华北克拉通北缘宣化一带中侏罗世双峰式火山岩的发现及其地质意义

Discovery of Middle Jurassic bimodal volcanic rocks in the Xuanhua district on the northern edge of the North China Craton and their geological significances

双峰式火山岩对构造环境和地球动力学具有重要研究意义。笔者首次在宣化地区蟒头山-祁家庄和滴水崖一带髫髻山组下部发现了玄武岩与粗面岩的双峰式火山岩组合,对其开展了地质学、岩石学、岩石地球化学和同位素年代学研究,以期为中侏罗世时期华北克拉通地幔状态和构造环境研究提供新的地质证据。结果显示,玄武岩分为两类,下部玄武岩的SiO_(2)含量为42.91%~46.63%,K_(2)O含量为0.28%~0.37%,TiO_(2)含量为1.42%~1.51%;Mg^(#)为40~61,属于拉斑系列;ΣREE为56.40×10^(-6)~85.85×10^(-6),La_(N)/Yb_(N)为1.55~2.01,轻重稀土弱分馏;δEu为1.04~1.09,具弱正铕异常。上部玄武岩的SiO_(2)含量为39.49%~49.79%,K_(2)O含量为2.08%~2.74%,TiO_(2)含量为2.32%~3.39%;Mg^(#)为28~42,属于碱性玄武岩系列;ΣREE为248.60×10^(-6)~329.56×10^(-6),La_(N)/Yb_(N)为7.24~8.22,轻重稀土分馏强烈,δEu为1.08~1.20,具正铕异常。粗面岩SiO_(2)含量为61.45%~63.79%,K_(2)O含量为5.22%~5.81%,Na2O含量为4.31%~4.7%,A/CNK为0.96~0.99,属于偏铝质岩石,属于碱性系列,ΣREE=262.55×10^(-6)~323.85×10^(-6),La_(N)/Yb_(N)=8.59~9.98,δEu=0.91~0.99,无铕异常。粗面岩的锆石U-Pb年龄为(162.5±1.3)Ma。髫髻山组下部玄武岩与粗面岩构成双峰式火山岩组合,年龄为(162.5±1.3)~(164.3±2.6)Ma,时代为中侏罗世末期,形成于大陆裂谷环境。研究认为中侏罗世末期华北克拉通北缘因受太平洋板块俯冲作用而发生破坏使其地幔转变为大洋型地幔,自下部玄武岩至上部玄武岩,形成其岩浆的地幔源区物质的部分熔融程度降低,地幔源区的深度变大,地幔源区为受古亚洲洋俯冲作用和陆壳富集组分改造的大洋型地幔。粗面岩形成于加厚地壳背景,由碱性玄武岩与高分异新太古代地壳物质混合物部分熔融形成。

The bimodal volcanic rocks have important significance for studying their tectonic environment and geodynamics.The authors have conducted geological,petrological,petrogeochemical,and isotopic chronological studies of the bimodal volcanic rock combination including basalt and trachyte which were firstly discovered in the lower part of the Tiaojishan Formation in the Mangtoushan-Qijiazhuang-Dishuiya area of the Xuanhua district,in order to provide new geological evidences for studying the mantle state and tectonic environment of the North China Craton during the Middle Jurassic period.The results show that there are two types of basalt including the lower basalt and upper basalt.Samples of the lower basalt have SiO_(2)contents ranging from 42.91% to 46.63%,K2O contents ranging from 0.28% to 0.37%,TiO_(2)contents ranging from 1.42% to 1.51%,Mg^(#)values ranging from 40 to 61,and ΣREE contents varying from 56.40×10^(-6) to 85.85×10^(-6),with weak fractionation of light and heavy rare earth elements(La_(N)/Yb_(N)=1.55~2.01)and weakly positive Eu anomalies(δEu=1.04~1.09).The lower basalt belongs to the tholeiitic basalt series.Samples of the upper basalt have SiO_(2)contents ranging from 39.49% to 49.79%,K_(2)O contents ranging from 2.08% to 2.74%,TiO_(2)contents ranging from 2.32% to 3.39%,Mg#values ranging from 28 to 42,and ΣREE contents of 248.60×10^(-6)~329.56×10^(-6),with strong fractionation of light and heavy rare earth elements(La_(N)/Yb_(N)=7.24~8.22),and positive Eu anomalies(δEu=1.08~1.20).The upper basalt belongs to the alkaline basalt series.Samples of trachyte have SiO_(2)contents ranging from 61.45% to 63.79%,K_(2)O contents ranging from 5.22% to 5.81%,Na_(2)O contents ranging from 4.31% to 4.7%,A/CNK ratios ranging from 0.96 to 0.99 ΣREE contents of 262.55×10^(-6)~323.85×10^(-6),La_(N)/Yb_(N) values of 8.59~-9.98,andδEu values of 0.91~0.99,with no obvious Eu anomalies.The trachyte belongs to the alkaline metaaluminous rock series.The zircon U-Pb age of trachyte is(162.5±1.3)Ma.The basalts and trachytes of the bimodal volcanic rock combination in the lower part of the Tiaojishan Formation have zircon U-Pb ages ranging from(162.5±1.3)Ma to(164.3±2.6)Ma.It is believed that they were formed in the continental rift setting during the end of Middle Jurassic.Period It is also believed that the lithospheric mantle in the northern edge of North China Craton was destroyed and transformed into the oceanic lithospheric mantle due to the subduction of Paleo-Pacific Ocean Plate at the end of Middle Jurassic.From the lower basalt to the upper basalt,the degrees of partial melting of mantle source materials for forming those basaltic magmas are decreased,and the depths of mantle source materials are increased.The mantle sources are oceanic mantle which were modified by the subduction of the Paleo-Asian Ocean Plate and the enriched continental crust components.Trachyte was formed in the thickened crust setting by magma derived from the partial melting of basalt mixed with highly differentiated Neoarchean crustal materials.