Geochemistry of Cenozoic volcanic rocks from Kirin Province, northeast China

抄録

The major volcanic activity in Kirin Province, northeast China occurred in Changpaishan area and lasted from the Miocene to Holocene. The volcanic rocks include basanite, alkali basalt, tholeiite, trachyte, and rhyolite. Ultramafic xenoliths and megacrysts occur in the basanite and alkali basalt. The compatible (Ni, Cr, Sc, V, Co) and incompatible elements (Rb, Li, Th, Hf, La, Lu, Nb, Zr, Y) vary systematically with 100MgO/(MgO + ΣFeO) in the volcanic rocks. Fractional crystallization clearly was important in the magmatic evolution. The volcanic rocks and their ultramafic xenoliths have characteristic Sr and Nd isotopic ratios which fall between DMM and EM-I or EM-II. There are two types of spinel-lherzolite xenoliths in the basalts from Kirin Province: (1) an E-type enriched in LREE with (La/Yb)_N varying from 6.6 to 8.15; (2) a D-type depleted in LREE with (La/Yb)_N varying from 0.2 to 0.7 indicating of mantle heterogeneity in the Kirin region. The E-type lherzolites contain higher TiO₂, K₂O, P₂O₅, Rb, Sr, Ba, Y, Nb, Ta, Th and U than the D-type lherzolites. The former may have resulted from mantle metasomatic processes. The tholeiite, alkali basalt and basanite may have been derived from an isotopically similar mantle source through different degree of partial melting at different P-T conditions. The trachyte may be derived from the basalt through fractional crystallization of plagioclase, olivine, clinopyroxene and magnetite.