GEODYNAMICS OF GRANITOID MAGMATISM IN KAZAKHSTAN

Ermolov P.V.*, Heinhorst J.**, Kroner A.***, Sidorenko I.S.*

*Institute of Mineral Resource Combined Use, Karaganda, Kazakhstan, permolov@ipkon.kz

**Technische Universitat, Clausthal-Zellerfeld, Germany, nobra@silicone-dream.ru

***Institut fur Geowissenschaften, Mainz, Germany, kroener@uni-mainz.de

Kazakhstan is a good example of accretionary tectonic occurrence. The character of granitoid magmatism in the early stages of development (Cambrian – Early Carboniferous) is largely determined by the position of specific terrane in global structure “open basin-island arc-continent”, and its deep structure. In that period, there existed crust blocks with the following geodynamic regimes:

1. Vend-Early Paleozoic crust with big part of mantle component. The young island arc is the suggested geodynamic regime. In the Kazakhstan present structure, the region is located in the area of the North Pre-Balkhash from latitude of the Tokrau River in eastern direction through the deposits Aktogai – Aidarly to the western rim of the Dzhungar plate in China. The model ages are from 0.52 to 0.58 Ga (area of the Valley deposit); from 0.5 to 0.67 Ga (Aktogai deposit); 0.64 Ga in south-western part of the Dalabuta ophiolite belt in China (Hu et al. 2000). Mantle contribution to crust formation is estimated as 75-85%: εNd(t) are 4.52-5.54; 4.41-5.94; 6.7 at the same points. The present field is spatially combined with the Itmurundinsky ophiolite belt, the Dalabuta ophiolite belt in China being its eastern continuation (Zhang et al. 1993). The granitoids exclusively belong to Type I. An abundance of deep melanocratic inclusions and relict parageneses of main composition (clinopyroxene + Labrador ± orthopyroxene) is their distinctive visual peculiarity. Metallogenic specialization: copper, gold.

2. Late Riphean crust of mixed composition with predominance of mantle component. The suggested geodynamical regime is mature island arc. The central and eastern parts of North Kazakhstan gold ore province spatially belong to it. Granitoid magmatism is represented by small bodies of gabbro-diorite-plagiogranite composition, and large batholiths of granodiorites and monzo-granodiorites. In contrast to the above point (1), granitoids belong to the purer line, i.e. deep inclusions are mostly represented by rocks of middle composition and not so abundant. The average model age is 790 Ma within the range 744-890 Ma by 9 datings (Heinhorst et al. 2000; Kroner et al. 2008). εNd(t) =3.49; 3.13; 2.83; 3.53; 3.12; 2.82; 4.20; 2.5; 2.0, that almost corresponds to 60% of mantle material in the source. Metallogenic specialization: gold combined with silver, tellurides and mercury.

3. Archean-Proterozoic crust subjected to amalgamation by mantle material. The suggested regime is median mass in the island archipelago. It spatially corresponds to the outcropped part of the Kokchetav exposure of metamorphic rocks and its slopes that are buried under the Ordovician flysch. Isotope characteristics of the Zerendinskaya series that composes the surface are typically platform ones: εNd(0) = -3.66 – (-)24.7; T_DM(Ga) = 1.84-2.65 Ma (Shatsky et al., 1999), while characteristics of the Zerendinsky and Borovsky granitoid complexes (O₃-S) demonstrate a significant contribution of mantle material to the crust at the level of magmatic hearths of these complexes, and as a consequence, the isotope rejuvenation of granitoid protoliths: εNd(t) = -2.63; 1.40; 1.01; T_DM = 1.23; 1.03; 1.07 Ga (Shatagin et al. 2001). While the crust model with the mantle material content of the first tens percent is right for the Zerendinskaya series, this index is not less than 50-60% for the Zerendinsky and Borovsky complexes. The direct evidence of amalgamation is in the Stepnyak gold deposit that occurs on the slope of the Kokchetav massif that is overlapped by the Ordovician flysch.

