Early cretaceous igneous activities in the north flank of the North China Craton: the Shouwangfen complex example

The Early Cretaceous igneous complexes in the north flank of the North China Craton (NCC) provide a window to investigate the Mesozoic magmatism in the NCC. Here we report the precise timing of Early Cretaceous magmatism and magma petrogenesis of the different rock types in the north flank of the NCC based on petrology, mineral chemistry, geochemistry, zircon geochronology and Sr-Nd isotopes. Zircon U-Pb dating reveals that the Shouwangfen complex was crystallized at 130–128 Ma. The rocks display enrichment in large ion lithophile elements (LILE) and LREE, and depletion in high field strength elements (HFSE) and HREE, with relatively high Sr/Y and La/Yb values, typical of subduction-related magmatic rocks. The mafic microgranular enclaves show typical igneous textures, acicular apatites, sieve-texture of plagioclase phenocrysts and overgrowth of amphibole around the pyroxene, corresponding to magma mixing and mingling and thermal exchange. The quartz monzonites display normal zoning of the plagioclase, low and homogeneous εNd(t) values and linear co-variations in two-component diagrams. The quartz monzodiorites are characterized by high and homogeneous εNd(t) values. Petrologic feature and geochemical data suggest that the quartz monzonites represent lower crustal magma with minor contribution of enriched melt of the sub-continental lithospheric mantle (SCLM), followed by fractional crystallization. The mafic microgranular enclaves were sourced from enriched SCLM, followed by mixing and mingling with the host quartz monzonitic magma. The quartz monzodiorites are mainly SCLM-derived and subsequently mixed with lower crust melts. The high water contents (≥3%) of these Early Cretaceous igneous complexes suggests a hydrous SCLM beneath the NCC. The Early Cretaceous igneous complexes formed within an extensional tectonic setting which were related to the retreat and dehydration of the paleo-Pacific slab. The hydration of the lithosphere induced extensive crust-mantle interaction and large-scale water-rich magmatism, leading to lithospheric thinning in the NCC during the Mesozoic.

华北克拉通（North China Craton, NCC）北缘的早白垩世火成岩杂岩为探讨该克拉通的中生代岩浆作用提供了重要研究窗口。本文基于岩石学、矿物化学、地球化学、锆石年代学以及Sr-Nd同位素数据，报道了华北克拉通北缘不同岩石类型的早白垩世岩浆作用精确时限及岩浆成因。锆石U-Pb定年结果显示，寿望坟杂岩的结晶时代为130~128 Ma。该类岩石表现出大离子亲石元素（large ion lithophile elements, LILE）和轻稀土元素（LREE）富集、高场强元素（high field strength elements, HFSE）和重稀土元素（HREE）亏损的特征，且具有较高的Sr/Y和La/Yb比值，属于典型的俯冲相关岩浆岩。镁铁质微粒包体具有典型的火成结构、针状磷灰石、斜长石斑晶筛状结构以及辉石周边角闪石次生生长边，指示了岩浆混合、混杂与热交换过程。石英二长岩显示斜长石正常环带、低且均一的εNd(t)值，以及二元图解中呈线性协变的特征。石英二长闪长岩则以高且均一的εNd(t)值为特征。岩石学特征与地球化学数据表明，石英二长岩起源于下地壳岩浆，仅受到大陆岩石圈地幔（sub-continental lithospheric mantle, SCLM）富集熔体的少量贡献，随后经历了分离结晶作用。镁铁质微粒包体来源于富集型大陆岩石圈地幔，之后与寄主石英二长岩岩浆发生混合与混杂。石英二长闪长岩主要起源于大陆岩石圈地幔，随后与下地壳熔体发生混合。这些早白垩世火成岩杂岩的高含水量（≥3%）指示华北克拉通下方存在含水的大陆岩石圈地幔。早白垩世火成岩杂岩形成于与古太平洋板片后撤及脱水作用相关的伸展构造环境中。岩石圈的水化作用引发了广泛的壳幔相互作用和大规模富水岩浆作用，进而导致华北克拉通在中生代发生岩石圈减薄。