Granites and Earth Evolution.
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THE SHIVEY ALKALINE GRANITE – QUARTZT-SYENITE MASSIF (EASTERN TUVA)


Sugorakova A.M.*, Yarmolyuk V.V.**

*Tuvinian Institute for Exploration of Natural Resources SB RAS, Kyzyl, Russia, samina51@inbox.ru

**Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Moscow, Russia volya@igem.ru

The Shivey massif has been distinguished as alkaline granite – quartz-syenite firstly by the authors. The massif is a part of the Kaakhem large (over 30000 km2) granitoidal polychronic batholyth (Rudnev et al., 2006). It is situated mostly on the left bank of the Maly Yenisey River (the Shivey, Mos, and Chinge rivers). It is the largest place of alkaline rocks development in this batholyth (over 500 km2). The massif is surrounded mostly by the Early Paleozoic granitoids (451 and 450 million years). It has a complicated configuration in plan and contains xenoliths and roof pendants. The north marginal parts of the massif are characterized by chilled zones in endocontacts, with large development of porphyraceous facies with gradient transition from fine-grained granite to granophyric and felsitic differences. All massif and host rocks contain dikes of aplites and pegmatites.

The massif is built by rocks of alkaline and subalkaline series. Each series is presented by quartz syenite, granite, and leucogranites families. As a mineral composition, all of them have sodapotash feldspar (perthite), quartz, albite, and dark-coloured and accessory minerals. Sodapotash feldspar – perthite gives transverse jet, maculous jet, streaky, and microstreaky structures of disintegration. Antiperthite structures occur rarely. Albite and potash feldspar ratio in perthite is 40:60-50:50 in average. Albite borders perthite or it forms xenomorphic, sometimes idiomorphic crystals with quartz in the intergranular opening. It is difficult to identify potash feldspar in perthite; when it is identified it can be microcline or orthoclase. Rocks are differentiated to alkaline and subalkaline by dark-coloured mineral absence or presence. These minerals are as follows: aegirine, riebeckite, arfvedsonite, and hastingsite in rocks with substantial amount of alkalis. Alkaline rocks contain 1-5% of alkaline minerals, rarely – up to 10% and a bit higher. Riebeckite and aegirine occur more frequently, than arfvedsonite which replaces riebeckite. Aegirine can also replace riebeckite. Ore and accessory minerals are as follows: magnetite, ilmenite, zircon, apatite, sphene, orthite, fluorite, monazite, and rarely – chevkinite, columbite tantalite, and thorite.

Subalkaline and alkaline granites and leucogranites. Mineral composition: quartz (20-40%), albite (5-15%), sodapotash feldspar – perthite (50-70%), biotite (1-3%), amphibole (0-5%), and pyroxene (0-5%).

Quartz-syenites and alkaline quartz-syenites. Mineral composition: quartz (3-15%), albite (5-10%), sodapotash feldspar – perthite (65-80%), biotite (1-5%), hastingsite (1-5%), aegirine, riebeckite, arfvedsonite, and clinopyroxene (0-10%).

To study petrochemical peculiarities of the Shivey massif, 158 samples have been analyzed. The rocks contain 62-77% of SiO2 and 8.1-12.9% of alkalis with potassium dominating sodium. Alkali presence increases from leucogranites (8.2-10.1%) to quartz syenites (8.1-12.9%), with the less silica, the more range of values in alkali amount. TiO2 content increases when silica reduces and P2O5 rises.

Amount of rare earth elements is 142-420 g/t at more steep inclination of light elements on the spidergram and almost horizontal inclination of heavy lanthanoids (9 samples). Composition of rare earth elements is close to OIB composition, excepting europium anomalies presence. It tells about differentiation of melts and selective character of melting processes. From six negative europium anomalies, the most intensive ones belong to alkaline granites, and the less intensive – to alkaline quartz-syenite. We can suppose that granites are the later differentiates of melts. From two weakly positive anomalies, alkaline quartz-syenite contains higher europium level because of great amount of alkaline pyroxene presence in amphiboles.

Extremely high Cr content is revealed in all rocks – 40-141 g/t, while the other elements of ferrum group rest within limits. Zr, Ba, Hf, and Nb content is also elevated and uneven – 246-1086, 330-1833, 6-19, and 7-39 accordingly. Sr, Rb, Ti, and Eu extreme minimums and Th and K distinct maximums are also noted.

All variety of rock of the massif has the same characteristics and smooth changing of mineral and petrochemical compositions.

Rocks varieties have complicated relationship. It is expressed frequently by gradient transitions with distinct changing of quartz and feldspar ratio and grains. Shadow breccias are observed frequently when sections of quarts syenites from 30-40 cm to 1-2 m in size occur in the ore field. The sections have borders distinct or indistinct by quartz content or grains, which occupies up to 50-60% of the rock volume. The reverse relationship is also described. The sections of quartz syenites or alkaline granite with sizes up to 30 km2, marked on the map, are conventional and differentiated by dominance of varieties.

When shapes of bodies disappear, sections with different level of crystallization in a large massif with long term of formation remelt and rework each other; it is convenient to indicate all these processes as facial transitions. Accordingly to G.L. Dobretsov et al. (Principles…, 1988), these facts of thermostatically controlled intrusive concealed contacts are quite typical to the close stages of massif formation. During a large massif formation, the following processes occur: multiple collapses and shattering of crystallized sections with repeated reworking by melt, numerous injections of melt in indurated or semi-indurated sections. Such kind of transitions of alkaline-granitoidal rocks were observed earlier on other massifs (Magmatic…, 1983).

Similarity of characteristic of the Shivey massifs REE with OIB REE and late-orogenic host granitoids with age 450 million years (Rudnev et al., 2006) permits supposition on the same mode of batholyth formation under influence of mantle plume.

The work has been carried out with financial support of Russian Foundation for Basic Research (grants 06-05-64235, 07-05-00601).

References

Magmatic mountain rocks (1983) v.1. part 2. M. Nauka.

Principles of granitoid intrusion dismemberment and mapping. Methodical recommendations (1988) L. 61p.

Rudnev S.N. et al. (2006) Kaakhemsky polychronical batholith E.Tuva): composition, age, sources and geodynamic position // Lithosphere. 2006. N2.