By composition, the Stepnyak massif is quite identical to small intrusions of central part in the North Kazakhstan gold ore province (point 1 in the text): gabbros, diorites, tonalities. However, its isotope characteristics are almost similar to those of the Zerendinsky and Borovsky complexes: εNd(t) = 0.04 – (-)1.0; T_DM = 1.1-1.21 Ga. A participation of ancient crust material in formation of magmatic hearth was proved by the presence of ancient cores in complex zircons: the age of rims is 480 Ma and that of cores – from 991 to 3890 Ma (Kroner et al. 2008). The isotope characteristics of the Early – MidPaleozoic granitoid complexes in the Kokchetav massif and model ages calculated based on them, are considered by us as indices of amalgamated continental crust.

4. All cases above are referred to the most widely spread aureole occurrence of island arc and orogenic granitoid magmatism in Kazakhstan. The narrow and extended (hundreds km) linear zones of riftogenic type anorogenic magmatism represent a peculiar case. They are small bodies and dikes of alkaline granites and granite-porphyries in the Tleumbet-Saur belt P₁ that lies along the border of the Hercynian Zaisanskaya and Caledonian Chingiz-Tarbagatai fold systems. The length of the belt in the Kazakhstan area is more than 350 km, its maximal width – 30 km and usual one – 8-10 km. All external features of joint zone of the two mentioned fold systems testify to rather thick crust. At the same time, the geochemical features indicate the closest association of alkaline-granite magmatism with young mantle: εNd = +5.3 to +7.86; T_(DM) = 0.24 to 0.6 Ga. However, it would be a mistake to state that they originated directly from the mantle substrate, as rubidium (90-180 g/t) is 2 and more times more in them than in oceanic pantellerites and granophyres (42-58 g/t; Ferrara, Treuil 1975). In the present case, we have the example of the Late Hercynian crust amalgamation.

References

Ferrara G., Treuil M. (1975) Petrological indication of trace element and Sr isotope distribution in basalt-pantellerite series//Bull Volcanol., 38. N3. P.548-574.

Heinhorst J., Lehmann B., Ermolov P. (2000) Paleozoic crustal growth and metallogeny of Central Asia: evidence from magmatic-hydrothermal ore systems of Central Kazakhstan//Tectonophysics, 328. P.69-87

Hu A., Jahn B-M., Zhang G., Chen Y., Zhang Q. (2000) Crustal evolution and Phanerozoic crustal growth in Northern Xinjiang: Nd isotopic evidence. Part I. Isotopic characterization of basement rocks // Tectonophysics, 328. P.15-51.

Kroner A., Hegner E., Lehmann B., Heinhorst J., Wingate M.T.D., Liu D.Y. and Ermolov P. (2008) Paleozoic arc magmatism in the Central Asian Orogenic Belt of Kazakhstan: SHRIMP zircon ages and whole-rock Nd isotopic systematics. Journal of Asian Earth Sciences (in press)

Shatagin K.I., Degtyarev K.e., Golubev V.I., Astrakhantsev O.V., Kuznetsov I.B. (2001) Vertical and lateral inhomogeneity of crust in North Kazakhstan: data of Paleozoic granitoid geochronological and isotope-geochemical study // Geotectonics, 5. P.26-44.

Shatsky V.S., Jagoutz E., Sobolev N.V., Kozmenko O.A., Parkhomenko V.S., Troesch M. (1999) Geochemistry and age of ultrahigh pressure metamorphic rocks from the Kokchetav massif (Northern Kazakhstan) // Contrib Mineral Petrol., 137. P.185-205.

Zhang C., Zhai M., Allen M.B. (1993) Implication of Paleozoic ophiolites from West Luggar, NW China for the tectonics of Central Asia // L.Geol.Soc. Lond., 150. P.551-561.