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REFLECTION OF PECULARITIES OF CENTRAL KAZAKHSTAN WEST SIALIC CRUST
STRUCTURE IN COMPOSITION OF EARLY PALEOZOIC GRANITOID COMPLEXES
*Geological Institute RAS,
Moscow, Russia, degtkir@ginras.ru
**Institute of Geology of
Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, Moscow,
Russia shat@igem.ru
Lower
Paleozoic magmatic complexes, mainly granitoids, are widespread in
Paleozoides of Central Kazakhstan.
In the western part of Central Kazakhstan Early Paleozoic granitoids
participate in the structure of large Stepnyak-Northern Tien-Shan
volcanic-plutonic belt. It is formed by Middle-Upper Ordovician
island arc volcanic complexes, which formation took place within
ensialic island arc system. Intrusion of large granitoid massif
manifested terminal stage of ensialic arc evolution. These
granitoids intrude both volcanic sequences of the belt and complexes
of Precambrian sialic massifs, locating in its back-arc part. The
largest sialic massifs are Kokchetavsky and Aktau-Dzhungarsky ones.
The latter was displaced along the system of large sinistral
strike-slips from the back-arc part of the belt to the position
before its frontal part at the beginning of Silurian (Degtyarev,
2003).
In the northern part of
Stepnyak-Northern Tien-Shan belt and within Kokchetavsky massif,
located westward, granitoids are represented by granodiorites and
granites of Krykkuduksky and granites, granodiorites of Zerendinsky
complexes consequently. Carried our in the last years Rb-Sr and U-Pb
datings showed similar age of granitoids, nearly 450 m.a., which
corresponds to the middle of Late Ordovician (Shatagin et al., 2001;
Letnikov et al., in press).
Granitoids
of both complexes compose large intrusive bodies of harpolith-,
lopolith- and plate-like forms up to 10-15 km in thickness. The
study of composition of Krykkuduksky and Zerendinsky complexes
granitoids showed that they have both similar features and principal
differences. As a whole silica and potassium contents in granitoids
increase from the rocks of Krykkuduksky complex to Zerendinsky one.
In the latter porphyre-like granites and granodiorites prevail,
whereas in Krykkuduksky complex granodiorites, tonalites and
plagiogranites of the second intrusive phase compose 70% of entire
volume. This difference results in high-K calc-alkaline character of
Zerendinsky complex granites and sodium calc-alkaline character of
Krykkuduksky complex rocks. Granitoids of both complexes belong to
I-type granites, though some granites of Zerendinsky complex may be
referred to S-type granites. Rocks of both complexes are
characterized by chondrite-normalized REE patterns with slight
enrichment in LREE and small negative Eu-anomaly in granites of
Zerendinsky complex. Late Ordovician complexes essentially differ in
Nd isotopic composition. Granodiorites of Krykkuduksky complex have
stable positive εNd(Ò)
values nearly +3.5, whereas granites and granodiorites of
Zerendinsky complex are characterized by variable εNd(Ò)
values from -2.6 to +1.5.
According
to structural position and composition the Akzhalsky complex of
Aktau-Dzhungarsky sialic massif is similar to Zerendinsky complex.
Akzhalsky complex comprise quartz monzodiorites and quartz diorites
of Berkutinsky and several small massifs, and also
granodiorite-granite Akzhalsky massif. U-Pb dating of Akzhal massif
granodiorites showed that this complex has younger Late
Ordovician-Early Silurian age (Degtyarev et al., 2006). Granitoids
of Akzhalsky complex belong to calc-alkaline series and low-Al
granites of I-type. They have REE patterns with slight enrichment in
LREE without Eu-minimum. These features indicate insignificant
extent of melts fractionation. On different discriminant diagrams
these granitoids with those of Krykkuduksky and Zerendinsky
complexes fall in the volcanic arc granites field, which is
confirmed by trace elements spiderdiagrams. The latter are
characterized by high contents of LIL elements (Cs, Rb, Ba), a
number of HFSE elements (Th, U) and low contents of HREE and Nb. The
study of Nd isotopic composition in granitoids of Akzhalsky complex
showed that εNd(Ò)
values changes from -0.27 in quartz diorites of Berkutinsky massif
to -2.37 in granites of Akzhalsky massif.
Obtained
data on structural position, age, petro-geochemical and isotopic
composition of Late Ordovician-Early Silurian granitoids allow
suggesting that their formation was related to terminal stage of
evolution of Middle-Late Ordovician Stepnyak-Northern Tien-Shan
ensialic island arc. All these granitoids belong to low-Al island
arc tonalite-trondhjemite-granodiorite series and probably have
lower crustal source. Differences in petro-geochemical and isotopic
composition of rocks of different complexes may be due to maturity
and thickness of sialic crust, in which primary melts intruded and
extent of their contamination. Minimum thickness and high
destruction probably was typical for the crust of the northern part
of Stepnyak-Northern Tien-Shan belt, where intrusives of
Krykkuduksky complex are located. The crust of Kokchetav and
Aktau-Dzhungarsky sialic massif was characterized by significant
thickness and maturity, so its effect on the granite genesis is
maximum. This resulted in variability of ratios of some trace
elements (Y/Nb, Zr/Nb, Zr/Ce), Rb/Sr versus silica content increase
from quartz diorites to granites, and also in wide variability of Nd
isotopic composition.
Work is supported by RFBR (project 06-05-65311),
Programme of Earth Sciences Department of Russian Academy of
Sciences ¹10.
References
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Degtyarev K.E.,
Shatagin K.N., Kotov A.B., Salnikova E.B., Luchitskaya M.V.,
Yakovleva S.Z., Plotkina Yu.V., Fedoseyenko A.M. Early Paleozoic
granitoids of Aktau-Dzhungar microcontinent (Central Kazakhstan) //
Doklady RAS. 2006. Vol. 411. ¹ 1. P.80–84. (in Russian)
Letnikov F.A., Kotov A.B., Degtyarev K.E., Salnikova E.B., Levchenkov O.A., Shershakova M.M., Shershakov A.V., Rizvanova
N.G., Makeyev A.F., Tolkachev M.D. Late Ordovician granitoids of
Northern Kazakhstan: foundation of age and structural position (in
press, in Russian)
Shatagin K.N.,
Degtyarev K.E., Golubev V.N., Astrakhantsev O.V., Kuznetsov N.B.
Vertical and lateral heterogeneity of Northern Kazakhstan crust:
data of geochronological and isotopic-geochemical study of Paleozoic
granitoids // Geotectonics. 2001. ¹ 5.
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