Geodynamic regime of the southeastern North China Craton before the Tectono-Magmatic Lull: evidence from ca. 2510 Ma and 2430 Ma igneous rocks in the Xuhuai micro-block

The Tectono-Magmatic Lull (TML) refers to the period between 2.37 and 2.24 Ga when magmatic activity declined globally, the cause of which remains debated. The North China Craton (NCC), which was formed by assembling several micro-blocks in Neoarchean to Paleoproterozoic, preserves the most records of magmatism across the TML. In this contribution, we report petrological, geochemical, and zircon U-Pb-Hf isotopic data of Paleoproterozoic basement rocks from the Xuhuai (XH) micro-block in the southeastern part of the NCC, and use them to constrain the geodynamic regime at the onset of the TML. Our data reveal that the central part of the XH micro-block is mainly composed of biotite granodioritic gneiss and plagioclase amphibolite, which is divided into a ca. 2510 Ma suite of low- and medium-pressure TTGs and a ca. 2430 Ma bimodal igneous suite including high-pressure TTG and plagioclase amphibolite. Variable magmatic zircon ε_Hf(t) values (1.9 to 7.2), Nb, Ta and Ti anomalies for the 2510 Ma TTGs indicate that the parental magma, which was involved with mantle materials, was generated in an active subduction environment in this pre-TML era. The felsic component of bimodal igneous rocks with high Na/Ta ratios (27 to 30) and stable Mg# (41 to 44) suggests their magma source region was as deep as the rutile stability field, and the delamination-derived asthenosphere mantle ascending. The influx of hot mantle induces thermal weakening and mechanical stretching of the overlying crust, leading to back-arc extension. The results of this study support the hypotheses that the NCC formed from micro-block amalgamation and the trigger of the TML could be the large-scale delamination of the lower crust into the mantle.

构造-岩浆宁静期（Tectono-Magmatic Lull, TML）指全球岩浆活动显著衰退的2.37~2.24 Ga时段，其成因迄今仍存争议。华北克拉通（North China Craton, NCC）于新太古代至古元古代通过多个微地块拼合形成，保存了全球范围内最为完整的TML期岩浆活动记录。本文报道了华北克拉通东南部徐淮（Xuhuai, XH）微地块古元古代基底岩石的岩石学、地球化学及锆石U-Pb-Hf同位素数据，并以此约束TML起始阶段的地球动力学背景。数据显示，徐淮微地块中部主要由黑云花岗闪长片麻岩与斜长角闪岩组成，可划分为两套岩石组合：一是约25.1亿年的低压-中压TTG岩系，二是约24.3亿年的双峰式火成岩组合，后者包含高压TTG岩系与斜长角闪岩。25.1亿年TTG岩系的岩浆锆石ε_Hf(t)值变化范围为1.9~7.2，且具有铌、钽、钛异常，指示其母岩浆起源于TML前的活动俯冲环境，且有地幔物质参与。双峰式火成岩的长英质组分具有较高的钠钽比值（27~30）与稳定的Mg#值（41~44），指示其岩浆源区深度达到金红石稳定域，且与拆沉作用形成的软流圈地幔上涌有关。热地幔的注入引发上覆地壳发生热弱化与机械拉伸，进而形成弧后伸展环境。本研究结果支持两项假说：一是华北克拉通由微地块拼合形成，二是TML的触发机制可能是下地壳大规模拆沉进入地幔。