【作者】 王晓蕊；

【导师】 高山；

【作者基本信息】 西北大学 ， 岩石圈地球化学， 2005， 硕士

【摘要】 华北克拉通在中生代发生了下地壳和岩石圈地幔的拆沉作用,造成辽西地区广泛出露中生代火山岩。辽西四合屯地区早白垩世义县组由于含有大量长羽毛或毛状皮肤衍生物的恐龙和早期鸟类等珍稀动物化石而备受世界注目。四合屯地区义县组主体由火山岩组成,中上部夹含化石的湖相沉积岩。该剖面总厚度为300-400米,其中火山岩约300米,沉积岩约50～100米。前人已对其中含化石的沉积地层开展了大量高水平研究,对相关火山岩和火山碎屑岩进行了详细的同位素定年,结果表明四合屯地区义县组时代为120～130Ma。对占义县组主体的火山岩则研究不多,尤其缺乏系统的地球化学研究。 本文沿四合屯新开岭至化石观察站义县组标准剖面系统采集并分析了85件火山岩样品,结果表明该剖面火山岩仅底部为玄武岩,其余主要由高镁安山岩组成。通过对四合屯义县组安山岩详细的岩石学、地球化学研究,得出以下认识: 1.四合屯安山岩大部分是高镁（Mg#>50）的,Mg#为38～69。 2.四合屯高镁安山岩具有埃达克岩的地球化学特征（SiO₂=52.82～59.31wt%,Al₂O₃=14.15～16.35wt%,Sr=620～1323μg/g,Yb=1.03～1.88μg/g,Y=12～20μg/g,La_N/Yb_N=10～25,Sr/Y=32～88）,同时四合屯高镁安山岩具有轻重稀土明显分异,无Eu异常（Eu/EU^*=0.96～1.05）,高MgO、Ni和Cr含量等地球化学特征。在微量元素蛛网图上,表现出明显的Nb、Ta负异常和Pb、Sr正异常。初始Nd-Sr同位素组成为:¹⁴³Nd/¹⁴⁴Nd（130Ma）=0.5118～0.5119,ε_Nd（130Ma）=-11.6～-13.8,⁸⁷Sr/⁸⁶Sr（130Ma）=0.7058～0.7064,与该区晚侏罗世兴隆沟组火山岩具有相似的地球化学特征,但具有较低的Nd同更多还原

【Abstract】 Delamination of lower continental crust and lithospheric mantle occurred within the North China craton during the Mesozoic, which accompanied extensive magmatism formation in western area of Liaoning Province. The Sihetun Yixian Formation has attracted considerable attention over the last two decades, due to a wide range of well preserved ’feathered’ dinosaurs and primitive birds fossils have been yielded from the Sihetun Yixian Formation. The 400m thick Yixian Formation mainly consists of volcanic rock, which interlayered with lake facies sedimentary rock that contain fossils on the top of section. Previous researchers developed abundant studies for the sedimentary section containing fossils and made detailed studies of isotope geochronology using Sihetun volcanic and volcaniclastic vocks. The age of Sihetun Yixian Formation is 120 to 130Ma. In contrast, the Yixian volcanic rocks lack systemic geochemical research.85 volcanic rocks have been collected and analyzed from the Sihetun standard section that began at Xinkailing and ended at fossil observation station. The analytic results of major and trace elements indicate that the Sihetun volcanic rocks are high magnesium andesites, besides some basalts on the bottom of section.Integrated detailed petrological and geochemical studies of the Sihetun andesites lead to the following conclusions:1. Sihetun volcanic rocks are mostly high magnesium andesites, whose Mg# is 38 to 69;2. The Sihetun high magnesium andesites (SiO2=52.82~59.31wt%, Al2O3=14.15~16.35wt%) show many characteristics of adakites such as depletion in heavy rare-earth elements(HREE;Yb=1.03~1.88μg/g) and Y(12~2(μg/g) and high Sr(620~1323μg/g) and Sr/Y(32~88), with high LaN/YbN ratio(10~25). Other striking geochemical features of Sihetun lavas are high whole-rock Cr (187~580μg/g) and Ni(89~290μg/g) contents, because of the presence of chromite. The Sihetun lavas also exhibit depletion of Nb, Ta and enrichment of Pb, Sr and without Eu anomaly (Eu/Eu*=0.96~1.05). The initial Sr-Nd isotopic compositions of high-Mg andesites are 143Nd/144Nd (130Ma) = 0.5118-0.5119, εNd(130Ma) =-11.6-13.8 and 87Sr/86Sr(130Ma) =0.7058-0.7064. The high-Mg andesites from the Yixian Formation at Sihetun have similar geochemical characteristics with high-Mg andesites from the Xinglonggou Formation except for more lower Nd isotopic ratio.3. Sr content and Sr/Y ratio indicate significant negative correlation with SiO2 in the Sihetun high-Mg andesites with SiO2 contents > 56%. These indicate Sr content and Sr/Y ratio of Sihetun high-Mg andesites were affected by fractional crystallization of mainly plagioclase. So the affection of fractional crystallization on volcanic Sr and Sr/Y ratio cannot be ignored.4. The lacustrine shale from the fossil observation station is also high-Mg with Mg# being remarkably higher than the average Archaean shale of the North China craton. This suggests that the whole Sihetun lavas are high in magnesium.5. The high-Mg andesites from the Yixian Formation at Sihetun have similar major and trace element characteristics to high-Mg andesites from the Xinglonggou Formation. They were interpreted in a way that eclogite that formed at the base of thicken Archaean lower crust of the North China craton foundered into the convecting mantle and subsequently melted and interacted with peridotite and experienced significant crustl contamination during eruption. Compare with the Xinglonggou lavas (εNd(130Ma)=1.9 to -2.9), the Sihetun andesites have obviously low initial εNd values (-11 to -14) that require the magma source contain more ancient continental crust substance with evolved Nd isotopic compositions.The age of Sihetun Formation was 120 to 130Ma, and this indicated that delamination was lasted to the Early Cretaceousperiod.更多还原