Geochemistry of Cenozoic alkali basaltic lavas from Shandong Province, eastern China

Abstract

Cenozoic alkali basaltic rocks from the central and eastern parts of Shandong province, eastern China include alkali basalts, basanites and olivine nephelinites, some of which contain spinel peridotite xenoliths and megacrysts predominantly of clinopyroxene and anorthoclase. The abundances and ratios of incompatible elements in the basalts are similar to those of many continental and oceanic island alkali basalts. The magmas were derived from metasomatised garnet-clino pyroxene-bearing peridotites by variable but small degrees of melting. A metasomatism which produced an enrichment of strongly incompatible elements was superimposed on a depleted source shortly prior to a melting event. The basaltic rocks from tbe central and eastern regions have distinct major and trace element characteristics and Sr-Nd-Pb isotopic compositions. The Eastern Shandong lavas have higher abundances of Na, P and incompatible trace elements but are lower in Mg and compatible trace elements than those from the central part. The source of the Eastern Shandong basalts was relatively homogeneous in Sr and Pb isotopic compositions and was metasomatically enriched, at least in part, by carbonate-rich fluids. Phlogopite and probably apatite remained in the residue after the extraction of Eastern Shandong nephelinites. The basaltic rocks of Central Shandong were generated from a heterogeneous multicomponent source. <51SO values negate a significant role of upper crustal contamination in the genesis of the Shandong basalts. The compositional similarities between the Shandong continental alkali basaltic lavas and continental and oceanic island alkali basalts suggest that their mantle parents were metasomatically enriched by fluids derived from the same source—rising asthenospheric plumes. © 1991, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.