Paleoproterozoic Rift-Related Volcanism of the Xiong'er Group, North China Craton: Implications for the Breakup of Columbia

The Paleoproterozoic Xiong'er Group in the North China craton is composed of mafic to felsic volcanic rocks and minor sedimentary rocks (4.3%), and crops out over an area of 60,000 km2. The volcanic and sedimentary strata, which vary from 3 km to 7 km in thickness, unconformably overlie an Archean and Paleoproterozoic crystalline basement, and are unconformably overlain by Meso-Neoproterozoic terrigenous clastic rocks and carbonates. Xiong'er volcanic rocks include basaltic andesite + andesite and dacite + rhyolite dated at 1760 Ma. The andesitic rocks contain clinopyroxene and plagioclase with minor olivine and orthopyroxene, but lack amphibole and biotite, suggesting crystallization from an anhydrous magma. All analyzed volcanic rocks are enriched in large-ionlithophile elements (LILE; e.g., K, Rb, Ba) and light rare-earth elements (LREE), and are depleted in high-field-strength elements (HFSE; e.g., Nb, Ta, and Ti). They have high LILE/HFSE ratios and low Ti/Zr (40 to 8) and Sr/Y (16 to 2) ratios and 147Sm/144Nd (0.09792 to 0.1332), 143Nd/144Nd (0.511082 to 0.511480), εNd(t = 1.76 Ga) (-3.8 to -9), and TDM (2.45 to 3.07 Ga). The geochemical and isotopic compositions indicate derivation from an enriched subcontinental lithospheric mantle previously contaminated by a subducted slab. The magmas exhibit the effects of low-pressure assimilation and fractional crystallization. The Xiong'er Group is contemporaneous with 1760 Ma mafic dike swarms, massif-type anorthosites, and rapakivi granites in the northern part of the North China craton. It formed in a continental-rift environment, marking the initiation of the breakup of the surpercontinent, Columbia.