山东邹平青山下亚组玄武安山岩源区性质及成因

Genesis and Source Characteristics of Basalitic Andesite for the Lower Qingshan Formation in Zouping Volcanic Basin,Shandong Province

本文通过岩石的显微观察、全岩元素地球化学及Sr-Nd-Pb同位素组成等综合研究,参照前人的研究成果,探讨邹平火山岩盆地内青山下亚组玄武安山岩的岩石地球化学性质、物质来源及岩石成因。研究表明,岩石富钠、富镁,具有大离子亲石元素(Rb、Sr、Ba)和轻稀土元素(LREE)富集,高场强元素(Nb、Ta、Ti)相对亏损的地球化学特征。全岩Sr同位素初始比值(87 Sr/86 Sr)i为0.705674~0.705876;全岩Nd同位素初始比值εNd(t)为-10.0^-10.2,Nd同位素二阶段模式年龄T2DM为1.74~1.75Ga;全岩铅同位素初始比值(206Pb/204Pb)i介于16.917~17.095之间,(207Pb/204Pb)i介于15.306~15.396之间,(208Pb/204Pb)i介于36.750~37.208之间;较低的Sr-Nd初始比值及较均一的低放射性铅同位素组成,可能为EMI型富集地幔的部分熔融所致。晚中生代加厚的华北岩石圈发生大规模拆沉作用,拆沉的大陆中下地壳部分熔融形成富硅熔体与古生代岩石圈地幔相互作用形成富集地幔,随后在岩石圈伸展与区域热异常作用下减压部分熔融,形成的玄武安山质岩浆沿着深大构造侵入地壳上部或喷发至地表,形成青山下亚组玄武安山岩,但是岩浆上侵过程中地壳物质混染不强。该研究对深化鲁西地区晚中生代构造-岩浆作用的认识有一定的理论意义。

This paper studies the whole-rock element geochemistry and Sr-Nd-Pb isotopic compositions of basalitic andesite for the Lower Qingshan formation in Zouping volcanic basin. With reference to previous research, we discuss petrochemistry property and material sources. The study shows that the basalitic andesite is characterized by sodium-rich and magnesium-rich, as well as rich in Light Rare Earth Element （LREE） and Large Ion Lithophile Element（LILEs, Rb. Sr,Ba）, while depletions in High Field-Strength Element（HFSEs, Nb , Ta, Ti）. The initial ratios of Sr isotope （87 Sr/86 Sr）varies from 0. 705674 to 0. 705876, and the initial ratios of Nd isotope e Nd （t） ranges from -- 10.00 to -- 10.20. The initial ratios of Pb isotope （206pb/204pb） has a range of 16. 917 to 17. 095 , while （207Pb/204pb） ranges from 15. 306 to 15. 396, and （208Pb/204pb）0 values 36. 750 to 37. 208. Lower initial Sr-Nd ratios and relatively uniform low- level radioactive Pb isotopic composition suggests that the basalitic andesite may be derived from the partial melting EMI-type enriched mantle. During Late Mesozoic, the thickening North China lithospheric delamination formed silica-rich melt in the lower continental crust, while the interaction between silica-rich melt and Paleozoic lithospheric mantle formed enriched mantle. With the action of lithospheric extension and regional thermal abnormity, enriched mantle decompressed and partially melted, forming basaltic andesitic magma. The magma invaded along the deep structure, emplaced in the upper crust or erupted to the surface, and then formed the basaltic andesite, while the crustal contamination is not obvious during the magma intrusion process.