PETROGENESIS OF POST-ACCRETIONARY GRANITES OF THE KIZIL COMPLEX, SOUTHERN URALS: GEOCHEMISTRY, Sr-Nd ISOTOPIC DATA, U-Pb AGE

The Late Devonian – Early Carboniferous intrusive magmatism in the West Magnitogorsk zone of the Southern Urals is associated with post-island arc restruction of the earth’s crust during the assembly of the Laurasia supercontinent. Magmatic bodies usually have a submeridional orientation, and the rocks are compositionally variegated and display mixed geochemical characteristics of supra-subduction and intraplate magmatism. A small proportion of magmatism falls within plagiogranites, the earliest of which correspond to the Kizil complex, which records the continental-type crust formation at the base of the Magnitogorsk island-arc terrane. It has been found that thin dikes and sills of the Kizil complex occur widely in the central part of the West Magnitogorsk zone and are represented by metasomatized plagiogranites with wide variations in alumina content (12–18 %), iron content (Fe# 0.7–0.9) and total REE (117–347 ppm). The ID TIMS method yielded four U-Pb zircon ages of which the 345±6 Ma value agrees most satisfactorily with the geological data; other values may be associated with the assimilation of host rocks and metasomatism of granites. Based on the microelement (variable minimums for Eu and Sr, minimums for Ti, Nb and Ta) and isotopic composition of Sr, Nd (εNd (345) =5.9–6.0, 87 Sr/ 86 Sr (345) =0.7041–0.7051), it was concluded that that the main source of the Kizil granitoids could be amphibolized mafic rocks of the ophiolite association, thrust onto the margin of the Laurussia paleocontinent along the Main Ural Fault zone to the west of the West Magnitogorsk zone.