鲁西地区闪长质岩石的成因:年代学、岩石地球化学和Sr-Nd-Hf同位素的证据

Petrogenesis of the diorite rocks in western Shandong:Evidence from geochronology,petrogeochemistry and Sr-Nd-Hf isotopes

本文对鲁西地区金牛山岩体和金星头岩体中生代闪长质岩石进行了详细的岩相学、锆石U⁃Pb年代学、全岩主微量元素、Sr⁃Nd同位素以及锆石Lu⁃Hf同位素的研究,旨在厘定闪长质岩石的形成时代、岩浆源区及岩石成因。结果表明,金牛山辉石闪长玢岩及斑状闪长岩中岩浆锆石的^(206)Pb/^(238)U加权平均年龄分别为130 Ma和126 Ma,形成于早白垩世。金牛山岩体闪长质岩石具有较高的MgO含量(4.35%~6.78%)、高的Mg^(#)值(56~70)以及高的Cr、Ni含量,金牛山岩体中的辉石闪长玢岩I_(Sr)值为0.707519,ε_(Nd)(t)值为-15.67,ε_(Hf)(t)值在-21.6~-15.9之间。金星头岩体闪长质岩石SiO_(2)含量为54.02%~65.02%,Al_(2)O_(3)含量在16.00%~17.61%之间,MgO含量(1.11%~2.68%)和Mg^(#)值(37~46)较低,高Sr/Y比值(29~136),具有埃达克质岩特征。研究表明,金牛山岩体闪长质岩石是古老下地壳再重熔并混染富集地幔部分熔融产物的结果;金星头岩体闪长质岩石为地幔源区的岩浆在上升侵位过程中被陆壳物质同化混染形成的埃达克质岩。受古太平洋板块向亚欧板块俯冲的影响,金牛山岩体和金星头岩体闪长质岩石形成于伸展的构造环境。

In this paper,detailed petrography,zircon U⁃Pb geochronology,major elements,trace elements,Sr⁃Nd isotopes and zircon Lu⁃Hf isotopes of Mesozoic diorite rocks from Jinniushan pluton and Jinxingtou pluton in western Shandong are studied,aimed to investigate the genesis and magma source of the diorite rocks.The weighted mean ^(206)Pb/^(238)U ages of the magmatic zircon are 131 Ma and 126 Ma in the pyroxene diorite porphyrites and porphyritic diorites of Jinniushan pluton,respectively,indicating their formation during the Early Cretaceous.The diorite rocks in Jinniushan pluton are characterized by high MgO(4.35%~6.78%),Mg^(#)(56~70),Cr and Ni.The pyroxene diorite porphyrites have rich Sr⁃Nd⁃Hf isotopes,I_(Sr)=0.707519,ε_(Nd)(t)=-15.67 andε_(Hf)(t)=-21.6~-15.9.The diorite rocks in Jinxingtou pluton have the higher contents of SiO_(2)(54.02%~65.02%),Al_(2)O_(3)(16.00%~17.61%)and lower contents of MgO(1.11%~2.68%),Mg^(#)(37~46)and higher Sr/Y ratio(29~136),which characteristics are similar to the adakitic.It can be concluded that the diorite rocks of Jinniushan pluton are the result of the remelting of the ancient lower crust and the mixing of partial melting products enriched in the mantle.And the diorite rocks of Jinxingtou pluton are adakite rocks formed by the assimilation and mixing of continental crust materials during the ascending emplacement of mantle magma.Influenced by the subduction of the ancient Pacific plate to the Eurasian plate,the diorite rocks of Jinniushan pluton and Jinxingtou pluton were formed in an extensional tectonic environment.