La, Ce, Nd, Sm, Eu, Gd, Tb, Yb, Lu, Cr and Sc contents have been determined by instrumental neutron activation analyses (INAA) in high-K calc-alkaline and shoshonitic volcanic rocks from the Upper Cretaceous volcanic belt of Eastern Srednogorie (Bulgaria). The high-K calc-alkaline rocks are characterized by fractionated LREE and unfractioned HREE patterns. The shoshonitic rocks have higher overall REE abundance, higher La/Yb ratio and they display a variable degree of HREE fractionation. The absolute abundance of REE and their distribution patterns are consistent with the hypothesis that both calc-alkaline and shoshonitic magmas were derived by partial melting of mantle enriched in LREE. The flat HREE patterns of calc-alkaline rocks indicate that they represent magma formed by melting of spinel peridotite. The variable HREE fractionation of shoshonites can be explained by non-modal melting of a garnet peridotite with differing proportions of garnet in the residuum. The low Ni, Cr and Sc contents of the analyzed rocks indicate that the primary shoshonitic and calc-alkaline magmas underwent fractionation involving removal of olivine and some spinel and pyroxene during their ascent to the surface.
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