【摘要】 辽西北票晚侏罗世蓝旗组火山岩由下段安山质角砾熔岩夹凝灰岩和上段粗安岩组成.通过火山岩的地球化学研究,安山质角砾熔岩具有埃达克质火山岩特征,其中,SiO2含量为56.99%～58.22%,含有较高的Al2O3(>15.00%),较高的MgO(≤4.22%,Mg#=0.44～0.49)和较低的K2O(<2.20%)含量,较高的微量元素,如Sr(>500.00μg/g)、Cr(>125.00μg/g)和Ni(>47.00μg/g)及Sr/Y比值(>28.00),和较低的∑REE(<112.00μg/g)、U(<0.30μg/g)、Th(<2.00μg/g)和重稀土(Y<20.00μg/g,Yb<1.60μg/g),且轻稀土分馏明显((La/Yb)N=10.49～10.59),具有明显的Sr正异常和弱的Eu负异常(Eu/Eu*=0.62～0.99);粗安岩不具有埃达克质火山岩特征,其中,SiO2含量为52.09%～61.08%,含有较高的Al2O3(>16.60%)和K2O(>2.00%),较低的MgO(≤2.64%,Mg#=0.08～0.38)、Cr(<28.00μg/g)和Ni(<19.00μg/g)及Sr/Y比值(≤23.00),和较高的∑REE(>180.00μg/g)、U(≥0.80μg/g)、Th(>4.00μg/g)和Sr(>350.00μg/g),且轻稀土分馏明显((La/Yb)N=10.22～16.28),具有明显的Sr和Eu负异常(Eu/Eu*=0.74～0.80).这些火山岩普遍具有Ba、Pb富集和Nb、Ta亏损的特征,Nd、Sr同位素具有中等的εSr(t)(20.15～23.34),TDM(1.45～1.50Ga)和低的εNd(t)(-7.69～-8.62),表明蓝旗组安山质岩浆和粗安质岩浆均起源于下地壳玄武质岩石的部分熔融.其中,安山质岩浆起源较深,位于壳幔过渡带,并发生明显的壳幔相互作用.蓝旗组火山岩由早期的安山质岩浆到晚期的粗安质岩浆演化可能预示着地壳由厚变薄的过程,这对理解华北克拉通中生代岩石圈减薄的深部过程具有重要意义.更多还原

【Abstract】 The Late Jurassic volcanic rocks of the Lanqi Formation in the Beipiao area,the western Liaoning Province,are composed of the lower andesitic breccia lava with tuffs and the upper trachyandesite.Based on the study of geochemistry,andesitic breccia lava are from 56.99% to 58.22% in SiO2,high Al2O3(>15%),high MgO(≤4.22%,0.44 to 0.49 in Mg#)and low K2O(<2.2%) characterizes by high Sr(>500 μg/g),Cr(>125 μg/g),Ni(>47 μg/g) contents and higher Sr/Y ratio(>28) and low REE(<112 μg/g),U(<0.3 μg/g),Th(<2 μg/g) and HREE(Y<20 μg/g,Yb<1.6 μg/g) contents and by strongly fractionated LREE(10.49 to 10.59 in(La/Yb)N) and distinct positive Sr anomalies and negligible negative Eu anomalies(0.62 to 0.99 in Eu/Eu*),which are similar to those of the adakitic rock;and trachyandesite are from 52.09% to 61.08% in SiO2,high Al2O3(>16.6%) and K2O(>2%),low MgO(≤2.64%,0.08 to 0.38 in Mg#) characterizes by high REE(>180 μg/g),U(≥0.80 μg/g),Th(>4 μg/g) and Sr(>350 μg/g) contents,and low Cr(<28 μg/g),Ni(<19 μg/g) contents and low Sr/Y ratio(≤23) and by strongly fractionated LREE(10.22 to 16.28 in(La/Yb)N) and distinct negative Eu and Sr anomalies,which differ from those of the adakitic rock.All the volcanic rocks also show positive Pb and Ba anomalies,and negative Nb-Ta anomalies,and have moderate εSr(t)(20.15 to 23.34),TDM(1.45 to 1.50 Ga) and low εNd(t)(-7.69 to-8.62),which are similar to the features of the EMI-like component and of the ancient continental lower crust of the North China craton and the Late Jurassic-Early Cretaceous volcanic rocks in Yanshan orogen belt.In this paper,author thinks that andesitic magmas of Lanqi Formation are derived partial melting of foundered lower continental basaltic crust near the transition between the crust-mantle and occur the interaction of the mantle and crust,and trachyandesitic magmas from partial melting of foundered lower continental basaltic crust.The magma evolution of Lanqi Formation indicates that the process of the crust thinning from the thicken crust,it is an important significance to understand the deep process for the lithospheric thinning of the North China Craton during the Mesozoic era.更多还原