Early Cretaceous bimodal volcanic rocks in the Yinshan belt, North China Craton: age, petrogenesis, and geological significance

Abstract

Determining the age and petrogenesis of the Mesozoic magmatic rocks may provide crucial evidence regarding the tectonic setting and evolution of the North China Craton (NCC). In this paper, we present new zircon U–Pb ages and geochemical data for bimodal volcanic rocks from Zhuozi area in the eastern Yinshan belt, NCC, to discuss their petrogenesis and tectonic implications. The volcanic rock suite consists of basalts, rhyolites, and trachytes. According to the results of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the rhyolitic and trachytic rocks have zircon U–Pb ages of 136.9 ± 0.8 Ma and 134.0 ± 1.7 Ma, respectively, representing their eruption ages. The basalts are enriched in large-ion lithophile elements (LILE; Ba, Rb, Sr, and Th) and light rare-earth elements (LREE), and are relatively depleted in high field strength elements (HFSE; Nb, Ta, and Ti). In addition, the basalts are characterized by high fractionations of rare-earth elements (REE), with (La/Yb)N = 24.87–42.85. These features suggest that they were derived from low-degree partial melting of enriched lithospheric mantle metasomatized by subduction-related components. The rhyolitic rocks have low mean Al2O3 (12.26 wt%) and TiO2 (0.14 wt%) contents and Mg# (14.92) values, with relatively low total REE abundances. They have high alkali and Zr, Ce, Y, and Nb contents and high Ga/Al ratios, with notably positive Pb and negative Sr, Ba, and Eu anomalies observed in the primitive mantle-normalized trace element diagram. These features indicate that the rocks exhibit characteristics of A1-type granite and likely derived from the partial melting of a crustal source. The trachytic rocks are show moderate trace elements and SiO2, MgO, Fe2O3T, and TiO2 contents. They were possibly generated by fractional crystallization of the underplating basaltic magmas and assimilated minor crustal materials. The geochemical characteristics, in combination with the regional geology, suggest that the bimodal volcanic rocks formed in an intraplate extensional setting likely associated with the retreat of the subducted Paleo-Pacific plate.