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GRANITOID MATTER
COMPOSITION IN BASEMENT OF THE UZBEK AREA IN SHAIM
REGION
(WEST SIBERIA)
Ponomarev V.S., Ivanov K.S., Erokhin Yu.V.
Institute
of Geology and Geochemistry UrB RAS, Ekaterinburg,
Russia, p123v@yandex.ru
The
Shaim oil- and gas-bearing district is located in the
Khanty-Mansijskiy autonomous region within the Sovetskiy and
Kondinskiy districts near the border with the Sverdlovsk region. Two
main regional structures, i.e.: 1. Triassic Danilovskiy graben that
extends for more than 330 km in width ranging from 45 to 90 km in
submeridional direction through the whole region. The graben has
tectonic borders with the Paleozoic volcanogenic, sedimentary and
metamorphic complexes to the west and east from it. Serpentinite
bodies are observed along these faults. 2. Late Paleozoic
“granite-schist axis” known as Shaim-Kuznetsovskiy
meganticlinorium of the Trans-Urals uplift (its central, axial part)
is located to the east of Danilovskiy graben. This structure also
extends through the whole Shaim oil- and gas-bearing region (and
farther for hundreds km in submeridional direction) in width 30-45
km (Ivanov et al., 2003).
The
Uzbek area localizes in central part of the Shaim oil- and
gas-bearing region. Previously, pluton of oval-shaped acid rocks 9.5
x 6.5 km in size was revealed in the area. The massif itself lies in
tectonic zone that separates the Danilovskiy graben and
“granite-schist axis” among rocks of ophiolite
association. Its central part is composed by monzodiorites (drilling
hole 10352). The rocks have massive, locally taxite texture (spots
are formed by segregations of colour ninerals), fine- and
middle-grained diorite, sometimes monzonite (poikilitic) structure.
Chemical composition of granitoid from depth 1780 m (laboratory of
IGG, Urals Branch, analyst N.P.Gorbunova) in mass. %: 54.05 SiO2;
2.64 TiO2;
4.39 Fe2O3;
13.58 Al2O3;
0.50 P2O5;
2.90 MgO; 0.24 MnO; 8.70FeO; 5.71 CaO; 11.21 K2O;
4.17 Na2O;
2.79 LOI; 100.87 in total; f = 0.67. Rock mineral composition
comprises 50-70% plagioclase (Ab61-66),
10-15% mica (annite, f = 0.68), 10-15% amphibole
(ferroedenite-ferroactinolite, f = 0.61-0.64), 5-8% quartz,
microcline, orthopyroxene (ferrosilite, Fs60).
Among secondary minerals are chlorite that partially replaces
amphibole. The accessory minerals are apatite, zircon and ore
minerals (ilmenite and pyrite).
The
marginal part of the massif is composed by the massive texture
granitoids (drill holes 10381 and 10350). The rock structure is
hypidiomorphic-grained, locally diorite; middle- and fine-grained.
Chemical composition of granitoid (drill hole 10381, depth 1969 m)
in mass.%: 70.17 SiO2;
0.24 TiO2;
1.64 Fe2O3;
13.40 Al2O3;
0.04 P2O5;
0.82 MgO; 0.09 MnO; 1.55FeO;; 0.72 CaO; 3.75 K2O;
4.50 Na2O;
2.00 LOI; 98.90 in total; f = 0.61. By petrochemical parameters
(70.17 mass.% SiO2;
8.25 mass.% K2O
+ Na2O),
the rocks belong to subalkaline granites. Mineral composition of
rock: 35% quartz, 25% albite-oligoclase, 25% microcline, 10% mica,
amphibole 5%. Amphibole and biotite are completely replaced by
chlorite and carbonate (siderite with 0.56 to 0.85 f. u. Fe). The
accessory minerals are apatite, zircon and pyrite.
