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Abstract Volume
workshop
Craton Formation and Destruction with
special emphasis on BRICS cratons
University of Johannesburg, South Africa
21-22 July 2012
Craton Formation and Destruction
21-22 July 2012
First data on the composition and age of the lower crust of the
central part of the Aldan-Stanovoy Shield: results of study of
xenoliths from Mesozoic plutons
1
1
2
1
1
Kravchenko A.A. , Smelov A.P. , Popov N.V. , Zaitsev A.I. , Beryozkin V.I. , Dobretsov V.N.
1
1
Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, Yakutsk, Russian Federation
– [email protected]
2
Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russian
Federation
Keywords: Lower crust, Aldan-Stanovoy Shield.
To date there has been almost no published data
on the composition and age of the lower crust of the
North-Asian craton (NAC) (Smelov and Timofeev,
2007). According to Rudnick and Fountain (1995),
the lower crust, 20-25 km thick, is composed of
metamorphic rocks of granulite facies. Within the
limits of the NAC, rocks of granulite facies are
widespread in the Aldan-Stanovoy and Anabar
Shields and constitute Early Precambrian terranes of
different age and composition (Rosen and Turkina,
2007; Smelov et al., 2010). Analysis of PTparameters of granulite metamorphism of some
terranes of the Central-Aldan superterrane (CAST)
show that they form a PT-path of isothermal
decompression and are typical for granulite
complexes formed as a result of crustal thickening
(Harley, 1989). These terranes are felsic and cannot
represent the lower crust. Rocks of high density
occur at depths of 20-45 km according to
geophysical data. The first data on the composition
and age of the pre-Mesozoic lower crust were
obtained during study of xenoliths of metamorphic
rocks from the Mesozoic syenite plutons which
intrude CAST granulite rocks.
greywacke. Sm-Nd isotopic data show that the
protoliths of high-alumina gneisses formed from
rocks of ~ 3.06 to ~2.85 Ga in age, while those of
the biotite-hypersthen gneisses were derived from
rocks of ~2.4 to ~2.33 Ga in age. The Fedorov
Group includes amphibole, diopside-amphibole, and
two
pyroxene-amphibole
plagiogneisses,
corresponding to basalt, andesite-basalt and
andesite. Rocks of the Fedorov Group formed by
metamorphism of protoliths with Nd model ages of
~2.3 to ~2.0 Ga (Smelov and Timofeev, 2007). UPb age of zircons from pyroxene-amphibole
plagiogneisses is 2006±3 Ma (Velikoslavinsky et al.,
2006). Age of granulite metamorphism (T=700830ºC, P = 5-6 Kb) is estimated to be 2.0-1.90 Ga.
Xenoliths of metamorphic rocks in syenite
plutons are quite rare. Their number is from 500 to
1000 pieces per1 km2, and size varies from 5 to 20
cm. Their petrographic composition is the same in
the different plutons. Xenoliths are represented by
amphibolites, amphibole gabbros (metagabbro),
garnet-amphibolites, anorthosites, and pyroxenites,
this suite having no analogues among rocks exposed
at the surface.
The CAST consists of the Nimnyr and Sutam
terranes. Plutons of Mesozoic syenites intrude
metamorphic rocks of the Nimnyr terrane. The
Nimnyr granulite-orthogneiss terrane is composed of
domes of granite-gneiss. The cores of the domes are
granite-, charnockite- and enderbite-orthogneisses,
covering more than 50 percent of the terrane.
Orthogneisses with Nd model ages of ~2.5 to ~2.3
Ga contain xenoliths of granite-gneisses and
tonalite-trondhjemite gneisses, which are older than
~3.35 Ga (Nutman et al., 1992). The external parts
of the domes are composed of a paragneiss complex
consisting of the Kurumkan and Fedorov Groups.
The Kurumkan Group includes quartzite and highalumina gneisses. The high-alumina gneisses are
similar in chemical composition to pelite and
siltstone and are interlayered with biotitehypersthene plagiogneisses, chemically similar to
Amphibolites and garnet-amphibolites have
foliation, revealed by amphibole orientation.
Amphibole gabbro, anorthosites and pyroxenites
show weakly manifested metamorphic textures.
There are gradual transitions from amphibole
gabbros
to
garnet-amphibolites,
then
to
amphibolites. In some cases, garnet-amphibolites
have massive structure and zonal plagioclase,
connecting them with amphibole gabbros. The
anorthosites are light-grey coarse-grained rocks with
a distinct lineation caused by alignment of darkcolored minerals.
Garnet-amphibolites consist of Grt+Cpx±Opx+
Hbl+Pl±Qtz. Garnet and clinopyroxene are often
surrounded by amphibole rims, which separate them
from plagioclase. Garnets show a narrow interval of
composition variation: Alm – 51.0-59.0%, Pyr –
Craton Formation and Destruction
21-22 July 2012
20.0-30.0%, Sps – 1.5-2.5, Grs – 11.0-20.0%, Adr –
0-7.0%. In clinopyroxenes Na2O content = 0.7-1.0
wt. %, Al2O3 = 3.1-5.5 wt. %. In orthopyroxenes
Al2O3 content = 0.4-2.3 wt. %. Amphibole
compositions are the most variable. Their TiO2
content varies from 0.1 to 2.3 wt. %, Na2O+K2O
content – from 1.9 to 4.5 wt. %. Anorthite content in
plagioclases varies from 40 to 55 %. The PT-path of
metamorphism shows isobaric character and
indicates that temperature varies from 900º to 700ºC
at almost constant pressure of 11.0 kb.
