ABSTRACT

An integrated study of the petrology, zircon U–Pb geochronology, whole–rock geochemistry, and in situ zircon Hf isotopic compositions of middle–late Permian granitoids from the Diaobingshan region in the northern margin of the North China Craton (NCC) has been implemented, with the aim of investigating their geochronological framework, petrogenesis, and tectonic significance. The studied granitoids are widely distributed on the southern side of the Changchun–Yanji suture zone and comprise mainly monzogranite and syenogranite, with minor quartz monzonite. New zircon U–Pb data measured by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) yield emplacement ages of 262–256 Ma, reflecting middle–late Permian magmatism. Geochemically, the rocks contain 60.53–73.33 wt.% SiO₂, 12.60–18.35 wt.% Al₂O₃, 3.50–8.10 wt.% K₂O, 0.18–0.91 wt.% TiO₂, and 0.22–1.01 wt.% MgO, and exhibit weakly peraluminous (A/CNK = 0.86–1.01) and high-K calc–alkaline and shoshonitic characteristics, indicating I-type granite affinities. They are also characterized by low Sr and high Yb contents, reflecting low-pressure magma source conditions with residual plagioclase (i.e. amphibolite facies). These granitoids also exhibit geochemical characteristics typical of arc–type magmatic rocks, with enrichment in large ion lithophile elements (LILEs; e.g. Rb, Ba, and K) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs; e.g. Nb, Ta, Ti, and P) and heavy rare earth elements (HREEs), indicating that they were formed in a subduction-related setting. Moreover, magmatic zircons from the studied rocks have relatively homogeneous Hf isotopic compositions (ε_Hf(t) = – 1.4 to +15.1) and corresponding two–stage model (T_DM2) ages of 1839–348 Ma, indicating that they were generated by partial melting of Paleoproterozoic–Phanerozoic accreted lower crust, with minor contributions of ancient crustal materials. Their geochemical characteristics and specific lithological assemblage, together with the temporal–spatial distribution of Permian granitic magmatism, indicate that the formation of middle–late Permian granitoids in the northern margin of the NCC was related to southward Paleo–Asian oceanic subduction. Data from this and previous studies lead us to favour a double-sided, early–late Permian subduction model, with the Paleo–Asian Ocean closing after the late Permian.

摘要

已对华北克拉通（NCC）北缘中冰山二叠纪中晚期花岗岩的岩石学，锆石U-Pb地质年代学，全岩地球化学和原位锆石Hf同位素组成进行了综合研究。，目的是研究它们的地质年代学框架，岩石成因和构造意义。研究的花岗岩类广泛分布在长春-延吉缝合带的南侧，主要由辉石花岗岩和正花岗岩组成，少量的石英辉石。通过激光烧蚀-电感耦合等离子体质谱法（LA-ICP-MS）测量的新锆石U-Pb数据产生的年龄为262-256 Ma，反映了中晚期的二叠纪岩浆作用。从地球化学角度讲，岩石中含有60.53–73.33 wt。％的SiO ₂，12.60-18.35重量％的Al ₂ ö ₃，3.50-8.10重量％的K ₂ O，0.18-0.91重量％的TiO ₂，和0.22–1.01 wt。％的MgO，并表现出弱的铝质（A / CNK = 0.86–1.01）以及高K的钙碱性和钾铁矿化特征，表明I型花岗岩具有亲和力。它们的特征还在于低Sr和高Yb含量，反映出低压岩浆源条件和残留斜长石（即闪石相）。这些花岗石还表现出弧型岩浆岩特有的地球化学特征，其中富含大型的离子亲石元素（LILE；例如Rb，Ba和K）和轻稀土元素（LREE），而高场强元素（HFSEs）则耗竭；例如Nb，Ta，Ti和P）和重稀土元素（HREE），表明它们是在俯冲相关环境中形成的。此外，研究岩石中的岩浆锆石具有相对均一的H同位素组成（ε_Hf（t）= – 1.4至+15.1）和相应的两阶段模型（T _DM2）年龄1839–348 Ma，表明它们是古元古代—杂多生时期下地壳的部分熔融而产生的，古地壳的贡献较小材料。它们的地球化学特征和特定的岩性组合以及二叠纪花岗岩岩浆作用的时空分布表明，NCC北部边缘的中晚期二叠纪花岗岩体的形成与南亚古亚洲俯冲有关。来自这项研究和先前研究的数据使我们倾向于采用双面早晚二叠纪俯冲模型，古亚洲洋在二叠纪晚期之后就关闭了。