Geochemical and Isotopic Study of the Xiong’er Volcanic Rocks at the Southern Margin of the North China Craton: Petrogenesis and Tectonic Implications

The Xiong’er volcanic rocks constitute a large Paleo-Mesoproterozoic volcanic belt along the southern margin of the North China Craton. The volcanic rocks can be petrographically and geochemically classified into the basaltic andesite–andesite and dacite-rhyolite series, with the former being the dominant component of the Xiong’er volcanic rocks. The volcanic rocks of the dacite-rhyolite series have been derived from the wet and low-temperature differentiation of the basaltic andesite–andesite series at shallow depth, suggested by regional geological, lithological, and geochemical studies. In addition, crustal assimilation was involved in the magmas to form volcanic rocks of the dacite-rhyolite series, as suggested by low ϵNd(t) values (−8.09 to ∼−9.99) and high Th/Yb ratios (1.7–2.7). Some of volcanic rocks from the basaltic andesite–andesite series show features of high-Mg andesites (HMA), being characterized by high MgO contents and Mg numbers, representing the most primitive end-member of the magmatic series in the Xiong’er volcanic rocks, whereas other volcanic rocks of the basaltic andesite–andesite series (low-magnesium andesites) can be assigned to evolved andesites because of their low MgO and relatively high SiO2 contents. In the crystallization sequence of Opx → Cpx → Pl from the HMAs, through low-magnesium andesites, to dacite-rhyolite series, inferred by the covariation of major and trace elements, the early crystallization of plagioclases could have been suppressed as a result of high water content. The Xiong’er volcanic rocks are characterized by arclike geochemical signatures (large ion lithophile element and light rare earth element enrichments and negative Ta-Nb-Ti anomalies) as well as remarkably high Fe-Ti and high field strength element concentrations. These features can be interpreted in terms of the subduction-related hydrous melting of a mantle that experienced siliceous melt-metasomatism during the Archean-Paleoproterozoic hot subduction in the Trans-North China Orogen. Thus, we interpret the Xiong’er volcanic rocks to have formed at an active continental margin. This implies that the North China Craton, like other continental components (e.g., North America, Greenland, Baltica, Amazonia, and other cratons) of the Columbia supercontinent, also underwent a subduction-related outgrowth along its southern margin during Paleo-Mesoproterozoic time.