Devonian rodingite from the northern margin of the North China Craton: mantle wedge metasomatism during ocean–continent convergence

The northern margin of the North China Craton (NCC) was an active convergent margin during Palaeozoic and preserves important imprints of magmatic and metasomatic processes associated with oceanic plate subduction. Here, we investigate the mafic–ultramafic rocks in the Xiahabaqin–Sandaogou complexes from the northern NCC including pyroxenite, hornblendites, hornblende gabbro, and their rodingitized counterparts within a serpentinite domain. We present petrological, zircon U–Pb geochronological, and geochemical data to constrain the nature and timing of the magmatic and metasomatic processes in the subduction zone mantle wedge. The rock suites investigated in this study are characterized by low contents of SiO₂, Na₂O, and K₂O, with high CaO, FeO, Fe₂O₃, and MgO. The rodingitized rocks show markedly high CaO and lower MgO compared to their ultramafic protolith, suggesting extensive post-magmatic infiltration of Ca-rich, Si-poor fluids derived by serpentinization of mantle peridotite. The enrichment of large ion lithophile and light rare earth elements such as Ba, Sr, K, La, and Ce with relative depletion of high field strength elements like Nb, Ta, Zr, and Hf in the ultramafic rocks collectively suggest metasomatism of a fore-arc mantle wedge by fluids released through dehydration of subducted oceanic slab and subduction-derived sediments. Dehydration and decarbonation leading to metasomatic fluid influx and serpentinization of mantle wedge peridotite account for the enriched geochemical signatures for the rodingitized rocks. The zircon grains in these rocks show textures indicating magmatic crystallization followed by fluid-controlled dissolution–precipitation. Magmatic zircons from altered pyroxenite, hornblendite, and rodingitized pyroxenite in Xiahabaqin yield protolith crystallization ages peaks at 396 Ma and 392 Ma and metasomatic grains show ages of 386 Ma, 378 Ma, and 348 Ma. The zircons from hornblendite and basaltic trachyandesite indicate protolith emplacement during 402–388 Ma. Metasomatic zircon grains from rodingitized hornblende gabbro in Sandaogou complex show a wide range of ages as 412 Ma, 398 Ma, 383 Ma, and 380 Ma. The common magmatic zircon ages peaks at 398–388 Ma in most of the rocks suggest a similar time for magma crystallization in the Xiahabaqin and Baiqi during Middle Devonian. Subsequently, repeated pulses fluids and melts resulted in metasomatic reactions in mantle wedge until early Permian. The Lu–Hf analysis of the zircon grains from these rocks display markedly negative εHf(t) values ranging from −22.4 to −7.7, suggesting magma derivation from an enriched, hydrated lithospheric mantle through fluid–rock interaction and mantle wedge metasomatism. Rodingitization processes are associated with exhumation of ultramafic mantle wedge rocks within a serpentinized subduction channel close to the subducted slab in response to slab roll back in a long-lasting subduction regime. This study offers insights into magmatic and metasomatic processes of ultramafic rocks in the fore-arc mantle wedge which were exhumed and accreted to an active continental margin during the southward subduction of the Palaeo-Asian oceanic lithosphere beneath the NCC.

华北克拉通（North China Craton, NCC）北缘在古生代时期为活动汇聚大陆边缘，保存了与洋壳俯冲相关的岩浆作用与交代作用的重要印记。本文针对华北克拉通北缘夏巴秦-三道沟杂岩体中的镁铁质-超镁铁质岩石（包括辉石岩、角闪石岩、角闪辉长岩，以及蛇纹岩域内的罗丁岩化岩石）展开研究。我们通过获取岩石学、锆石U-Pb地质年代学与地球化学数据，约束了俯冲带地幔楔中岩浆与交代作用的性质与时限。
本研究涉及的岩石组合具有低SiO₂、Na₂O、K₂O含量，高CaO、FeO、Fe₂O₃、MgO含量的特征。与原岩超镁铁质岩石相比，罗丁岩化岩石的CaO含量显著升高，MgO含量降低，指示其经历了由地幔橄榄岩蛇纹岩化形成的富钙、贫硅流体的广泛岩浆期后渗透作用。
超镁铁质岩石中富集大离子亲石元素与轻稀土元素（如Ba、Sr、K、La、Ce），相对亏损高场强元素（如Nb、Ta、Zr、Hf），整体表明弧前地幔楔被俯冲洋壳板片及俯冲沉积物脱水释放的流体所交代。引发交代流体注入与地幔楔橄榄岩蛇纹岩化的脱水与脱碳作用，是罗丁岩化岩石地球化学特征富集的成因。
这些岩石中的锆石兼具岩浆结晶结构与流体控制的溶解-再沉淀结构。夏巴秦地区蚀变辉石岩、角闪石岩及罗丁岩化辉石岩中的岩浆锆石原岩结晶年龄峰值为396 Ma与392 Ma，交代成因锆石的年龄则为386 Ma、378 Ma与348 Ma。角闪石岩与玄武粗安岩中的锆石指示原岩侵位时代为402~388 Ma。三道沟杂岩体中罗丁岩化角闪辉长岩的交代成因锆石年龄跨度较大，分别为412 Ma、398 Ma、383 Ma与380 Ma。
多数岩石中共同的岩浆锆石年龄峰值为398~388 Ma，表明夏巴秦与白旗地区的岩浆结晶时代均为中泥盆世。随后，持续的流体与熔体脉冲事件导致地幔楔发生交代作用，直至早二叠世。对这些岩石中锆石的Lu-Hf同位素分析显示，其εHf(t)值显著为负，范围为-22.4~-7.7，指示岩浆来源于经历了流体-岩石相互作用与地幔楔交代作用的富集、水化岩石圈地幔。
罗丁岩化作用与超镁铁质地幔楔岩石在靠近俯冲板片的蛇纹岩化俯冲通道中的折返有关，这是长期俯冲体制下板片回卷的响应。本研究揭示了古亚洲洋岩石圈在华北克拉通之下南向俯冲过程中，被折返并增生至活动大陆边缘的弧前地幔楔中超镁铁质岩石的岩浆与交代作用过程。