Evolution of Sr and O Isotopic Composition During Two Different Granite Types Formation Within Sangilen Highland (South-East Tuva, Russia)

A. Yu. Petrova Institute of Mineralogy, Geochemistry, and Crystallic Chemistry of Rare Elements (IMGRE),

Veresaeva str, 15, Moscow 121357 Russia


Yu. A. Kostitsyn Institute of Mineralogy, Geochemistry, and Crystallic Chemistry of Rare Elements (IMGRE),

Veresaeva str, 15, Moscow 121357 Russia

B. G. Pokrovskiy Geological Institute of RAS (GIN), Pyzhevskiy str, 7, Moscow Russia

Isotopic investigations of Sangilen granites have been intended to study a possible mechanism of their origin. Matter evolution from source rocks to granitic composition was also interesting for us.

West Sangilen is unique because different types of granites are present within quite small area. There are autochthonous granites with gneissic stripe-like structures and without sharp contacts here. On the other hand there are allochthonous granitic rocks having normal intrusive relationships with country rocks. Chemical composition of the autochthonous granites is constant and in congruence with normal ones.
The allochthonous granitic rocks are differentiated from granodiorites to alaskites. Country rocks for the autochthonous and the allochthonous ones are schists and garnet-bearing gneisses. There are also a lot of big mafic bodies and dykes of different chemical composition in this region.

In previous works of other investigators metamorphic rocks have been related to the most ancient rocks. An age interval between metamorphism and granitic rocks formation was proposed upto 700-900 Ma. New Sr and O isotopic results for more than 30 samples, including metamorphic, granitic and basic rocks are presented in this report.

Both the granitic and metamorphic rocks have been analyzed to evaluate age relationships between metamorphism and granites origin. Three Rb-Sr isochrones were constructed: two mineral ones for a garnet-bearing gneiss (468±6 Ma, 0.7094±3) and for an allochthonous granite (476±6 Ma, 0.70611±10) as well as whole-rock isochrone for alaskites (468±12 Ma, 0.7047±35). These three lines should be considered as parallel with common age 472±4 Ma. Four points of whole-rock gneisses also fit the mineral isochrone for gneiss. The points of the autochthonous granites and the schists don't form isochrones, but they trend to received three isochrones. So metamorphism and granite formation within Sangilen were a result of the same heat event with the age 472±4 Ma.

Average strontium initial ratios calculated for 472 Ma decrease in the following sequence: the schists (0.7108±15), the gneisses (0.7093±7), the autochthonous granites (0.7091±14), the allochthonous granitic rocks, including the alaskites (0.7047±9), the basic rocks from dykes (0.7039±6). The isotopic ratios in the granitic rocks, especially in the allochthonous ones, are lower than in the metamorphic rocks. Therefore some matter influx from other sources occurred during granite formation.

Sr and O isotopic data permit to construct a model of different sources interaction during granites formation. Apparently mantle basic magmas were the source of energy and some matter in the process. The model presumes two types of interaction between basic melts and crustal rocks: direct one and interaction through hydrothermal solutions. Points of the schist, the gneisses, the allochthonous granitic rocks in (87Sr/86Sr)0- d18O coordinates lie along the immediate mixing line of basic magmas with country rocks matter. Points of the autochthonous granites and some metamorphic rocks are close to the line of hydrothermal mixing.

Apparently melting of country rocks occured in the neighborhood of basic magma chambers. In this way, melts from partly molten country rocks were mixed with some components of basic melts. This process could result in origin of initial granitic magmas. Then, redistribution of some elements between two coexisting liquids (basic and granitic magmas) and contemporaneous magma differentiation led to formation of more evolved products as alaskites with the most primitive strontium isotopic composition.

The second result of the crust heating was metamorphism and autochthonous granites formation. These granitic melts also interacted with basic magma and some element exchange occurred through crustal water.