Geochemical Characteristics and Geological Significance of the Shangsu Mafic Dikes in Jiangxi

Abstract

Mafic dikes are developed in the Shangsu region of Jiangxi, including Nanyuan Mountain and Mianhuaao. These dikes intrude into the Upper Triassic Anyuan Formation and are primarily composed of pyroxene, plagioclase, and opaque minerals, exhibiting a dolerite texture. The mafic dikes were found to have an SiO2 content ranging from 48.38% to 50.53% and the (K2O + Na2O) content ranging from 2.88% to 4.16%. The Na2O/K2O ratio was found to be between 2.56 and 3.99. The Eu anomaly was absent (δEu = 0.91–0.95), suggesting no influence of plagioclase fractionation during the rock formation process. The Ce anomaly was found to be absent (δCe = 1.04–1.08). The Mg# was found to range from approximately 40.06 to 45.06, indicating minimal crystal fractionation within the magma. The mafic dikes are enriched in light rare earth elements and large ion lithophile elements (LILE) and Th, while they showed significant depletion of high field strength elements (Nb, Ta, Ti). The Ti depletion may be related to the separation and crystallization of Ti-bearing minerals such as amphibole and ilmenite. Zr and Hf anomalies were evident, likely due to the contamination of zircon minerals in the samples. The mafic dikes were most likely products of the Middle–Late Yanshanian magmatic intrusion, formed in an extensional setting within the intracontinental rift environment caused by the collision of the Pacific Plate and the Eurasian Plate. The lithospheric mantle source was influenced by subduction zone fluid, and the magma source region exhibited mixed crustal materials.