EVOLUTION OF MIDLE TO LATE PALEOZOIC GRANITIC MAGMATISM AT KOKCHETAV AND AKTAU-DZUNGARIAN MASSIFS (KAZAKHSTAN)

Degtyarev K.E.*, Shatagin K.N.**, Tretyakov A.A.*, Luchitskaya M.V.*

*Geological Institute RAS, Moscow, Russia, degtkir@ginras.ru

**Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry RAS, Moscow, Russia shat@igem.ru

Sialic massifs play an important role in the structure of Central Kazakhstan. The massifs are large blocks of Precambrian continental crust separated from the surrounding early Paleozoic complexes by tectonic contacts. During middle and late Paleozoic time the massifs made up heterogeneous basement of marginal continental volcano-plutonic belts, and underwent there intensive tectonic and magmatic reworking. Voluminous granitic magmatism was one of the manifestations of this reworking.

Kokchetav and Aktau-Dzungarian sialic massifs are from the largest massifs of that kind. Their basement comprises gneisses, quartzites, shales, and acid volcanics. Formation of the Precambrian continental crust was completed by intrusion of granites: 1128±12 Ma at the Kokchetav massif, and 917±6 Ma at the Aktau-Dzungarian massif (Letnikov et al., 2007; Degtyarev et al., 2008). Late Paleozoic platform mantle is composed of terrigeneous-carbonate-siliceous suites which are most representative at the Aktau-Dzungarian massif.

Among middle to late Paleozoic Andean-type active margin complexes the most widespread at both massifs are granites. Although differing in age, granites of both massifs comprise precisely the same succession of formations. At the Kokchetav massif the granites are middle Paleozoic in age, whereas at Aktau-Dzungarian massif there are also some late Paleozoic granitic plutons.

The oldest from the discussed granites at the Kokchetav massif is Silurian Borovskoy suite (Borovskoy and Zhekey plutons), at the Aktau-Dzungarian massif the oldest is Late Devonian Kyzylespinsky suite (East-Shaltas and Kosmurun plutons) (Shatagin et al., 2001; Degtyarev et al., 2007; Letnikov et al., in press). Main intrusion phases of these suites are Dy-normative, slightly metaluminous granites and adamelites. The granites have relatively steep REE pattern with moderate negative Eu-anomaly, Rb/Sr ratio and SiO₂ are frequently correlated which is indicative of small degree of initial magma differentiation. Variations in Y/Nb, Zr/Nb, Zr/Ce ratios suggest magma contamination in intermediate chambers. In general, according to their geochemical features Early Silurian granites of Kokchetav massif and Late Devonian granites of Aktau-Dzungarian massif are of I-type.

Relatively younger granitic suite and leucogranitic suite formed simultaneously, but differ in some composition peculiarities and mineralization. At Kokchetav massif these are Early Devonian Balkashinsky and Orlynogorsky suites, at the Aktau-Dzungarian massif – Late Carboniferous to Early Permian suites of granitic (Shaltas and Zhaman-Karabas plutons) and leucogranitic composition (Kyzyltau, Ortau, and Airtau plutons) (Negrey et al., 1991; Yudintsev and Simonova, 1992; Shatagin et al., 2001; Degtyarev et al., 2007).

From one side, plutons of the Balkashinsky suite are mainly leucogranitic and alaskitic. Late Carboniferous to Early Permian Shaltas and Zhaman-Karabas plutons consist of Bi-granites and Amf-Bi-granites. Although rocks of both suites are I-type granites, they differ in REE distribution. Balkashinsky suite leucogranites and alaskites have very large negative Eu-anomaly indicative of deep differentiation of parental granitic magma. Characteristic is that both suites don’t bear any valuable mineralization.

From another side, Orlinigorsky suite plutons, as well as Late Carboniferous to Early Permian Ortau, Kyzyltau, and Airtau plutons are composed of multiple leucogranitic intrusions. Yudintsev and Simonova (1992) subdivided Orlynogorsky granites to a standard and Li-F-geochemical types. The latter has highest Rb/Sr-ratio and differs from others by flat and even U-shape REE patterns with profound negative Eu-anomaly – all signs suggesting very high degree of differentiation. It is difficult to discriminate the granites between I and S-types. Also, high abundances of LILE and HFSE make the granites similar to that of A-type. Characteristic is that on both massifs these later granites are associated with rare-metal and tin (only at Kokchetav) mineral deposits.

Nd-isotope data suggest some differences in evolution of crustal sources for the granites.

Value of εNd(Т) in granites of the Kokchetav massif gradually decreases from +1,0 in Early Silurian Borovskoy suite to −0,8 in Early Devonian Balkashinsky suit and to −2,4 ÷ −4,7 in Orlynogorsky suite. In the same succession Nd-model age changes from 1000 to 1300 Ma (Shatagin et al., 2001). In general, Nd-model ages of Middle Paleozoic granites of the Kokchetav massif are similar to the age 1100 Ma of earlier granitoids formed at the end of continental crust formation at the region.

An inverse picture is seen at the Aktau-Dzungarian massif: there εNd(Т) increases from −1,7 in Late Devonian granites of Eastern Shaltas to +3,5 in Late Carboniferous to Early Permian leucogranites of Kyzyltau pluton. Respective decrease of Nd-model age is from 1200 to 740 Ma, which is quite different from the age of local Precambrian granites (920 Ma), completing continental crust formation of the massif.

The trends in Nd-isotope evolution pointed above could be interpreted as a result of changing of proportion between magma protoliths with different εNd in the sources of the granites. In case of the Kokchetav massif the protoliths most probably were lower and middle continental crust, which contributions changes with decreasing of magma chamber depths. In case of the Aktau-Dzungarian massif, if previous explanation could be accepted, reverse vertical distribution of protoliths with relatively high and relatively low εNd values should be suggested.

The research was supported by the RAS Division of Earth Sciences Program N 10 and RFBR grant 06-05-65311.

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