GRANITOID GEOCHEMICAL TYPIZATION OF ANGARA-VITIM BATHOLITH

Khrustaljov V.K.

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

The granitoid geochemical peculiarities of the Angara-Vitim batholith have been studied during GTS-200, GS-50 (1960-1986) and research work (1986-2008) (Khrustaljov 1970, 1990, 2000).

Granitoids of batholith being the main geological background where the whole picture of the Phanerozoic petrogenesis is observed, are widespread in West Transbaikalia. By matter composition and structural position, they are united into the two large groups, i.e. the allochthonous homogeneous granitoids that have the clearly cutting contacts with host rocks, and autochthonous ones with distinct features of formation as a result of sedimentary-metamorphic roof granitization. The regular-grained and porphyry-like biotite and more rarely leucocratic granites are mostly spread among the allochthonous granitoids. The autochthonous granitoids are characterized by mixed composition. They are represented by porphyry- and gneiss-like granites, granodiorites, monzonites, granosyenites, plagiogranites and adamellites.

We have studied the rocks of 13 autochthonous and 12 allochthonous massives in the central part and south-east protrusions of batholith. A geochemical characteristic of the most typical of them is presented in Table 1. The analysis of geochemical information on allochthonous and autochthonous granitoids shows that their most evident geochemical peculiarity is low, near to clark level of rare element concentrations. The check of estimates by asymmetry and excess indicates that distribution of elements in granitoids is mostly subordinate to normal law of distribution. As a result of rare element study, the characteristic tendencies of Li, Rb, Zn, Be, Sn, W and Mo distribution have been revealed. They unite the rocks of batholith, and at the same time, the significant differences of autochthonous and allochthonous granitoids are revealed: 1) strong positive associations of K with Li, Rb, Be, B and negative ones with Na and F are characteristic of the first ones, while significant associations lack at all in allochthonous granitoids; 2) high concentrations of Ba (1-2 clarks) and strong positive associations of W with Be, and those of Be with B are typical for autochthonous granitoids; in allochthonous ones, the below clark concentrations of Ba and strong positive associations of Li with W, Mo, Be and B, those of Sn with W and Be, of Be and Zn with B, and negative associations of Rb with Sn, W and Be, of U with Ba and Sr are characteristic.

Conclusions

1. The low, near clark level of rare element concentrations is the main geochemical peculiarity of the studied granitoids.

2. Close rock geochemical characteristics of the studied massives obtained for Li, Rb, Zn, Be, Sn, W and Mo testify to genetic affinity of granitoids.

3. Significant differences in distribution of Ba, Sr, Pb, and Bi in less extent are confirmed under the conditions of autochthonous and allochthonous granitoid formation.

4. Processes of emanational differentiation in granitoids are suppressed, and their role in rare element distribution is insignificant.

5. Areas of allochthonous and autochthonous granitoid development are barren, while metasomatically altered autochthonous granitoids in frontal paleostructures of ensimatic island arcs can be considered as potentially ore-bearing ones for rare elements and gold.

6. Average concentrations of the studied rare elements in granitoids are recommended to be used as regional clarks for the area of West Transbaikalia.

References

Khrustaljov V.K. (1990) Geochemistry and ore-bearing of the Vitim Plateau Paleozoic granitoids. Novosibirsk, Nauka, p.132

Khrustaljov V.K. (2000) Rb-Sr age of Barguzin complex granitoids. Izd-vo of Buryat Research Centre SB RAS, Ulan-Ude, p.41-54.

Khrustaljov V.K., Lositsky I.V., Kremenetsky I.G. (1970) About some peculiarities of the Upper River Turka and Kydzhimita Upper Paleozoic granitoids related to their Sn-bearing (West Transbaikalia). Notes of Transbaikalian Phil. Geographical Society of the USSR, issue 5, Chita, P.35-41.

Tauson L.V. (1977) Geochemical types and potential ore-bearing of granitoids. Nauka, 279p.

Table 1. Intrusive rock geochemical characteristics of the Ina, Kurba, Kydzhimit, Zaza, Turka river basins and Vitim right bank.