Alkaline
granites of the Syadatoyakhinsky massif (Polar Urals)
Udoratina O.V.
Institute
of Geology of Komi SC UB RAS, Syktyvkar, Russia
In
Polar Urals among numerous acidic magmatic rocks of the Kharbey block of
preuralides the alkaline granitoids of the Syadatayakhinsky massif are found.
The granitoids are related to the Syadatayakhinsky complex (Correlation …,
1985). The rocks of the massif are rare exception - they are poorly affected in
this zone by metasomatic silica-alkaline processes and actually magmatic
(intrusive formations) (Okhotnikov, 1985).
In
this zone the alkaline metasomatites with rare metal specificity are widespread
(Taykeyusky ore knot). The ore rocks are related to the formation of alkaline metasomatites
of fault zones and have paragenetic relation with magmatites. Recently the
material has been collected, which allow relating genetically the rare metal
deposits with alkaline source. The magmatic rocks of such bias are also
included in the studied massif, which does not yield, however, ore load.
The
granitoids, exposing in source of the Zaozerny brook, comprise sheet bodies
among green shales metamorphites of the Middle Riphean Nyarovey suite. The
absolute age of massif rocks on the basis of Rb-Sr (bulk) and U-Pb (zircon)
datings corresponds to Middle Cambrian (Andreichev, 2007). It should be noted
that figures of age of rocks, determined by different methods, are close. The
initial 87Sr/86Sr ratio makes 0.70490±41 and specifies
the abyssal nature of the matter.
The massif
is composed by arfvedsonite-biotite, biotite and micropegmatite granites, as
well as granophyres and granite-porphyries. The ratio with host rocks – layered
intrusion into metamorphite strata. The arfvedsonite-biotite granites are the
deepest, other varieties of rocks are located hypsometrically higher and represent
endocontact facies. Lenticular or rhythmical zoning is observed near endocontacts
and in apophysis bodies. The observable geological ratios – intertransitions of
rocks specify their near-simultaneous formation that allows relating them to the
products of one phase of intrusion (t– 600–700œ) (Okhotnikov, 1985).
The
ore load of the granitoids of the Syadatoyakhinsky massif, as N. Okhotnikov
emphasizes, does not represent practical value; generally it is a fine scattered
sulphidic mineralization (molybdenite, sphalerite, pyrite, pyrrotyne). He also
stated that the granites of lower and upper levels have different geochemical
specialization - atop the magmatic chamber tin, molybdenum and tungsten, and
below - tantalum-bearing granites with niobium (!) accumulated.
The
tested rocks of the massif (V. L. Andreichev’s collection) are represented by
medium-grained, massive alkaline arfvedsonite-biotite and biotite granites. Microscopic
studies reveal granite hypidiomorphic-granular texture, locally cataclastic.š
The
rocks are composed (%) of potassic feldspar (40-45), plagioclase (25-30),
quartz (20-25) and biotite (3-5). The constant presence of arfvedsonite is
characteristic. Accessory minerals are presented by allanite, sphene, zircon (rarer
by apatite). Ore minerals – magnetite, pyrite; secondary - epidote, limonite.
Salic
minerals are presented by potassic feldspar and plagioclase and quartz. Potassic
feldspar is presented by pertitized orthoclase and neogenic microcline
replacing orthoclase. Plagioclase - albite-oligoclase (An10–15)
idiomorphous prismatic, it forms hypidiomorphic-granular texture. It is often sericitized.
Quartz is poorly cataclized. Chemical compositions of plagioclase and potassic
feldspar are without impurities. Widespread femic minerals are represented by
micas (biotite and phengite), and also amphibole (arfvedsonite). In arfvedsonite-biotite
reveal taxitic distribution of dark minerals. The amphibole is dark green,
pleochroic from dark green to yellowish green; it is optically negative. The
biotite is presented by thin scale stilpnomelane. Often biotite is a unique
dark mineral in the rocks. At transition to micropegmatitic granites and also granophyres
and granite-porphyries muskovite (phengite) is found. Epidote and allanite are
observed as small isometric grains forming accumulations. Among accessory the structures
of zircons and sphene have been studied. Apatite and garnet, described by
V.N.Okhotnikov, are not found. Zircon is observed as small crystals of
prismatic yellow-brownish habitus. According to V. L. Andreichev the
catode-luminescent survey reveals concentric magmatic zoning (Andreichev,
2007). It is characterized by HfO2 presence, which content vary from
1.75 to 2.51 weight %. Sphene is observed as individual small extended brown crystals.
