辽西黄半吉沟早白垩世义县组高镁安山岩的成因

发布时间：2018-05-21 09:28

本文选题：橄榄石 + 熔体包裹体　；参考：《岩石学报》2017年01期

【摘要】：利用橄榄石和熔体包裹体,结合全岩的方法对辽西地区早白垩世义县组黄半吉沟火山岩的成因进行了研究。黄半吉沟火山岩SiO_2=53.41%~53.74%,MgO=8.15%~8.23%,Mg#=~70(Mg#=mol Mg/(Mg+Fe2+)),为高镁安山岩;全岩在TAS图解上,落在玄武安山岩范围内,属于亚碱性系列;它们具有较高Ni(119×10-6~125×10-6)和高Cr(467×10-6~521×10-6),显示幔源岩浆特征;在微量元素组成上,黄半吉沟高镁安山岩Sr=920×10-6~930×10-6,Y=16.1×10-6~16.4×10-6,Sr/Y=57~58;在微量元素原始地幔标准化蛛网图上,黄半吉沟高镁安山岩显示轻重稀土分异,明显的Nb-Ta-Ti和弱的Zr-Hf负异常,Ba、Sr和Pb正异常,这些特征与大陆下地壳非常相似。熔体包裹体MgO为6.5%~9.7%,SiO_2为51%~53%,不符合典型高镁安山岩的定义;在TAS图解上它们落在玄武粗安岩内,属于碱性系列;MgO与其它主量元素成分呈明显或者弱的负相关关系,说明它们的成分主要受控于橄榄石结晶分离过程。黄半吉沟高镁安山岩的橄榄石Fo值为75~91;CaO含量为0.10%~0.18%,NiO为0.05%~0.41%,Fe/Mn比值为60~80。黄半吉沟高镁安山岩的全岩和熔体包裹体成分存在显著差异,如熔体包裹体具有更高的Al2O3和更低的SiO_2。结合全岩微量元素特征,我们认为黄半吉沟高镁安山岩在地壳深度的岩浆演化过程中加入了来自下地壳的酸性熔体,是壳幔相互作用的结果。全岩较低Ni高Mg#,熔体包裹体低CaO并落在CATS-Olivine-Quartz相图的热障碍边界线富硅一侧,以及橄榄石低Ca和陡倾的Fo-Ni关系,指示黄半吉沟高镁安山岩的幔源岩浆是来自以斜方辉石为主辉石岩的源区。我们认为广泛发育于辽西地区的早白垩世义县组高镁安山岩可能经历了壳幔相互作用,因而不能作为拆沉作用导致岩石圈大规模减薄的重要证据。
[Abstract]:The origin of Huangbanjigou volcanic rock of early Cretaceous Yixian formation in western Liaoning was studied by olivine and melt inclusions combined with the whole rock method. The Huangbanjigou volcanic rock SiOSP (53.41) and MGO 8.15 (8.23) are Mg#mol Mg/(Mg Fe2 rocks, which belong to the subalkaline series of Black Tortoise andesite, and belong to the subalkaline series in the TAS diagram. They are of high Ni(119 脳 10-6 (125 脳 10-6) and high Cr(467 脳 10-6 (521 脳 10-6), indicating the characteristics of mantle magma, and the composition of trace elements. Huangbanjigou high mafic andesite Sr=920 脳 10-6 + 930 脳 10-6 + YT 16.1 脳 10-6 + 16.4 脳 10-6 Sr-P / Y5758.In the primitive mantle-normalized cobweb diagram of trace elements, the Huangbanjigou high-mafic andesite shows light and light rare earth differentiation, obvious Nb-Ta-Ti and weak negative anomaly Ba-Sr and Pb positive anomalies. These characteristics are very similar to the lower continental crust. The MgO of melt inclusions is 6.5and 9.7. SiOSCH2 is 51and 51.It does not accord with the definition of typical high-mafic andesite, and they fall into Black Tortoise trachyandrite on the TAS diagram, which belongs to the alkaline series and has an obvious or weak negative correlation with other principal element compositions. The results show that their composition is mainly controlled by the process of olivine crystallization and separation. The Fo value of olivine in Huangbanjigou high mafic andesite is 75 0. 91% Cao content is 0. 10 0. 18%, nio is 0. 05% 0. 41% Fe / mn ratio is 60% 80. There are significant differences between the whole rock and melt inclusions in Huangbanjigou high-mafic andesite, for example, the melt inclusions have higher Al2O3 and lower SiO2. Based on the characteristics of trace elements in the whole rock, we think that the acidic melt from the lower crust was added in the magmatic evolution process of Huangbanjigou high magnesium andesite, which is the result of the interaction of crust and mantle. The whole rock is low in Ni and high in mg, and the melt inclusion is low in CaO and falls on the thermal barrier boundary of the CATS-Olivine-Quartz phase diagram on the side of the thermal barrier boundary rich in silicon, as well as the relationship of olivine with low Ca and steep dip Fo-Ni. It is indicated that the mantle magma of the high-mafic andesite in Huangbanjigou is derived from the source region of clinopyroxene. We think that the early Cretaceous high mafic andesite of Yixian formation, which is widely developed in western Liaoning, may have experienced crust-mantle interaction, so it cannot be used as an important evidence of the large-scale thinning of lithosphere caused by desubsidence.
【作者单位】：中国科学院南海海洋研究所中国科学院边缘海地质重点实验室;中国科学院广州地球化学研究所同位素地球化学国家重点实验室;中国科学院大学;
【基金】：国家自然科学基金项目(41403024、91214202、41273042) 中国科学院广州地球化学研究所135项目(Y234051001)联合资助
【分类号】：P588.144

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