formation (Baisykhan River, left bank of the Vitim River). Thus, geological observations suggest Carboniferous as the time of formation of the Kydzhimit complex.

The orogenic stage of development was completed with emplacement of intrusions of alkaline granitoids of the Kunaley complex. In central BMR its distribution is limited, and some of them are exposed in the southern parts, e.g. basins of Kidzhimit, Ona, Uda and Kurba rivers. The massifs of Kunaley granitoids are basically composed of medium-grained single-feldspar granites.