Petrological and mineralogical features of volcanic rocks from the central Kuznetsk Basin (southern Siberia)

The Kuznetsk Basin volcanic rocks are close in age (from 252.3 ± 0.6 to 246.2 ± 1.4 Ma) to the traps of the West Siberian Plate and Siberian craton, which formed as a result of the Permo-Triassic plume activity. The geologic and petrographic features evidence that the andesitic basalts exposed in the Karakan and Elbak quarries are effusive rocks; most of them are andesitic basalts, and the rest are trachyandesitic basalts. The mineral composition is as follows: olivine Fo_59–66, plagioclase An_47–60, and clinopyroxene En_47–42Fs_25–12Wo_42–33; Mg# = 82–63. Using the COMAGMAT 3.5 program, the magma crystallization conditions during the andesitic-basalt formation were determined: 1109–1105 ºC, buffer QFM-NNO. The studied rocks are enriched in LREE ((La/Yb)_ch = 4.7–7.5) and are depleted in HREE ((Sm/Yb)_ch = 2.0–2.8). A specific geochemical feature of the rocks is strong Nb, Ta, Ti, and Eu negative anomalies ((La/Nb)_PM = 4.5–1.6, (La/Ta)_PM = 3.2–2.0, Eu/Eu? = 0.7) and a positive U anomaly on their normalized element patterns; ?_Nd(T) varies from +2.3 to +3.1. The HREE depletion of the Kuznetsk Basin volcanic rock points to the presence of garnet in the mantle source during their generation. The low Mg# indicates that the parental melts are not of the primary-mantle genesis but are probably the product of differentiation in deep-seated intermediate magma chambers.