Home – Home – Jornals – Russian Geology and Geophysics 2014 number 1

The paper presents new petrographic, geochemical, and petrological data from volcanic rocks of suprasubduction origin of the Char shear zone in eastern Kazakhstan. We discuss bulk rock composition (concentrations of major and trace elements), types of mantle sources and parameters of their melting, conditions of crystallization of mafic magma, and geodynamic settings of basalt eruption. According to the major element composition, the volcanic rocks are basalt, andesibasalt, and andesite of tholeiitic and transitional, from tholeiitic to calc-alkaline, series. They are characterized by low TiO ₂ (0.85 wt.% on average) and crystallization trends in MgO–major elements plots. In term of trace element composition, the volcanic rocks show moderately LREE-enriched rare-earth element patterns and are characterized by negative Nb anomalies present on the multi-element spectra (Nb/La _pm = 0.14–0.47; Nb/Th _pm = 0.7–1.6). The distribution of rare-earth elements (La/Sm _N = 0.8–2.3, Gd/Yb _N = 0.7–1.9) and the results of geochemical modeling in the Nb–Yb system suggest high degrees of melting of a depleted mantle source at spinel facies depths. Fractional crystallization of clinopyroxene, plagioclase, and opaque minerals also affected the final composition of the volcanic rocks. Clinopyroxene monomineral thermometry calculations suggest that the melts crystallized within the range of 1020–1180 °С. We think that this volcanic complex formed on the western active margin of the Paleo-Asian Ocean.