Isotope and trace-element geochemistry of alkali basalts and associated megacrysts from the Huangyishan volcano, Kuandian, Liaoning, NE China

Abstract

Trace-element abundances and Sr, Nd isotopic compositions have been measured on three megacrysts of clinopyroxene, three of garnet and one of amphibole, as well as their host alkali basalts from the Huangyishan volcano, Kuandian, NE China. The Ce isotopic compositions of three host basalt samples have been also analyzed. All megacrysts studied here are shown to be cognate with the host basalts mainly based on the isotopic compositions and major-element distributions between silicate crystal and melt. The source mantle of Huangyishan basalts is isotopically and chemically heterogeneous, and must have been enriched in Rb, La and Sm relative to Sr, Ce and Nd, respectively, for a relatively long time. Highly incompatible trace-element ratios are also suggestive of enrichment of large-ion lithophile elements in the mantle compared with the mantle source of MORB and most OIB. The fine structure, i.e. the double-concave shape of REE patterns commonly found in alkali basalts, is considered to be an important clue to the origin of alkali basalts. Based on the REE partition between the megacrysts and host basalts, evaluation of several theoretical models which can account for the double-concave shape of REE patterns shows that alkali basalts with such patterns must have been in equilibrium or quasi-equilibrium with both garnet and clinopyroxene solid phases. The small positive Eu anomaly commonly observed in REE patterns in alkali basalts, in most cases, is actually the turning point of the double-curved REE pattern. Partial melting of a garnet peridotite source is the most probable process which can generate the alkali basalts under consideration.