Lithospheric thinning of the North China Craton: Constraints from Mesozoic mafic dikes in the Jiaodong Peninsula

The North China Craton (NCC) has undergone substantial thinning of its Paleozoic lithosphere. The original lithosphere was supplanted by a juvenile one during the Mesozoic era. Nevertheless, ongoing scholarly discourse exists concerning the precise age and mechanism of this lithospheric thinning. In this paper, we examined U-Pb dates on gold-related diabase-porphyry along with whole-rock elemental and Nd-Sr isotopes. Three categories of diabase-porphyry have been identified based on their petrographic features and titanium content. These are the low-Ti diabase-porphyry from Shangzhuang, the medium-Ti lamprophyres from Huangbuling, and the high-Ti diabase-porphyry from Qilishan. Their TiO₂ content are <1.1, 1.1, and >1.1 wt% for the Shangzhuang, Huangbuling, and Qilishan, respectively. The Ti/Y ratio varies among them, with the highest value in the Qilishan diabase-porphyry, a lower value in the Shangzhuang diabase-porphyry (370), and medium values between 270 and 370 in the Huangbuling lamprophyres.

The geochemical nature of the low-Ti diabase-porphyry, which are LREE-rich and have a higher LILE/HFSE ratio, are evocative of volcanic arcs. The Nd-Sr isotopes of the low-Ti rocks closely resemble those of mafic dikes originating from a fertile mantle lithosphere in the Jiaodong Peninsula. Conversely, the high-Ti diabase-porphyry display OIB-like characteristics, featuring HFSE enrichment with no Nb-Ta depletion. Their Sr-Nd isotope compositions are similar to isotopic signatures from asthenosphere-derived rocks, suggesting that the high-Ti diabase-porphyry may have originated in an asthenospheric mantle. Lastly, the Sr-Nd compositions of the medium-Ti diabase-porphyry fall between those of the low and high titanium diabase-porphyry, and they show weak enrichments in LILE/HFSE and LREE. This implies that a mixed melt of lithosphere and asthenosphere may have generated the medium-Ti lamprophyres. Zircon U-Pb isotopes suggests that the medium-Ti lamprophyres record weak lithospheric thinning in response to the sinking and/or rollback of the subducting oceanic slab at 155 Ma. The coexistence of high-Ti and low-Ti diabase-porphyry signifies a rapid shift in the magma sources from those derived from the lithosphere to those derived from the asthenosphere. This suggests lithospheric detachment (at 75–85 km depth) just before 127 Ma beneath the Jiaodong Peninsula.