GEOCHEMICAL AND ND-SR-PB ISOTOPIC COMPOSITION OF MESOZOIC VOLCANIC ROCKS IN THE SONGLIAO BASIN. NE CHINA

Abstract

Mesozoic volcanic rocks exposed in the Songliao basin form a major part of the basin fill and are the important pertroleum reservoir rocks. Petrogenesis of these volcanic rocks and their tectonic setting, however, are poorly understood due to lack of sufficient chemical and isotopic data. These volcanic rocks cover a wide compositional spectrum ranging from basaltic trachyandesite to high-silica rhyolite. They can be divided into two formations, the upper Jurassic Huoshiling formation (157-146 Ma) with dominant dark gray andesite and the lower Cretaceous Yingcheng formation (136-113 Ma) with dominant yellowish rhyolite. Compositionally, these rocks are peraluminous to metaluminous, high-K to medium-K, calc-alkaline. They are enriched in large-ion lithophile and light rare earth elements, depleted in high field strength and heavy rare earth elements, and commonly display negative anomalies of Nb, Ti and P. Initial 87Sr/86Sr ratios and initial εNd-values of these volcanic rocks range from 0.7034 to 0.7106 and from -3.4 to 5.3, respectively, and their δ18O values vary from 3.2‰ to 14.8‰. Initial 206Pb/204Pb and 207Pb/204Pb ratios have ranges of 17.65 to 18.22 and 15.52 to 15.57, respectively. The characteristic in Nd and Sr isotopic composition probably indicates a mixing of enriched MORB-like material and crustal component. Primary magmas for both the Jurassic and Cretaceous volcanic rocks could be derived from metasomatized enriched MORB-like sources, but the Cretaceous rhyolites commonly show crustal assimilation indicated by high initial 87Sr/86Sr ratios and low initial εNd values. Whether the volcanic rocks in the Songliao basin formed related either to the post-collisional setting of the Mongolia-Okhotsk suture or the subduction of the western Pacific plate remains to be discussed.