MINERAL CHARACTERISTICS AND SETTINGS FOR FORMATION OF PRECAMBRIAN GRANITOIDS IN THE PRE-SAYAN UPLIFT

Levitsky V.I.*, Kotov A.B.**, Reznitskiy L.Z.***, Salnikova E.B.**, Kovach V.P.**, Levitsky I.V.*,

Barash I.G.***, Anisimova I.V.**

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

** Institute of Precambrian Geology and Geochronology, RAS, St. Petersburg, akotov@peterlink.ru

*** Institute of the Earth’ crust SB RAS, Irkutsk, Russia garry@crust.irk.ru

The Pre-Sayan marginal uplift of the Siberian platform basement consists of Irkutny, Kitoy, Bulunsky and Birjusa blocks, as well as the Urik-Iya proto-platform basin. The uplift is composed of some structural and mineral units:

1) Paleo-Archean low and moderately metamorphosed tonalite-trondjemite associations (ТТА) and greenstone belts lying within the Kitoy and Bulunsky blocks being part of the East Sayan granite-greenstone belt (ESGGSB). In this structure the granitoids have meta-intrusive batholith nature and make up its main volume.

2) The Paleo-Archean complexes of granulite facies (Sharyzhalgay series in Irkutny; Kitoy series in Kitoy and Bulunsky; Khailaminsky in the Birjusa blocks) united into Pre-Baikal gneiss-granulite area (PBGGA). The substratum of complexes is made up of the protolith for ultra-metagenic granitoids.

3) The non-Archean granitoids of for ultra-metagenic Kitoy complex sitting in the zone of juncture of PBGGA and ESGGSB. These are plagioclase and K-spar migmatites, autochthonous and allochthonous granitoids, migmatite-granites of different schistosity.

4) Paleo-Proterozoic complex of ultra-metagenic migmatites and granitoids developed granulite-facies through associations (Sharyzhalgay and Kitoy series) in PBGGA and amphibolite facies in ESGGSB, and in the Birjusa block.

5) Paleo-Proterozoic batholiths of post-kinematic post-collision granitoids of the Sayansk, Shumikhinsky, Premorsky and other complexes occurring within PBGGA and ESGGSB rocks, as well as in proto-platform volcanogenic-sedimentary troughs. The largest massifs occur in the zones of their juncture.

6) Paleo-Proterozoic low-metamorphosed volcanogenic-terrigenous-sedimentary depressions (Urik-Iya and Tumanshet grabens) hosting granitoid massifs of the Sayansk and Ignoksky complexes, with which formation of rare-metal granites is associated.

The tonalite-trondjemite associations occurring in the blocks (up to 20*40 kmм) make up the basement of the Gargansky block. ТТА consist of meta-tonalite biotite-amophbole and biotite plagiogneisses. Quite specific are early banded trondjemites making up massifs and batholiths, and late small crossing bodies of massive trondjemites, tonalities, plagiopegmatites. Their primary intrusive nature is undoubtful. In places they underwent K-feldspar migmatization. ТТА represent aluminous high-magnesian tonalities and trondjemites with strongly-fractionated distribution of REE. The age of trondjemites was defined from zircons by U-Pb method as 3250 Ma (Bibikova et al., 1982), for tonalites it is 3287 Ma (Bibikova et al., 2002), from zircons by U-Pb method by the ion microprobe as 3386 Ma (Bibikova et al., 2006). The values (Rb/Sr)₀ of ТТА correspond to BABI (0.699-0.701), have values Nd(T) = -2 – +0,4 and model age T(DM)=3.47-3.56 Ga. The petrochemical and geochronological evidence of ТТА allows them to be referred to the most ancient complexes of the world.

The age of granulite facies rocks od Sharyzhalgay and Kitoy series defined from the isochron data by Rb-Sr method is measured as 3.4-3.7 Ga, from detrital zircons by U-Pb it was defined to be 3.3-3.4 Ga (Poller et al., 2005). Protolith was formed in the oceanic and island arc settings.

The neo-Archean syn-collisional granitoids of ultra-metagenic Kitoy complex are proved to be more widespread than it is commonly believed. They are derived through ultra-metamorphic transformations of both TTA in ESGGSB, and granulite facies rocks in PBGGA. Their compositions vary from plagiomigmatites and plagiogranites to K-feldspar migmatites and granites.

The dimensions of their bodies range from 1*5 m to 3*10 km, and the age of formation lies within the interval 2.48 (ESGGSB)- 2.649 - 2.557 (PBGGA) Ga. The model age T(DM) varies from 2.9 -3.6.

The Paleo-Proterozoic age of granulite facies rock transformations in PBGGA and amphibolite facies in amphibolite facies in ESGGSB is distinguished by formation of diverse ultra-metagenic migmatites and granites. The age of ultrametagenic granitoids defined from the data of U-Pb method from zircons reaches 1853-1868 Ma.

The post-kinematic post-collisional granitoids of the Sayansk, Shumikhinsky ans Premorsky complexes are widespread in Irkutny, Kitoy, Bulunsky and Birjysinsky blocks, Urik-Iya graben occur as massifs lying in a sub-parallel manner with striking zone of the Main Sayan Fault.

Their structure consists of 3 phases from Early granodiorites (rarely tonalites) to leucogranites. Granites show predominance of K over Na, Fe over Mg, as well as heightened contents of К, Ba, Sr, LREE, Zr, Co, Cr and low V, essentially differentiated pattern of REE distribution. On the discrimination diagrams the compositions of the main phases, showing the composition of the parent magma, are largely located in the field A-type granites. Their general petrological-geochemical characteristics and age indicate formation of granitoids by the same mechanism and from the same source. The age of emplacement of granitoids of the Sayansk, Shumikhinsky, Premorsky and Ignoksky complexes (1855-1871 Ma) coincides with the period of formation of ultrametagenic granitoids in the Sharyzhalgay series (1853-1868 Ma). It is evident that in Early Proterozoic the ultrametagenic granite formation in the Sharyzhalgay series and formation of post-kinematic intrusive granitoids are related in time and paragenetically. This is confirmed by a symbatic mode of lines on the spider-grams of average-weighted compositions of ultra-metagenic granitoids of the Sharyzhalgay complex and post-kinematic granitoids normalized from the clarkes of low-calcium granites. Thus, granite formation in the Pre-Sayany marginal uplift of the Siberian platform basement reflects emplacement in the region of mature continental crust on account of the early primitive crust composed of TTA and granulite complexes.

References

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Bibikova Е.V., Levitsky V. I., Reznitskiy L.Z. et al. Archean tonalite-trondjemite association of Pre-Sayan uplift of the basement of the Siberian platform: U-Pb, Sm-Nd, and Sr isotope data// Geology, geochemistry and geophysics on the border of XX and XXI ages. RFBR in the Asian part of Russia. Irkutsk: IGC SB RAS. 2002. P.175-176. (in Russian)

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