【摘要】 辽西中侏罗世海房沟组火山岩是燕山造山带中生代火山岩的重要组成部分.海房沟组火山岩主要由粗安岩组成,其次为英安岩和安山岩,属于准铝质岩石和高钾钙碱性火山岩系列岩石.地球化学资料表明,辽西中侏罗世海房沟组火山岩具有低镁埃达克质火山岩的特征,如SiO2≥51.98%,Al2O3≥15.0%,MgO<4.18%(Mg#<0.48),Sr≥436×10-6,Yb≤2.25×10-6,Y≤23.14×10-6,Cr≤81.09×10-6,Ni≤34.66×10-6,Y/Yb≤14.25,Sr/Y≥39和Nb/Ta≈20;轻稀土元素富集,Ba、U、Sr和Pb等相对富集,而重稀土元素和高场强元素(如Nb、Ta、Ti)相对亏损,轻重稀土元素强烈分馏((La/Yb)N≥10.37),具有弱的负铕异常或正异常(0.79~1.05).另外,Nd、Sr同位素具有较低的(143 Nd/144 Nd)初始值(0.511 603~0.511 733,εNd(t)=-15.84~-13.30)、适中的87Sr/86Sr初始值(0.704 9~0.705 6,εSr(t)=8.64~18.11)和较高的亏损地幔模式年龄(TDM=1.81~1.99Ga),显示出富集地幔端员(EMI,Enriched Mantle End-member)特点.上述特征表明,海房沟组埃达克质岩浆起源于较厚下地壳榴辉岩部分融熔,其成因与玄武质岩浆的底侵作用有关.结合中生代火山岩的地球化学及其成因,认为早-中侏罗世是燕山造山带从古亚洲洋构造体系向古太平洋构造体系的转折时期,而中侏罗世海房沟组火山岩的形成(174Ma)标志着燕山造山带进入了古太平洋构造体系的演化阶段.这对进一步理解燕山运动的深部过程及其岩石圈减薄提供重要信息.更多还原

【Abstract】 The Middle Jurassic volcanic rocks of Haifanggou formation from the western Liaoning Province is an important com- ponent of Mesozoic volcanic rocks in the Yanshan Orogenic belt.The volcanic rocks of Haifanggou formation,which are mainly composed of trachyandesite with dacite and andesite,are metaluminous calc-alkaline and high-K series.Geochemical data indi- cate that the Middle Jurassic volcanic rocks of Haifanggou from the western Liaoning province have the low Mg adakitic volcan- ic features,such as SiO2 ≥51.98%,Al2 O3 ≥15.0%,MgO <4.18%(Mg#<0.48),Sr≥436×10-6,Yb≤2.25×10-6,Y≤ 23.14×10-6,Cr≤81.09×10-6,Ni≤34.66×10-6,Y/Yb≤14.25,Sr/Y≥39,and Nb/Ta≈20.The rocks are enriched in light rare earth elements(LREE),Ba,U,Sr and Pb,and are depleted in heavy rare earth elements(HREE)and high field strength elements(such as Nb,Ta,Ti),and strongly fractionated((La/Yb) N ≥10.37)with weak negative Eu anomaly or positive Eu anomaly(0.79to 1.05in Eu/Eu*).In addition,these adakitic volcanic rocks also have that low(143 Nd/144 Nd)ini- tial ratios(0.511 603to 0.511 733,-15.84to-13.30in epsilonεNd(t)),moderate(87 Sr/86 Sr)initial ratios(0.704 9to 0.705 6,8.64to 18.11in epsilonεSr(t))and a higher depleted mantle model ages(1.81to 1.99 Ga in TDM),showing enriched mantle end-member(EMI)characteristics.These features indicate that Haifanggou adakitic magma were derived from the partial melting of a thickened eclogite from the lower crust,and its origin is related to underplating of the basaltic magma. Combining with geochemistry of Mesozoic volcanic rocks in Yanshan Orogenic belt,the Early-Middle Jurassic in the Yanshan orogenic belt is a conversion period from the ancient Asian Ocean tectonic system to the paleo-Pacific tectonic system,while thegeneration of the Middle Jurassic adakitic volcanic rocks of Haifanggou formation(174Ma)shows that the Yanshan orogenic belt has come into the evolution stage of the paleo-Pacific tectonic system.The research results can facilitate the understanding of the deep mechanism of Yanshan movement and lithospheric thinning process.更多还原