THE EAST SAYAN ADAKITE GRANITOID MAGMATISM, GENETIC PECULIARITIES AND SOURCES OF SUBSTANCE

Efremov S.V.

Institute of Geochemistry SB RAS, Irkutsk, Russia,, esv@igc.irk.ru

The middle and moderately acid varieties of calc-alkaline volcanic and plutonic magmatic rocks that have original geochemical characteristics are called adakites (Defant, 1990; Martin, 2005). Genetic ideas about this type rock formation are rather reserved. There are three most likely mechanisms, i.e. 1) melting the subdued oceanic lithosphere (Defant, 1990); 2) melting the metabasalts in the base of thick continental crust (Petford, 1996); melting the rocks of mantle cline that are metasomatized by adakite melt (Brandon, 2002).

The combination of conditions that are needed for adakite formation occurs rather rarely that results in their insignificant distribution compared to other calc-alkaline granitoids, and origin of the various age adakite magmatism is considered unique. The East Sayan is one of such unique regions, where adakites occur within the Vendian (Kuzmichev, 2004) and Early Paleozoic (Efremov, 2007) granitoids.

The two conceptual approaches can be used for explanation of geochemical characteristics inheritance in time, i.e. the concept of single geochemical reservoir that generated adakite magmas in periods of large tectonic reconstructions; concept of adakite magmatism association with geodynamic events of various age.

The first concept seems most attractive, as spatial association of various age granitoids hardly appears accidental. And it seems quite substantiated, taking into consideration their reference to the Archean sulphur-gneiss stratas, geochemical analogues of adakites. However, this concept requires the distribution of “single source” through the whole area of northern Tuva-Mongol microcontinent (distribution of granitoid massifs with adakite geochemical characteristics) that contradicts to the modern geodynamic reconstructions. Their authors consider the region as collage of varios age terranes that are of various genetic origin (Kuzmichev, 2004 and references herein).

The concept of adakite magmatism association is also in bad agreement with the present geodynamic reconstruction. If the formation of the Vendian adakites can be associated with formation of new subduction zone that appeared after collision of the Gargan block and Dunzhugur island arc (Kuzmichev, 2004), the formation of the the Early Paleozoic adakites cannot be logically elucidated. Taking into account the present day geodynamic reconstructions, we should relate the formation of these granitoids to the process of the Tuva-Mongol micricontinent collision with Siberian craton. But “collisional” adakites are unknown in the world. The only example described in literature is associated with melting the detached slab immediately after the collisional event (Sajona, 2000). Theoretically, adakite formation in collisional zones is possible by all three (see above) genetic models. However, this problem requires special studies. The most likely are slab melting in the Early Paleozoic zone of subduction; remobilization of sulphur-gneiss basement during the collisional event; remobilization of lithospheric source formed in one of the previous stages of the regional geological development (buried slab, lithospheric mantle metasomatized by adakite melts).

In context with the above material, the data on sources of substance for the Vendian and Early Paleozoic adakites acquire the key value. We can solve the most above problems, with the information on composition, location and age of these sources being available. The data can be obtained based on the present genetic models with use of isotope and geochemical characteristics of various age adakites.

We can definitely suggest extractustal source of the rock substance. Geochemical characteristics of the Early Paleozoic adakites allows to refer them to the LSA-type (Martin, 2005), associating their formation with melting the lithosphere mantle metasomatized by adakite melts. The Vendian adakites more correspond to the HAS-type by their matter composition (Martin, 2005) and could have “slab” source.

The available isotope data (Sr,Nd) give the possibility to calculate protolith age for granitoids of both types. By the obtained calculations, they are rather similar and range within 2500-2550 Ma that permits to relate the formation of these granitoids with the single source of substance.

The obtained results show that the ancient specialized source existed within the East Sayan. Its remobilization resulted in the various age adakite magmatism. In addition, according to the present models of high niobium basite and adakite formation, it can be in lithospheric mantle of the region. It excludes the possibility of adakite formation at account of sulphur-gneiss strata remobilization, but does not exclude the possibility that all granitoids having the adakite specificity (including grey gneisses) formed at account of single source of substance.

This problem is rather difficult and requires more thorough study. It should be considered that the conclusions about lithospheric position of source are only based on the hypothetical models of forming this or that type of rocks that can be altered in time. However, existence of the source itself is quite evident. Its position is fixed by plutons of adakites and adakite granites. Theoretically, it can be spread through the whole northern Tuva-Mongol microcontinent.

Both the age of the source and its distribution indicate existence of single continental block, at least, from the boundary of Archean and Proterozoic (2500 Ma) that contradicts to the present geodynamic reconstructions. Thus, the obtained results indicate the necessity of correcting the available ideas about geological structure of the region and making the schemes of geodynamic development more exact.

References

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Efremov S.V. (2007) Early Paleozoic adakites of East Sayan. Geochemical peculiarities and sources of substance // Problems of endogenic process geochemistry and environment. Materials of conference. Irkutsk. V.2. p.91-97.

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