Regarding chemical composition, xenoliths
correspond to basic and intermediate rocks and
belong to a layered gabbro-diorite-anorthosite
complex of normal and sub-alkali series. The
position of the majority of composition points on the
AFM diagram shows a single tholeiitic trend. The
several points of analyses in the calc-alkalic field are
explained by plagioclase fractionation. The LaN/YbN
mean value = 8.7, with different concentrations of
REE in rocks. Heavy REE concentration exceeds
chondrite in 10-20 times, and light REE – in 30-120
times. All basic and intermediate xenoliths have
characteristic
negative
Eu
anomalies.
In
anorthosites, the level of relations of heavy REE to
chondrite < 1, light REE = 116, LaN/YbN = 197. Eu
positive anomalies are recorded by anorthosites.
Trace element and lead isotope compositions
were determined in three zircon grains from one
sample of amphibolite (Cpx+Hbl+Pl+Ilm) by the
LAM-ICP-MS
method. According
to
the
classification and regression trees of Belousova et al.
(2002), such zircons are typical for mafic rocks. The
Pb-Pb model age of zircons varies within the
interval 1900-1963 Ma, and probably reflects time
of emplacement and metamorphism, which is
confirmed by estimations of zircon crystallization
temperature by Watson et al. (2006) at 815-7300C.
Thus, we can draw the following conclusions:
1. Chemical and petrographic composition of
xenoliths, specifically the presence of early Hbl with
high TiO2 content, in association with Grt, Cpx, Opx
and Pl, indicate that these rocks are derivatives of
basalt melt crystallization.
2. PT-parameters of metamorphism show that,
magma crystallization occurred at a depth of ~30
km. Trends of isobaric rock cooling support the fact
that, the lower crust of the central part of the AldanStanovoy Shield was formed as a result of magmatic
mafic underplating.
3. Estimation of the average composition of the
Paleoproterozoic lower crust, taking into account
percentage ratio of different petrographic types of
rocks and their size, indicates the following values:
SiO2 – 53.61, TiO2 – 0.98, Al2O3 – 16.35, Fe2O3 –
9.56, MnO – 0.21, MgO – 4.5, CaO – 7.86, Na2O –
3.21, K2O – 1.58, P2O5 – 0.21, LOI – 1.57 (wt.%)
and Ba – 897.59, Rb – 27.79, Sr – 642.57, Y –
22.28, Zr – 198.1, Nb – 7.69, Th – 1.72, Ni –
132.12, V – 167.44, Cr – 419.96, Hf – 4.84, Ta –
0.41, Co – 27.19, U – 0.73, La – 24.98, Ce – 48.27,
Pr – 5.96, Nd – 24.69, Sm – 5.02, Eu – 1.18, Gd –
4.79, Tb – 0.66, Dy – 4.17, Ho – 0.86, Er – 2.48, Tm
– 0.33, Yb – 2.35, Lu – 0.35 (ppm).
4. Formation of the lower crust was at the final
stage of collision processes between terranes of the
North-Asian craton, as the part of the Nuna
(Columbia) supercontinent. Presumably, formation
of the lower crust contributed to the rapid collapse
of the collision orogen, exhumation of the granulites
of CAST and transition to a platform stage of the
North-Asian craton development at the turn of 1.81.7 Ga.
References
Belousova E.A., Griffin W.L., O’Reilly S.Y., and Fisher
N.I., 2002. Igneous zircon: trace element composition
as an indicator of source rock type. Contributions to
Mineralogy and Petrology, 143: 602–622.
Harley S.L., 1989. The origin of granulites: A
metamorphic perspective. Geological Magazine, 126:
215-247.
Nutman A.P., Chernyshev I.V., Baadsgaard H. and
Smelov A.P., 1992. The Aldan Shield of Siberia,
USSR: the age of its Archean components and
evidence for widespread reworking in the midProterozoic. Precambrian Research, 54(4): 195-209.
Rosen O.M. and Turkina O.M., 2007. The Oldest Rocks
Assemblages of the Siberian Craton. In: Van
Kranendonk M.J., Smithies R.H., and Bennett V.C.
(Editors), Earth’s Oldest Rocks, Amsterdam, Elsevier:
793-838.
Rudnick R.L. and Fountain D.M., 1995. Nature and
composition of the continental crust: a lower crustal
perspective. Reviews of geophysics, 33: 267-309.
Smelov A.P., Yan H., Prokopiev A.V., Timofeev V.F.,
and Nokleberg W.J., 2010. Chapter 4. Archean
through
Mesoproterozoic
Metallogenesis
and
Tectonics of Northeast Asia. In: Warren J. Nokleberg
(Editors), Metallogenesis and Tectonics of Northeast
Asia, U.S. Geological Survey Professional Paper
1765: 56.
Smelov, A.P. and Timofeev, V.F., 2007. The age of the
North Asian cratonic basement: An overview.
Gondwana Research, 12: 279-288.
Velikoslavinsky S.D., Kotov A.B., Salnikova E.B.,
Kovach V.P., Glebovitsky V.A., Zagornaya N.Yu.,
Yakovleva S.Z., Anisimova I.V., Fedoseenko A.M.,
and Tolmacheva E.V., 2006. Protoliths of the
metamorphic rocks of the Fedorov Complex, Aldan
shield: Character, age, and geodynamic environments
of origin. Petrology, 14(1): 21-38.
Watson E.B., Wark D.A. and Thomas J.B., 2006.
Crystallization thermometer for zircon and rutile
Contributions to Mineralogy and Petrology, 151: 413433.