沂沭断裂带北段早白垩世高钾钙碱性火山岩地球化学特征与岩石成因

沂沭断裂带内发育了大量的中生代火山岩，其成因研究对剖析断裂带活动规律具有重要意义。本文对断裂带北段潍坊、安丘、鄌郚地区中生代火山岩进行了详细的岩石学、锆石U-Pb年代学和主量元素、微量元素、Sr-Nd-Pb同位素分析。探讨了沂沭断裂带中生代火山岩成因及其形成的地球动力学过程。断裂带北段火山岩年龄集中在131~124Ma之间，喷发于早白垩世。岩石为一套高钾钙碱性火山岩，包括玄武安山岩、粗安岩、粗面岩、英安岩。火山岩富集Rb、Ba等大离子亲石元素，相对亏损Nb、Ta、Ti等高场强元素。稀土元素含量高（∑REE=140.5×10^-6~433.1×10^-6），富集轻稀土、亏损重稀土（∑LREE/∑HREE=9.8~31.0），无明显的Eu负异常。火山岩具有富集且变化范围较大的Sr-Nd-Pb同位素组成，（⁸⁷Sr/⁸⁶Sr）_i=0.708054~0.711692，ε_Nd（t）值为-15.3~-7.4，（²⁰⁶Pb/²⁰⁴Pb）_i=17.213~17.962，（²⁰⁷Pb/²⁰⁴Pb）_i=15.425~15.590，（²⁰⁸Pb/²⁰⁴Pb）_i=37.597~38.365。研究表明，高钾钙碱性火山岩的源区与区域基性岩源区一致，均为富集岩石圈地幔。玄武安山岩、粗安岩、粗面岩经基性岩浆分离结晶作用形成，英安岩则遭受了华北下地壳强烈混染。三叠纪末期，由于扬子板块向华北克拉通俯冲，扬子地壳析出的流体/熔体交代了华北古老岩石圈地幔，形成富集岩石圈地幔。早白垩世，受古太平洋板块俯冲和后撤作用，富集岩石圈地幔部分熔融形成基性岩浆，岩浆在上升过程中发生分离结晶作用，受沂沭断裂深切作用影响，部分地区遭受华北下地壳混染，最终形成区内各类高钾钙碱性火山岩。

Geochemistry and petrogenesis of the Early Cretaceous high-K calc-alkaline volcanic rocks in the northern part of Yishu fault, East China

A large number of the Mesozoic volcanic rocks developed in the Yishu fault, whose origin are of great significance to analyze the activities of the fault. In this paper, the petrology, zircon U-Pb geochronology, major elements, trace elements and Sr-Nd-Pb isotopes of the volcanic rocks in Weifang, Anqiu and Tangwu areas are studied, aimed to investigate the genesis and geodynamics of these volcanic rocks. Zircon U-Pb dating results of the volcanic rocks yield U-Pb ages concentrating at 131~124Ma, indicating their eruption during the Early Cretaceous. The volcanic rocks are high-K calc-alkaline (HKCA) series, including basaltic andesite, trachyandensite, trachyte and dacite. They are enriched with large ion lithophilic elements such as Rb and Ba and relatively depleted in high field strengths such as Nb, Ta and Ti. The content of rare earth elements is high (∑REE=140.5×10-6~433.1×10-6), enrichment of light rare earth elements and depleted heavy rare earth elements (∑LREE/∑HREE=9.8~31.0), without obvious Eu negative anomaly. The volcanic rocks have rich and highly variable Sr-Nd-Pb isotopes, (87Sr/86Sr)i=0.708054~0.711692, εNd(t)=-15.3~-7.4, (206Pb/204Pb)i=17.213~17.962, (207Pb/204Pb)i=15.425~15.590, (208Pb/204Pb)i=37.597~38.365. It can be concluded that the HKCA volcanic rocks have the same magma source with the basic rocks in the Yishu fault. Both of them were derived from enriched lithosphere mantle. Basaltic andesite, trachyandensite and trachyte were formed by crystallization of basic magma. Dacite were suffered from strong contamination of the lower crust of North China Craton. Triassic, the ancient lithospheric mantle was subjected metasomatsim to form enriched lithosphere mantle by the fluid or melt from subducting Yangtze continental crust. Up to the Early Cretaceous, due to the subduction and rollback of the ancient Pacific plate towards Eurasian continent, the basic magma was generated by the partial melting of the enriched lithospheric mantle. HKCA volcanic rocks were formed through crystallization of basic magma. During the rise of magma, partial magma was suffered from crustal material contamination along the Yishu fault.