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New data on the geodynamic
nature and age of granitoids of the
Billyakh
pluton (Anabar
Shield)
Smelov A.P.*,
Berezkin V.I.*, Salnikova E.B.**, Kovach V.P.**, Kravchenko A.A.*, Dobretsov V.N.*
*Diamond and Precious Metal
Geology Institute SB RAS, Yakutsk, Russia, a.p.smelov diamondysn.ru
**Institute of Geology and
Geochronology of Precambrian RAS, St. Petersburg, Russia
The
Anabar shield represents an uplift of the Precambrian crystalline
basement in the northern Siberian platform. Three large terranes are
recognized within its structure: Magan, Daldyn, and Khapchan. They
are composed of various associations of gneisses, crystalline
schists, quartzites, and marbles metamorphosed to the granulite
facies, with the age ranging from 3.2 to 2.4 Ga. The terranes are
bounded by the tectonic mélange
zones (Lamuika, Kotuykan, Billyakh, etc) made of diaphthorites
formed after granulites, the amphibolite facies rocks, and
anorthosite and granitoid plutons. In the context of geodynamics,
the zones are interpreted as the root parts of collisional sutures,
and the age of collision is estimated at 1.9 Ga (Rosen et al., 2000;
Smelov et al., 2002). Assumptions as to the collisional nature and
the age of the zones have been made mainly on the basis of a wide
distribution of various granitoid types within their limits. There
are, however, only fragmentary data on the geological and structural
settings of the granitoids as well as their petrographic and
chemical compositions. No geochronological and geochemical data
obtained by up-to-date methods are available. The paper reports the
results of studying porphyry granitoids of the Billyak pluton
located within the zone under the same name.
The
Billyakh zone, separating the Daldyn and Khapchan terranes, extends
northwestward for 250 km and ranges up to 30 km wide. The Daldyn
terrane is composed mostly of enderbites with subordinate
garnet-biotite plagiogneisses, gneisses, quartzites, and basic,
rarely ultrabasic, crystalline schists. Characteristic rocks of the
Khapchan terrane are marbles, calciphyres, and calc-silicate rocks
in association with various garnet-bearing and garnet-free
plagiogneisses, gneisses, and crystalline schists. The structure of
the Billyakh zone is defined by a system of closely spaced and
intersecting faults. In between the faults there are blocks and
plates of variably metamorphosed rocks of the contacting terranes.
The faults are traced by blastomylonite and cataclasite zones with
an upthrow or strike-slip fault kinematics (Lutz, Oxman, 1990).
Mapping of the boundaries of the Billyak granitoid pluton (60x8 km)
showed that their forms are defined by the superposed folds produced
by strike-slip motions. Parallel to the axial planes of the folds
occur linear zones of blastomylonites with the bodies of weakly
schistose alaskite granites and aplites.
The
granitoids of the Billyakh pluton are weakly gneissose. Almost all
the granitoid varieties contain K-feldspar porphyroblasts and the
groundmass with a granoblastic or lepidogranoblastic texture. The
porphyroblasts are relict magmatic porphyry phenocrysts oriented
along the axes of superposed schistosity. In addition to feldspars,
hornblende, biotite, and quartz are present in various proportions.
The melanocratic rocks are dominated by green or brown-green
hornblende associated with brown biotite. Biotite is the principal
dark-colored mineral in the leucocratic rocks and the only one in
the leucogranites. The mineral assemblages correspond to the
amphibolite facies conditions.
Chemistry
of the the granitoids of the Billyakh pluton matches that of quartz
monzonites, quartz syenites (granosyenites), subalkaline granites,
granodiorites, and, rarely, quartz diorites. On Harker diagrams, the
bulk of the analytical points form a single differentiation trend.
Trace elements and REE were determined in only a few samples.
According to these data, the amounts of Zr, Nb, Ta, Y, Hf, Sr, Ba,
V, and Co and the total REE content decrease in the differentiation
series. Rb, Cs, U, Th and Ni contents are low, and there is no
tendency for their change depending on the acidity of the rocks. The
granitoids display fractionated spectra of REE distribution (LaN
/YbN=19-52)
with an Eu minimum.
In terms of chemical
composition, the granitoids of the Billyakh pluton do not match
one-to-one with M, I, S or A-type granites. However, they contain a
large total amount of alkalies and in this they are close to A-type
granites. At the same time, their relatively low Rb, U, Th, Zr, Y,
Nb and V values make them comparable to S- or I-types granites. As
seen from the A/Nk-A/CNK diagram, the granitoids belong to the
metaluminous I-type. Judging from the Rb/Y+Nb and Nb/Y ratios, they
could have formed in a volcanic arc environment as confirmed by the
A/NK-A/CNK diagram in which the composition points of the rocks plot
mainly into the fields of granites of island arc and continental
margin settings.
Until
recently, there were only rare K-Ar ages (1.80-1.81 Ga) available
for the granitoids of the Billyakh pluton (Krylov et al., 1963).
Using the isochron U-Pb method we determined the age of zircons of
magmatic habit from the porphyry granites, which yielded 1983±2.9
Ma. Studies of the isotopic Nd composition of the granitoids showed
that the source for them were the rocks with TNd(DM)-2206-2412
Ma and εNd
(T)=+1.0 - -1.4, i.e. compositionally similar to weakly depleted
mantle or lower crust. The limited isotopic data available for the
basic granulites of the Daldyn terrane do not contradict this
assumption (TNd(DM)
–2422 Ma and εNd
(T)= +1.2).
Thus,
it seems likely that at about 1.98 Ga the eastern margin of the
Daldyn terrane still was an active continental margin, and that the
Billyakh pluton granitoids were formed above the subduction zone.
The ages of 1.80-1.81 Ga seem to reflect the time of completion of
the collision processes between the Daldyn and Khapchan terranes.
The work was completed with the
financial support of the Russian Foundation for Basic Research,
under the project N 07-05-00695a.
References
Krylov, A.Ya., Vishnevsky, A.N. Silin, Yu.I. et
al.,1963. Absolute age of the Anabar shield rocks //Geokhimiya,
no.12, P.1140-1144 (in Russian).
Lutz, B.G. and Oxman, V.S., 1990. Deeply eroded fault
zones of the Anabar shield. –Moscow: Nauka, 260p. (in
Russian).
Rosen, B.G., Zhuravlev,
D.Z., Sukhanov, M.K. et al., 2000. Isotope-geochemical and age
characteristics of terranes, collision zones and the related
anorthosites in the northeastern Siberian craton //Geologiya i
Geofizika, 41, no.2, P.163-180 (in Russian).
Smelov, A.P., Berezkin,
V.I., Zedgenizov, A.N. et al., 2002. New data on the composition and
ore content of the Kotuykan tectonic mélange zone //Otechestvennaya
Geologiya, no.4, P.45-49 (in Russian).
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