Petrochemically the rocks of the
massif correspond to the family of alkaline granites. The content (table) of
silica acid varies (weight %) from 67 to 74, and alkalis from 8.5 to 10.5, at
the alumina content at 14-15, but can reach 21 in granosyenites (Okhotnikov,
1985; Andreichev, 2007). The agpaite index varies from 0.7 to 1 except for one
test.
The geochemically studied rocks are
characterized by the raised content in relation to clarke for acid rocks Rb,
Zr, Hf, reduced content of all the light (except for La) and medium rare
earths, also Cr, Sr, Ba, Zn, Th, Y, the contents of heavy rare earths Y, Nb and
U are observed at the clarke level. The rocks are characterized by low REE
content with predominance of light over heavy and distinct europium anomaly,
(La/Yb)N varies from 13 to 47. By the substrate type the studied
rocks are related to A-granites. Thus, the geochemical features of the rocks,
their abyssal nature and age values specify that the geodynamic conditions of
formation correspond to postcollision settings prior to riftogenesis in the north
Urals.
The work is performed under financial support
of project îû-3763.2008.5 Leading scientific schools.
Table
Chemical composition (weight %) and content of
rare elements (g/t)
Component |
368/1 |
368/3 |
368/4 |
368/5 |
368/7 |
368/8 |
368/9 |
SiO2 |
66.78 |
71.12 |
71.38 |
71.50 |
70.94 |
70.82 |
73.90 |
TiO2 |
0.10 |
0.15 |
0.19 |
0.14 |
0.18 |
0.20 |
0.13 |
Al2O3 |
21.85 |
13.12 |
14.36 |
14.18 |
15.95 |
15.67 |
12.72 |
Fe2O3 |
0.99 |
1.34 |
1.31 |
1.05 |
1.58 |
1.47 |
1.08 |
FeO |
0.45 |
0.76 |
0.73 |
0.68 |
0.56 |
0.76 |
0.58 |
MnO |
0.02 |
0.02 |
0.04 |
0.03 |
0.03 |
0.05 |
0.02 |
MgO |
0.07 |
0.80 |
0.20 |
0.70 |
0.33 |
0.31 |
0.31 |
CaO |
0.56 |
0.56 |
0.69 |
0.67 |
0.62 |
0.48 |
0.67 |
Na2O |
3.88 |
4.64 |
4.60 |
4.80 |
3.90 |
4.62 |
4.17 |
K2O |
4.59 |
5.84 |
5.47 |
5.39 |
4.58 |
5.49 |
4.95 |
P2O5 |
– |
– |
0.03 |
– |
0.02 |
0.02 |
0.01 |
LOI |
0.88 |
1.27 |
0.90 |
0.83 |
0.85 |
0.61 |
1.04 |
Total |
100.17 |
99.62 |
99.90 |
99.97 |
99.54 |
100.50 |
99.58 |
Na2O+ K2O |
8.47 |
10.48 |
10.07 |
10.19 |
8.48 |
10.11 |
9.12 |
Na2O/ K2O |
0.85 |
0.80 |
0.84 |
0.90 |
0.85 |
0.84 |
0.84 |
P.I. |
0.52 |
1.06 |
0.94 |
0.97 |
0.71 |
0.86 |
0.96 |
La |
38.0 |
23.2 |
19.7 |
48.5 |
95.4 |
83.4 |
3.87 |
Ce |
65.3 |
35.0 |
30.1 |
71.0 |
150.0 |
138.0 |
5.0 |
Pr |
6.35 |
3.08 |
2.74 |
6.9 |
13.1 |
12.2 |
0.4 |
Nd |
21.7 |
9.88 |
8.52 |
20.4 |
40.3 |
40.2 |
1.03 |
Sm |
4.76 |
2.0 |
1.82 |
4.2 |
8.43 |
8.19 |
