ANGARA-VITIM AREAL-PLUTON: TO PROBLEM OF GENESIS AND AGE

Mitrofanova N.N., Mitrofanov G.L.

ESSRGGMS, FSURPGE “Irkutskgeophizika”, Irkutsk, Russia

The term “areal-pluton” “which is sometimes used for the Angara-Vitim batholith” was introduced by L.I. Salop (1967) for the distribution area of granites of Late Proterozoic Barguzinskiy complex. At that time, besides of Barguzinskiy complex, the Late Proterozoic Mamsko-Oronskiy and Telmamskiy, Early Paleozoic Vitimkanskiy and Konkudero-Mamakanskiy complexes were marked there. The ages and the distribution borders of the complexes were relative and sometimes – administrative ones. V.S. Malykh, and A.L. Aleksandrov (1967) offered to combine Mamsko-Oronskiy, Telmamskiy, and Vitimkanskiy and Konkudero-Mamakanskiy complexes into Konkudero-Mamakanskaya association with gradual decrease in the substrate influence.

As the result of geological survey and subject work on stratigraphy and magmatism of 70-s years, the Paleozoic age of granitoids and intertransition of the complexes into each other (Litvinovsky, Zanvilevich, 1972; and others) were proved. On the basis of isotopic estimation VSEGEI (Murina et al., 1978) in the absence of the direct geological data, the age of the major part of granitoids was accepted as 320-280 mln.years – Middle-Late Carboniferous.

In recent years, the opinion about the unvaried composition of batholith and its crystallization from magma during the short period of time, i.e. 320-290 Ma predominates in publications lately. In several publications, the Carboniferous age of batholith is debated, and the substantiations for its Early Paleozoic age are also given there. New geological data of batholith composition have not been practically published by now.

Summarizing our own materials accumulated for 40 years on different parts of Angara-Vitim areal-pluton and published data on geological structure and isotopy of this peculiar giant granitoid formation, we came to the conclusion that Angara-Vitim areal-pluton has a complicated internal composition, and it is a giant area of lithosphere fluidization (Letnikov, 1988), that continued discretely for a long period of time, from Ordovician-Silurian till the end of Paleozoic, and only the part of granitoids are magmatic formations which respond to the age-determination magmatic model according to the isotope composition to proportion. Block composition of Angara-Vitim areal-pluton can be easily tied up with the composition and combination of different-age heterogeneous granitized foundation and depth breakings of the predominating Noth-Eastern spread. In whole, the rejuvenation of geological formations from the North-West to the South-East can be observed. In the same direction, very roughly, inside Paleozoic areal-plutonic the following “blocks” can be singled out:

1. Mamskiy block of crystalline shales and gneisses, where the process of granitization stopped at the stage of magmatization and point-melting (Letnikov, 1981). The age of the melts, i.e. pegmatites is Ordovician-Silurian, the age of subsidiary processes (microclinitization, muscovitization) – Carbon.

2. Severo-Baikalskiy block, where both the Early- Paleozoic and Late Paleozoic granites can be defined. The granitoids of Early Paleozoic are registered in the form of separate massifs (Svetlinskiy and others), broken through by alkaline layers of Sinnirskiy complex, with zircon of Early Paleozoic age (U-Pb, Ritsk et al., 1998) or in the form of relics without clear limitations according to the data of isotopy (Rb-Sr) among later granitoids, evidently, of non-magmatic origin (Telmamskiy massif). For obtaining the isochrone of the age 429±15 Ma in Telmanskiy massif from the whole mass of the samples those which corresponded to the magmatic model of the relations Rb and Sr were selected (from those samples which were not culled the isochrone with approximate age - Carbon was obtained). Late Paleozoic granites are determined on the basis of breaking through of alkaline layers of Sinnirskiy complex and the age of zircon (Rytsk et al., 1998). Visually, it is impossible to refer granites to Early- or Late-Paleozoic age, and isotopic composition cannot be a mapping characteristic for the reference of the granite to the certain complex. That is why, in the absence of age proofs of the granitoids, in 1995, we offered the term “Angara-Vitim association” which included Svetlinskiy Early Paleozoic complex as well as Late Paleozoic Konkudero-Mamakanskiy one.

3. Barguzinskiy block of predominantly autochthonous granites with relics of Riphean formations. The diagrams of isotopic age determinations of granitoids do not suit the isochrone.

4. Central part of Vitim plateau is characterized by the presence of big areas of roof covering sags, xenoliths of sedimentary layers predominantly of Early Paleozoic age among the Late Paleozoic granites, being both autochthonous and allochthonous ones. The granitoids are preceded by the intrusions of Itakitskiy gabbro-syenitic complex, which are confined to the abyssal fracture of nepheline syenite age 457±33 and 463±20 Ma (Rb-Sr Gerasimov’s estimation). Petrographic layer-variety depended on the composition and the age of granitized substratum is marked in this block which indicates great activity of deep granitizing fluids. At the same time, large quantity of displaced magmatic granites with intensive contact influence on Early Paleozoic sedimentary formations is marked in this block. Unfortunately, Rb-Sr isochrones of displaced granites have not been obtained yet. K-Ar age of biotite, amphibole and plagioclase from granites falls into the interval of 316-270 Ma. The age of potash feldspar is 260 Ma and less.

5. In the North-East of the areal-pluton, the Ingamakitskiy complex of granitoids is represented only by displaced intrusions of Late Paleozoic among the layers of Early Proterozoic. The last to the north-east alkaline intrusion, being similar to the Itakitskiy-Sakunskiy complex is also marked here.

The distribution area of granitoids that belong to the Angara-Vitim areal-puton is a unique natural laboratory of the giant granitoid body formation both due to granitization of substratum varied by the age and composition and due to crystallization of displaced magmatic melt. The influence of Late Paleozoic granitization cannot be regarded as post-magmatic auto-metasomatic change of Early Paleozoic granites, since the volumes of granitized altered rocks exceed many times the size of magmatic formations. The determination of the age in granitized layers that correspond to granites and even to leucogranites by composition with use of Rb-Sr method is meaningless, since it does not meet the conditions of magmatic model.

Studying the Angara-Vitim areal-pluton at the up-to-date level is the task not for individual researchers and even individual organizations, investigation of this unique object must be made according to the complex program with attraction of all organizations that can contribute to investigation of this problem.

References

Letnikov F.A. Fluid regime of granite formation. Novosibirsk: Nauka, 1981. 185p.

Letnikov F.A. Petrology and fluid regime of continental lithosphere. Novosibirsk: Nauka, 1988. 184p.

Litvinovsky B.A., Zanvilevich A.N. New data on Angara-Vitim batholith (West Transbaikalia). Rep. SA USSR, 1972, volume 203, № 3. P.654-657.

Murina G.A., Rublev A.G., Shergina Yu. V., Kozubova L.A. Age of Vitimkanskiy complex granites according to radiological data. Rep. SA USSR 1978, volume 238, № 3 P.666-669.

Rytsk E.Yu., Neymark L.A., Amelin Yu.V. Age and geodynamic settings of Paleozoic granitoid formation in northern part of Baikal Fold Area. Geotectonics. V. II M.: Nedra, 1967, 698p.