闽中晚侏罗世火山–侵入杂岩岩石成因研究

Study on the Genesis of Volcanic-Intrusive Complex Rocks in the Late Jurassic of Central Fujian

摘要: 闽中屏山火山–侵入杂岩由流纹岩与花岗斑岩组成，是揭示华南板块东南缘晚中生代构造–岩浆演化的关键载体。本次研究对该杂岩开展系统的全岩主量、微量元素地球化学分析，约束其岩石成因与岩浆演化过程。结果表明：流纹岩具高硅(SiO₂ = 75.39%~78.25%)、富钾(K₂O = 3.91%~5.70%)、过铝质(A/CNK = 1.11~1.42)特征，Ba、Sr、Eu、Ti等元素强烈亏损；花岗斑岩SiO₂含量相对偏低(64.39%~74.61%)，为准铝质(A/CNK = 0.97~1.46)，微量元素亏损程度显著更弱。二者稀土元素配分模式与微量元素蛛网图高度相似，指示源自同一岩浆系统。地球化学特征显示，杂岩初始岩浆源于幔源底侵诱发的壳幔混合岩浆，演化以分离结晶作用为主，经历了显著的晶体–熔体分离过程。分离结晶模拟表明，流纹岩为浅部岩浆储库抽提的高硅熔体，花岗斑岩为熔体抽离后的残余堆晶岩。结合区域构造背景，本文认为屏山杂岩体形成于古太平洋板块俯冲后撤诱发的弧后伸展环境，是壳幔相互作用下同一母岩浆经晶体–熔体分离的产物，为深化华南东南缘晚中生代岩浆系统演化研究提供了新的岩石学与地球化学约束。

Abstract: Pingshan Volcano in Central Fujian-the intrusive complex is composed of rhyolite and granophyre, serving as a key carrier for revealing the Late Mesozoic tectonic-magmatic evolution at the southeastern margin of the South China Block. This study conducted systematic whole-rock major and trace element geochemical analyses on the complex to constrain its petrogenesis and magma evolution. The results show that the rhyolite is characterized by high silica (SiO₂ = 75.39%~78.25%), high potassium (K₂O = 3.91%~5.70%), and peraluminous composition (A/CNK = 1.11~1.42), with significant depletion in elements such as Ba, Sr, Eu, and Ti; the granophyre has relatively lower SiO₂ content (64.39%~74.61%), is near-aluminous (A/CNK = 0.97~1.46), and shows significantly weaker trace element depletion. The rare earth element distribution patterns and trace element spider diagrams of the two rock types are highly similar, indicating derivation from the same magmatic system. Geochemical characteristics suggest that the initial magma of the complex originated from mantle-derived underplated induced crust-mantle hybrid magma, with evolution dominated by fractional crystallization and experiencing significant crystal-melt separation. Fractional crystallization modeling indicates that the rhyolite represents high-silica melt extracted from a shallow magma reservoir, while the granophyre is a residual cumulate rock after melt extraction. Considering the regional tectonic setting, this study proposes that the Pingshan complex was formed in a post-arc extensional environment triggered by the rollback of the paleo-Pacific Plate subduction, representing products of the same parental magma through crystal-melt separation under crust-mantle interaction. This provides new petrological and geochemical constraints for advancing the study of Late Mesozoic magmatic system evolution at the southeastern margin of South China.