TECTONIC SETTINGS OF FORMATION OF VOLCANIC AND SEDIMENTARY ROCKS OF THE ITMURUNDY ZONE, CENTRAL KAZAKHSTAN

The paper presents new petrographic and geochemical data from volcanic and sedimentary rocks and first U-Pb ages of detrital zircons from sandstones of the ltmurundy zone of central Kazakhstan. The volcanic rocks are aphyric and porphyric basalts, andesibasalt and andesite. The major element composition of tuff and sandstone are close to that of andesite. The poorly sorted greenish grey sandstones carry numerous fragments of volcanic and sedimentary rocks suggesting its greywacke nature which is probably due to. The greywacke probably formed by the destruction of undissected arc. The distribution of U-Pb ages of detrital zircons spanning 505 to 432 Ma has unimodal character peaked at 445 Ma suggesting formation of the sandstones by the destruction and subsequent transportation of clastic material from a late Ordovician intra-oceanic arc. In geochemical diagrams, the tuffs and sandstone plot close to the volcanic rocks. All chondrite-normalized REE spectra show enrichment in LREE (La-N =38-367, La/Yb-N=4.0-16.9, La/Sm-N=2.1-3.3) and moderate to weakly differentiated HREE (Gd/Yb-N=1.4-4.0). However, the level of REE concentrations in the volcanic rocks, in particular, in basalts, is significantly higher than that in the sandstone and andesite. The primitive mantle normalized trace-element diagrams show peaks at Nb (Nb/La-pm=0.9-1.6, Nb/Th-m=0.8-1.6) in most basaltoids, but troughs at Nb for andesite, tuff and sandstone (Nb/La-pm=0.25-0.31, Nb/Th-pm=0.17). The previous and new geochronological, petrographic and geochemical data show that the volcanic and sedimentary rocks of the ltmurundy zone formed in Ordovician time in an intra-plate oceanic setting and in a supra-subduction setting at a Pacific-type convergent margin.