The mafic lavas, exposed in the Rujigou area, Helan Mountain, showed a conformable contact with the Upper Triassic Yanchang Formation. Here, we present new major and trace elements of these lavas in order to reveal the geodynamics background and the tectonic setting. The lavas are mainly composed of tholeiite basalts characterized by low K2O（<1%）and Mg#（43%～58%）. All samples exhibit LREE-enriched patterns without obvious Eu anomalies（La/Yb）N = 3.46～4.71, δEu = 0.96～1.05）. In the primitive mantle-normalized spider diagram, the basalts are enriched in large ion lithophile elements（LILE）and high field strength elements（HFSE）. In the multiple tectonic environment discrimination diagrams, the high field strength element abundances indicate that the rocks were emplaced in an environment similar to a continental rift setting. The volcanic eruption occurred in the terminal stage of the basin rifting, which is likely a kind of passive rifting. The comprehensive analysis of geological and geochemical characteristics in this area suggests that the tholeiite rocks were formed by partial melting of mantle peridotite at the spinel-garnet transition zone, and experienced crystallization of olivine and clinopyroxene, which provides an evidence of the rifting event between the Alax massif and the North China plate in the Late Triassic-Early Jurassic.
Yue Yuangang Dong Yunpeng Wang Runsan Xu Jinggang Zhang Hongfa Liu Xiaoming He Dengfeng. Geochemical characteristics and their geological implications of the basalt from the Rujigou area, Helan Mountain[J]. Chinese Journal of Geology, 2018, 53(3): 1157-1170.