0.19 |
Eu |
0.26 |
0.046 |
0.22 |
0.036 |
0.53 |
0.23 |
0.11 |
Gd |
4.2 |
2.22 |
2.02 |
4.98 |
11.1 |
10.1 |
0.23 |
Tb |
0.53 |
0.29 |
0.3 |
0.71 |
1.73 |
1.58 |
0.037 |
Dy |
2.53 |
1.6 |
1.77 |
4.11 |
10.1 |
9.03 |
0.24 |
Ho |
0.44 |
0.33 |
0.39 |
0.87 |
2.24 |
2.0 |
0.054 |
Er |
1.01 |
0.82 |
1.02 |
2.26 |
6.26 |
5.58 |
0.16 |
Tm |
0.13 |
0.11 |
0.15 |
0.33 |
0.93 |
0.81 |
0.026 |
Yb |
0.56 |
0.57 |
0.8 |
1.71 |
5.03 |
4.5 |
0.14 |
Lu |
0.076 |
0.085 |
0.13 |
0.27 |
0.85 |
0.7 |
0.026 |
(La/Yb)N |
46.75 |
27.44 |
16.60 |
19.12 |
12.78 |
12.49 |
18.63 |
EuN/Eu*N |
0.17 |
0.07 |
0.35 |
0.02 |
0.17 |
0.08 |
0.61 |
Cs |
š – |
0.042 |
š0.049 |
š – |
2.4 |
š – |
1.14 |
Ba |
45 |
20 |
33 |
165 |
125 |
265 |
69 |
Sc |
2.57 |
1.1 |
1.33 |
3.31 |
4.87 |
4.61 |
0.48 |
Cr |
3.36 |
3.23 |
1.05 |
1.25 |
8.93 |
8.61 |
0.92 |
Co |
0.54 |
0.26 |
0.22 |
1.69 |
0.99 |
4.2 |
0.13 |
Ni |
410 |
570 |
110 |
260 |
380 |
360 |
140 |
Zn |
š – |
46.1 |
š – |
š – |
46.4 |
š – |
14.4 |
Se |
2.01 |
0.53 |
0.38 |
0.44 |
3.91 |
1.66 |
2.73 |
As |
6.68 |
š – |
3.37 |
3.17 |
5.95 |
2.35 |
2.96 |
Sb |
1.17 |
0.4 |
0.62 |
0.37 |
1.04 |
0.18 |
0.35 |
Th |
9.07 |
4.07 |
3.04 |
10.0 |
14.5 |
11.2 |
0.97 |
U |
2.97 |
0.39 |
2.57 |
2.72 |
4.92 |
2.17 |
2.1 |
Br |
1.02 |
0.5 |
0.4 |
2.36 |
0.89 |
0.54 |
0.33 |
Hf |
10.5 |
3.15 |
3.12 |
14.0 |
17.5 |
0.92 |
0.31 |
Ta |
š – |
0.58 |
š – |
0.88 |
4.31 |
2.12 |
– |
Au |
š – |
0.019 |
š – |
š – |
0.026 |
0.05 |
– |
Ag |
1.76 |
š – |
š – |
š – |
š – |
– |
– |
Nb* |
13 |
21 |
18 |
17 |
19 |
22 |
19 |
Zr* |
207 |
450 |
411 |
274 |
428 |
422 |
248 |
Y* |
24 |
37 |
34 |
29 |
39 |
36 |
31 |
Rb, ÍËÇ |
97.5 |
95.3 |
87.8 |
108.9 |
98.3 |
102.7 |
88.0 |
Sr, ÍËÇ |
85.2 |
75.7 |
27.0 |
9.2 |
25.4 |
12.3 |
14.8 |
Note. Rare and rare earth elements
are determined in the Institute of Geochemistry RAS by neutron-activation and
X-ray-radiometric (*) methods. The chemical contents and Rb, Sr data are
according to [1].
References
Andreichev
V. L., Larionov A. N., Litvinenko A.F. New Rb-Sr and U-Pb data on the age of
granitoids of Syadatayakhinskya intrusion (Polar Urals) // Lithosphera, 2007, ¹1,
pp. 147-154. (in Russia).
Correlation
of magmatic complexes of European northeast of the USSR / V.N. Okhotnikov,
V.I.Mizin, L.T.Belyakova et al. // Series of pre-prints «Scientific
recommendations for the national economy». Syktyvkar, 1985. edition 53.24 pp. (in Russia).
Okhotnikov
V. N. Granitoids and ore formation (Polar Urals). Leningrad: Nauka, 1985.184
pp. (in Russia).