Petrology, geochronology, and geochemistry of Paleoproterozoic volcanic rocks in the Xiong’ershan area, NE China: implication for petrogenesis and tectonic setting

The Xiong’er volcanic rocks (XVRs) on the North China Craton (NCC)’s southern margin are vital for understanding the NCC’s late Proterozoic evolution. This study focuses on the Xiong’ershan area’s volcanic rocks and performs detailed investigations on their petrology, chronology, and geochemistry, coupled with re-evaluation of prior geochemical data. Zircon U – Pb dating indicates their eruption ages of 1806–1764 Ma. These volcanic rocks are classified into basic – intermediate and felsic groups. Abundant quartz-filled vesicles and amygdales appear in basic – intermediate volcanic rocks, which can lead to an overestimation of SiO2 content in geochemical analyses. To remove this bias, the SiO2 content of the basic – intermediate rocks were re-evaluated by subtracting the quartz contributions from vesicles and amygdales. Our corrected data reveals a clear bimodal distribution (basalt + rhyolite/dacite), contradicting prior interpretations of continuous intermediate compositions. This supports that the XVRs formed in an intracontinental extensional environment, likely to have occurred due to the Columbia supercontinent’s break-up in the late Paleoproterozoic. All volcanic rocks have similar Sr-Nd-Hf isotope ratios and trace element patterns, indicating a common origin. The consistent isotopes and incompatible element ratios imply little crustal contamination during magma ascent. The relationships between MgO and other elements agree with the fractionation of clinopyroxene, olivine, and plagioclase before MgO decreases to ~2 wt%, and then the fractionation of ilmenite and apatite. Additionally, the rocks have lower water content and oxygen fugacity than arc magmas, and contain numerous inherited zircons with ages close to those of the Taihua group’s basement rocks. These results indicate that the XVRs originated from the reworking of the late Paleoproterozoic lithospheric mantle on the NCC’s southern margin, which was enriched by contamination of delaminated lower crust materials and metasomatism of subducted slab fluids before the late Proterozoic.

华北克拉通（North China Craton, NCC）南缘的熊耳火山岩（Xiong’er volcanic rocks, XVRs）对于解析该克拉通的晚元古代演化历程至关重要。本研究以熊耳山地区的火山岩为研究对象，对其开展了岩石学（petrology）、年代学（chronology）与地球化学（geochemistry）的详细调查，并对已发表的地球化学数据进行了重新评估。锆石U-Pb定年（zircon U–Pb dating）结果显示，其喷发时代为1806~1764 Ma。这些火山岩可划分为基性-中性与酸性两个类别。基性-中性火山岩中发育大量充填石英的气孔（vesicles）与杏仁体（amygdales），该现象会导致地球化学分析中二氧化硅（SiO₂）含量被高估。为消除这一分析偏差，研究人员通过扣除气孔与杏仁体中的石英贡献，对基性-中性火山岩的二氧化硅含量进行了重新校正。校正后的数据清晰呈现出双峰分布特征，即玄武岩（basalt）+流纹岩（rhyolite）/英安岩（dacite），这与此前提出的连续中性组分的解释相悖。该结果支持熊耳火山岩形成于陆内伸展环境（intracontinental extensional environment），其成因可能与古元古代晚期（late Paleoproterozoic）哥伦比亚超大陆（Columbia supercontinent）的裂解事件相关。所有火山岩均具有相似的Sr-Nd-Hf同位素比值（Sr-Nd-Hf isotope ratios）与微量元素配分模式（trace element patterns），指示其具有统一的源区。一致的同位素与不相容元素比值表明，岩浆上升过程中几乎未发生地壳混染（crustal contamination）。MgO与其他元素的相关关系显示，在MgO含量降至约2 wt%之前，岩浆经历了单斜辉石（clinopyroxene）、橄榄石（olivine）与斜长石（plagioclase）的分离结晶作用，之后则发生钛铁矿（ilmenite）与磷灰石（apatite）的分离结晶。此外，这些火山岩的含水量与氧逸度均低于弧岩浆（arc magmas），且含有大量继承锆石（inherited zircons），其年龄与太华群（Taihua group）基底岩石的年龄相近。上述研究结果表明，熊耳火山岩起源于华北克拉通南缘古元古代晚期岩石圈地幔（lithospheric mantle）的再造作用，该岩石圈地幔在晚元古代之前因拆沉下地壳（delaminated lower crust）物质的混染与俯冲板片流体（subducted slab fluids）的交代作用（metasomatism）而发生了富集。