Variations of Oxygen Isotopic Composition in Magmas of Okhotsk-Chukotka Volcanic Belt

摘要

First data on oxygen isotopic composition in phenocrysts in volcanic rocks from Okhotsk-Chukotka volcanic belt (106-78 Ma, North-Eastern Russia) together with Sr and Nd isotopic data gives additional geochemical evidence on belt segmentation. Calculated delta O-18(melt) in calc-alkaline magmas of belt vary from normal-delta O-18 (from +5.2 to +6.9 parts per thousand) consistent with mantle-like signatures of radiogenic isotopes. Low-delta O-18 (from +4.7 to +4.8 parts per thousand) silicic magmas were found in Western-Okhotsk segment of the belt interpreted as assimilation of hydrothermally altered rocks from pre-caldera rifted basement. The highest delta O-18 values were obtained for Chukotka segment of the belt reaching +10.8 parts per thousand and these are correlated with elevated Sr-87/Sr-86 due to assimilation of relatively mature high-delta O-18 continental crust. Comparison with Kamchatka volcanic belt is performed.