GEOCHEMISTRY AND PETROLOGY OF PALEOPROTEROZOIC GRANITOIDS

OF THE EAST SAYAN PEGMATITE BELT

Makagon V.M.

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

The East Sayan pegmatite belt is located in the south-west margin of Siberian craton, it consists of three main zones: 1) south-eastern one, located to Urik-Iya graben, 2) central part in Neroy and Tepsa grabens and 3) north-western one in Elash graben. In Urik-Iya graben there are some intrusive massifs of sayansky complex (composed by series of). Two phases are distingwished in this complex rock formation. Gabbroids, monzonites, biotite-amphibole diorites, quartz diorites, biotite-amphibole and biotite granodiorites, plagiogranites and low-alkali granites are forming at the first stage. Second phase includes intrusions of biotite and muscovite-biotite granites, and veins of albite-microcline pegmatites without ore components. In the first place unusual geochemistry of some small bodies of granodiorites and tonalites is of interest. They contain anomalous high Li, Cs, Sn and Be concentrations and high contents Ba and Sr. This fact indicates that granite magma enrichment in granitophile elements can be going on in its seats on early stage before process of intensive crystallization fractionation. Evolution of magmatic rock chemical composition in studied massifs of Urik-Iya graben is close to obtained by V.M. Moskaleva regularity (Moskaleva, 1999) for salic rock series forming in result of core-mantle interaction, however hier alkali granites are absent, but rare-metal pegmatites with spodumene - Li analoge of alkali pyroxenes are widespread.

In the Elash graben of the Tagul-Tumanshet zone massifs of granitoids of sayansky complex are composed with granodiorites and low-alkali granites (the Elash-Tenishet massif, 1 phase) and small bodies of biotite and two-mica granites and pegmatites without ore components (2 phase). They were formed in the compression regime and geochemical peculiarities of this massif granodiorites indicate large role of core substance in granodiorite magma formation and properly hybrid origin of forming in result of core-mantle interaction granodiorite melt is most probable. Chemical composition and geochemical characteristic of granodiorites agree with ones for granodiorites in Urik-Iya graben. They relate to I-type granites. Large fields of rare-metal pegmatites occured in the Urik-Iya graben and spatially connected with massifs of granitoids belong to spodumene subformation.

In the Elash structure there are more young granites of the Toporok massif. They contain with leucocratic biotite and amphibole-biotite granites of geochemical type rapakivi. As it is known stretch regime is typical for rapakivi granite formation. Geochemical characteristic features indicate that role of mantle source in formation of these granites is increasing essentially in comparison with granitoids of sayansky complex. These granites belong to A-type. Located in the Elash graben Vishnyakovskoye pegmatite field belongs to the complex geochemical series of petalite subformation of rare-metal formation.

Table 1. Stages and geochemical characteristic features of granitoids and pegmatites

in the East Sayan pegmatite belt

Stages	Age (Ma)	Granitoids and pegmatites	Geochemical features
Early Palaeoproterozoic	2160 - ?	S-type granites	Ba, Sr Li, Cs,Co, Cr, F
		Muscovite pegmatites	Ba, Sr Li, Cs, Co, Cr
Palaeoproterozoic	1870 – 1820	I-type granitoids	Ba, Sr, TR, F Rb, Be, Nb, Ta,
Late Palaeoproterozoic	1820 – 1650	I-type leucogranites	Rb, Ta, Sn Ba, Sr, TR, Zr, F
		Spodumene pegmatites	Li, Cs, Ta, Sn Ba, Cr,TR, F
Palaeomesoproterozoic	1650 – 1300	Rapakivi granites of A-type	Ba, Rb, Zr, TR, Sr, Pb, Be
		Petalite pegmatites	Ta, Rb, Cs,Sn, F Ba, Pb, Cr, TR

Note: in last column there are elements in numerator, in which the rocks are enriched, and elements with lowered concentrations are in denomenator. TR are rare earth elements.

The fields of mica-bearing pegmatites are located in central zone including the Neroy and Tepsa grabens. Included in Neroy complex granitoids form small massifs and veins in these fields. They are biotite and two-mica pegmatoid granites of plagioclase and two-feldspar composition. Geochemical features of granites are high Ba content and low Li, Cs, Co, Ni, Cr and F concentrations. Granitoids and pegmatites were formed in result of anatexis and palingenesis processes in ultrametamorphism zones. They are S-type granites.

Sequence of granitoid and pegmatite formation in the East Sayan belt is proposed in result of their study. It is presented in table 1. Available data on age and geochemistry of granites and pegmatites and features of their geological position point out that it is necessarily to single out independant pegmatite stages in granitoid magmatic process in this region. Large rare-metal pegmatite fields were formed from pegmatite magmas originated in result of long fluid-magmatic interaction of low crust acid melts with mantle fluids (Makagon, Zagorsky, 2002).

References

Bryntsev V.V. Precambrian granitoids of the North-West Cissayan. Novosibirsk: Nauka, 1994. 184p.

Levitsky V.I., Melnikov A.I., Reznitsky L.Z. et al. Postkinematic Early Proterozoic granitoids of Siberian platform south-west part // Geologiya i geofizika, 2000. V.43, № 8. P.717-731.

Makagon V.M., Zagorsky V.Ye. Deep seats of rare-metal pegmatite magmas and problem of their connection with granites (East Sayan) // Deep-seated magmatism, magmatic sources and the problem of plumes. Irkutsk: Publishing House of IrGTU, 2002. P.265-275.

Moskaleva V.N. Mantle-crust magmatism and features of deep structure of continental riftogenesis region // Regionalnaya geologiya i metallogeniya, 1999. № 9. P.20-26