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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.
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