Geochemical and Sr–Nd isotopic constraints on the petrogenesis of late Cenozoic basalts from the Abaga area, Inner Mongolia, eastern China

Abstract

Over the past 30years, the Cenozoic basalts in eastern China have been the subject of many investigations, but their origin remains highly controversial. The Abaga–Dalinuoer volcanic field in Inner Mongolia consists of an approximately 10,000km2 lava plateau and more than 300 volcanic cones and extends northwestward to the adjacent Dariganga lava plateau of Mongolia, forming one of the largest Cenozoic volcanic fields in eastern Asia. In this paper, we concentrate on the Abaga volcanic field that comprises more than 200 monogenetic cones, including scoria cones, lava flows, and maars. Phreatomagmatism has contributed to the generation of several volcanic edifices, and the diameter of the maars craters can reach 3 to 6km, which are among the largest in eastern China. The volcanic rocks in the Abaga area can be divided into alkaline basalts and tholeiites. According to trace element and Sr–Nd isotopic characteristics, the Abaga magmas were primarily derived from a relatively homogeneous garnet peridotite source within the asthenosphere. Variable degrees and depths of partial melting played a key role in the genesis of magma from alkaline basalts to tholeiites. In contrast to the surrounding Cenozoic volcanic fields, such as Datong, Jining, and Hanuoba, in the western block of the North China Craton, the Abaga volcanic region in the southern part of the Central Asian Orogenic Belt is distinguished by the deepest melting and the thickest lithosphere. Negative Ba-, Rb-, K-, and Ti anomalies and relatively high 87Sr/86Sr ratios suggest the presence of residual phlogopite/amphibole during mantle melting. The origin of the Abaga magma most likely required involvement of fluids released from the stagnant Pacific slab in the mantle transition zone.