ABSTRACT

The palaeoproterozoic tectonic/geodynamic evolution of the Jiao-Liao-Ji Belt (JLJB), one of the three representative palaeoproterozoic tectonic units in the North China Craton (NCC), remains controversial. The palaeoproterozoic mafic dikes sampled in the Jiaobei terrane, situated in the southern segment of the JLJB, could provide significant evidence for the JLJB’s evolution. Their LA-ICP-MS zircon U-Pb dating data show that these dikes were emplaced at the period of ~2175 Ma. They have relatively low SiO₂ contents and relatively high contents of Al₂O₃, MgO, Cr, and Ni. Further, they exhibit high field strength elements (HFSE) (e.g. Nb and Ta) depletion and weak Eu anomalies. Also, they show limited initial ⁸⁷Sr/⁸⁶Sr ratios (0.700478–0.705069) and low ɛ_Nd(t) values (−2.07 to −0.09) with model ages (T_DM1) of 2722–2967 Ma. The magmatic crystal zircons yielded low ɛ_Hf(t) values of −9.22 ~ −3.82 relative to the corresponding Hf model ages of 2627–2870 Ma. These geochemical characteristics of the dikes indicate that they originated via 5–10% partial melting of the North China Craton’s enriched lithospheric mantle within the spinel stability field. In combination with contemporaneous A-type granites, mafic rocks, and other meta-volcanic associations, palaeoproterozoic Jiaobei mafic dikes formed in a back-arc extensional tectonic setting. This further indicates that the JLJB experienced back-arc extending triggered by slab rollback of subducted oceanic plate at ~2175 Ma ago. This study reports new evidence regarding the evolution of Precambrian tectonics of the North China Craton.

摘要

作为华北克拉通（NCC）三大古元古代构造单元之一的胶辽冀带（JLJB）的古元古代构造/地球动力学演化仍然存在争议。在位于JLJB南段的焦北地体中采样的古元古代基性岩脉可以为JLJB的演化提供重要证据。他们的 LA-ICP-MS 锆石 U-Pb 定年数据表明，这些岩脉是在 ~2175 Ma 时期就位的。它们具有较低的SiO ₂含量和较高的Al ₂ O ₃、MgO、Cr和Ni含量。此外，它们表现出高场强元素 (HFSE)（例如 Nb 和 Ta）耗尽和弱 Eu 异常。此外，它们显示有限的初始⁸⁷ Sr/ ⁸⁶Sr 比率 (0.700478–0.705069) 和低 ɛ _Nd (t) 值（-2.07 到 -0.09），模型年龄 (T _DM1 ) 为 2722–2967 Ma。岩浆晶体锆石产生低 ɛ _Hf(t) -9.22 ~ -3.82 的值相对于 2627-2870 Ma 的相应 Hf 模型年龄。这些岩脉的地球化学特征表明，它们起源于尖晶石稳定场内华北克拉通富集岩石圈地幔的 5-10% 部分熔融。结合同期的A型花岗岩、基性岩和其他变质火山组合，古元古代胶北基性岩脉形成于弧后伸展构造环境中。这进一步表明，JLJB 在~2175 Ma 前经历了由俯冲大洋板块的板片回滚触发的弧后延伸。本研究报告了有关华北克拉通前寒武纪构造演化的新证据。