HYBRID ROCKS INCLUDED IN THE ANGARA-VITIM BATHOLITH

Vrublevskaya T.T., Tsyrenov B.Ts., Tsygankov A.A., Burdukov I.V.

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

The interest in studying one of the world largest granitoid plutons, the Angara-Vitim batholith got higher because of the increased methodical possibilities in isotope geology and geochronology. The study of hybridization processes is significant both in view of determining the composition of parent magmas and clearing out the role of these processes in development of intrusives, effect on differentiation and ore-forming ability of magma. Two terms, i.e. assimilation and contamination are used to name the processes of hybridization. The assimilation represents full assimilation of xenoliths without preserving the material carcass relics of absorbed rocks. Assimilation also includes mixture of two magmas (Tirrel' 1932). The two types of assimilation are distinguished, i.e. 1) deep assimilation that occurs in the area of magma generation and contributes to differentiation and separation of hybrid magmas; 2) marginal one that comprises reworking the rocks in situ that leads to appearance of zonation and marginal facies in the crystallizing massif.

The hybrid rocks included in batholith were previously distinguished as well, starting from the geological surveys, based on the structural and textural peculiarities and melanocratic appearance of some varieties. However, the most typical feature of hybridization processes is the presence of fine-grained melanocratic inclusions in the intrusive rocks, their nature being various: 1) traces of acid and basic magma mixture; 2) result of xenolith reworking; 3) autoliths that formed in the early stage of granitoid crystallization; 4) refractory restites of substrate with the removed anatexial fusion.

When studying the hybrid rocks, we employed the newest methods (microprobe analysis of minerals, electron microscopy, thermobarogeochemistry, isotopy of oxygen) to clarify the nature of melanocratic inclusions and contribution of various processes of hybridization to formation of batholith on the whole. Within the Angara-Vitim aureole-pluton, the mixture of basic and acid magmas is characteristic of the massives of quartz-monzonite composition (Litvinovsky et al. 1993). The assimilation of host rock xenoliths took place in the Khangintui and Khasurta plutons.

The Khangintui massif occurs in the middle Kurba, between its tributaries, i.e. the Khangintui and Birkhe-Shibir' creeks. Its larger part is composed by the coarse-grained porphyry-like amphibole-biotite quartz syenites; granodiorites and granites are also observed. In the massif, the hybrid rocks that refer to the large xenoliths and relics of host sediments are widely developed. The rocks of endocontact zones that are represented by quartz diorites and monzodiorites contain fine-grained melanocratic inclusions reworked to various extent. The peculiarity of hybrid rocks is their enrichness in biotite and presence of porphyry-like extracts of sharply zonal plagioclase: labradorite is in the centre and oligoclase – in marginal zones. The xenoliths of host gabbroids and schists that intruded in magma are transformed to the amphibole-plagioclase hornfels. Their further transformation results in origin of fine-grained diorite-like amphibole-plagioclase rocks that are enriched in magnetite and sphene. The diorite-like inclusions in quartz-syenite melt are subjected to metasomatic reworking, during which recrystallization of minerals and replacement of amphibole by biotite are noted. The quantity of biotite subsequently increases in the diorite-like inclusions, then amphibole leaves the inclusions, and the amphibole-plagioclase paragenesis is changed by the biotite-plagioclase-quartz-magnetite one. Fine-grained biotite-magnetite aggregate often remains at the place of the inclusion, and only shadow of xenolith that is marked by magnetite small grains is only preserved. In addition, diorite-like inclusions are subjected to partial melting that the presence of hypidiomorphic leucocratic isolations with amphibole being poor in titanium and aluminum, and alkaline feldspar containing the maximal barium.

The second variety of hybrid rocks that is represented by quartz-containing biotite syenites appears in contact zone of quartz syenites with host rocks of higher aluminousity, i.e. biotite schists. Their hybrid character is confirmed by similiarity of mineral composition and contents of some admixture elements with host schists.

The Khasurta pluton occurs in interfluve of the Kurba and Uda rivers 140 km to the north-east of Ulan-Ude town. It is composed by fine- and middle-grained monzonites, monzodiorites, quartz monzonites, biotite-hornfels quartz syenites. Crystalline schists and carbonate rocks are host sediments that effected the specificity of assimilation conditions and composition of hybrid formations. In exocontact, the apodolomite magnesium scarns or plagioclase-quartz-biotite-cordierite hornfels that are formed on amphibole-biotite schists are developed depended on host rock composition. The studied hybrid rocks that origin at the contact with dolomites in this massif are represented by leucocratic coarse- and middle-grained pyroxene syenites, their real thickness being impossible to establish, but due to the size of some outcrops, it ranges within 20-30 meters. In the endocontact rocks, F.G.Reif also noted the presence of melanocratic schlieren and small aggregates of amphibole, plagioclase and biotite grains.

The mineralogical and petrographical peculiarities and chemical composition of endocontact syenites and rocks of the inner part of the massif significantly differ: the sharply zoned feldspar with the centre composed by plagioclase, margins – by alkaline feldspar is present in the pyroxene syenites. Among the rock-forming minerals, amphibole and biotite are absent, and magnetite – among the accessory ones, however, they are enriched in sphene up to 3-4 vol. %. The main specific feature of endocontact syenite chemistry is in higher alkalinity and aluminousity compared to other rocks of the massif. The endocontact syenites formed as a result of hybrid melt fractional crystallization at the temperature within 1100⁰C with maximal content of volatile components in the relatively reduced conditions.

The carried out studies showed that the processes of hybridization are various and significantly contribute to the formation of the Angara-Vitim batholith. Variations of rock composition from diorites to granites are caused not only by crystallization of the melts with various compositions, but assimilation as well, that includes both mixture of basic and acid magmas, and contamination by the host rock material.

References

Litvinovsky B.A., Zanvilevich A.N., Alakshin A.M., Podladchikov Yu.Yu. (1992). The Angara-Vitim batholith is the largest granitoid pluton. Novosibirsk, izd-vo of UIGGM, SB RAS, 141p.

Reif F.G. (1976) The physical and chemical conditions of large granitoid mass formation in East Prebaikalia. Novosibirsk, Nauka, 87p.

Tirrel' G.V. (1932) Fundamentals of petrology. M-L., GNTI, 328p.