鲁西临朐铁寨杂岩体锆石U-Pb年代学、地球化学特征及其地质意义

鲁西临朐铁寨杂岩体主要由石英二长玢岩和石英二长斑岩组成。为了精确厘定杂岩体的成岩时代和地球化学特征,对石英二长玢岩和石英二长斑岩进行了锆石U-Pb定年、全岩地球化学研究。结果表明,石英二长玢岩的锆石U-Pb年龄为(129.0±1.7) Ma,其含有的继承锆石年龄为(2 520±30) Ma;石英二长斑岩的锆石年龄为(125.0±1.6)Ma。两种侵入岩均具有富硅、富碱和低钙镁的特征,石英二长玢岩比石英二长斑岩的Mg~#值高;稀土总量较低,轻重稀土分馏明显,Eu基本无异常;Sr、Ba含量高,Nb、Ta和Ti等高场强元素负异常。岩石地球化学特征表明:鲁西铁寨杂岩体来源为富集地幔部分熔融、同化混染了部分古老的地壳物质,两种岩性的Mg~#值差异是磁铁矿分离结晶的结果。早白垩世滞留在地幔过渡带的太平洋板片脱水是岩石圈减薄的诱因,期间产生大量的岩浆岩,表明早白垩世是华北克拉通岩石圈减薄的峰期。

Zircon U-Pb Chronology,Geochemistry Characteristics of the Tiezhai Complex in Linqu County of Western Shandong and Their Geological Significance

The Tiezhai complex is located in Linqu County of western Shandong Province, and is mainly composed of grey and light-red quartz monzonite porphyries. To accurately determine the formation age and geochemical characteristics of the complex, we studied the major and trace elements (including REEs), and zircon LA-ICP-MS ages of the grey and light-red quartz monzonite porphyries. Our zircon U-Pb dating reveals that the grey quartz monzonite porphyry was formed in (129.0±1.7) Ma, and contains many Precambrian inherited zircons with ages of (2,520±30) Ma. The light-red quartz monzonite porphyry was formed in similar age at (125.0±1.6) Ma. The rocks are all rich in Si and alkali, but low in Ca and Mg. The grey quartz monzonite porphyry has a higher Mg^# value than its light-red counterpart.Their total REE contents are relatively low, and the LREE/HREE fractionation is high with no Eu anomaly. The rocks have high Sr and Ba contents, and with negative HFSE (high field strength elements,e.g.,Nb, Ta and Ti) anomalies.Geochemical characteristics of the two porphyries indicate that parental magma of the Tiezhai complex may have sourced from partial melting of the enriched mantle and the assimilation of some ancient crustal materials. The difference in Mg^# between the two porphyries was likely resulted from magnetite fractionation. Dehydration of the Pacific subducted slab resided in the mantle-crustal transition zone was likely the cause of the Early Cretaceous lithospheric thinning. Voluminous magmatic rocks were generated in this period, indicating that the lithospheric thinning had peaked during the Early Cretaceous in the North China Craton.