Microelement
composition of rocks was determined by ICP-MS (IGG, Urals Branch
RAS, analyst D.V.Kiseleva). Granitoids from the massif central
part(at normalizing to contents in primitive mantle) are
characterized by positive anomalies on Ba, Ce, Nd,Sm and negative
ones on RbTh, U, Sr, Hf, Zr, Ti. In normalizing to chondrite, rare
earths demonstrate predominance of easy lanthanoids over heavy ones
and presence of weak Eu anomaly. At the same time, granitoids from
the massif marginal part are characterized by positive anomalies on
Th, U, La, Ce, Nd Sm and negative ones on Ba, Nb, sr, Hf, Zr,Ti
when normalizing to primitive mantle. In normalizing to chondrite,
rare earths show domination of light lanthanoids over heavy ones,
and presence of clearly observed negative Eu anomaly. A variation of
microelements is observed in rocks of the massif marginal and
central parts. Rb (to 85 ppm, Zr (to 195 ppm), Ce (to 128 ppm, Th
(to 12 ppm) prevail in granites, and Li (to 35 ppm), Sc (to 36 ppm),
Ti (to 19393 ppm), V (to 267 ppm),Mn (to 2394 ppm), Co (to 31 ppm),
Sr (to 643 ppm) and Ba (to 732 ppm) - in monzodiorites
Thus,
acid pluton located in the Uzbek area has zonal structure. The
central part of the massif is composed by rocks of middle
composition (quartz-containing monzodiorites), and marginal one –
by acid rocks (subalkaline bifeldspar granites). By all features
(structure, chemical composition and geochemistry), this pluton
belongs to granitoids of monzodiorite-granite series that are widely
represented within “granite-schist axis” of the Shaim
district (Ivanov et al., 2007).
The
higher Fe content of the Uzbek massif minerals and rocks compared to
granitoids of the “granite-schist axis” is their
peculiar feature. Thus, Fe content of rocks (of minerals
respectively) that compose the Uzbek massif falls into the interval
0.6-0.7, and that of “granite-schist axis”granitoids –
0.4-0.5.Therefore, we suggest that the Uzbek pluton formed in the
deeper conditions, since the rocks belong to magnetite free
ferrofacies by mineral Fe contents (Fershtater et al., 1978),
whereas granitoids of “granite-schist axis” fall into
the magnetite ferrofacies. It is also confirmed by temperatures and
pressure calculated by the use of amphibole geothermobarometer
(Mishkin, 1990). Thus, the temperature equals 550-600oC
and pressure – 7 kbar for ferroedenite from the Uzbek area
granitoids that corresponds to depth 20-25 km, and they are
500-550oC
and 1-2 kbar for edenite from “granite-schist axis”
granitoids.
Therefore, the massif of
granitoids in the Uzbek area is deeper than granitoids of
“granite-schist axis”. The Uzbek pluton was likely to
form not only deeper, but earlier than granitoids of”granite-schist
axis”, and was put in the same erosional truncation due to
tectonic movements.
The study has been carried out
with financial support of RFBR (grant 08-05-00019) and integrational
program SO-UrO RAS.
References
Fershtater G.B.,
Borodina N.S., Chashchukhina V.A. (1978) Ferrofacies of granitoids
//Geochemistry. N2. P.147-160.
Ivanov K.S., Erokhin Yu.N., Ponomarev V.S. (2007)
Matter composition of “granite-schist axis” plutons
(Shaim district, West Siberia) // Endogenic mineralization in mobile
belts. XIII Readings of A.N.Zavaritsky. Ekaterinburg. IGG UrO RAS.
P.9-13.
Ivanov K.S., Kormiltsev
V.V., Fedorov Yu.N., Pogromskaya O.E., Erokhin Yu.V., Knyazeva I.V.
(2003) Main features of Pre-Jurassic basement structure in the Shaim
oil- and gas-bearing district. // Ways of gas-oil potential
realization of KhMAO. Khanty-Mansijsk. V.1. P.102-113.
Mishkin M.A. (1990)
Amphibole geothermobarometer for metabasites // Doklady AN SSSR.
V.312. N4. P.944-946.
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