中文摘要：

地球化学分析结果表明：北秦岭斜峪关群基性熔岩的LREE轻度富集,（La/Yb）N=1.31-3.27,中—酸性熔岩的LREE显著富集,（La/Yb）N=4.22-26.54。草滩沟群基性火山岩LREE中度富集,（La/Yb）N=2.50-2.97,中—酸性熔岩的LREE显著富集,（La/Yb）N=4.57-10.15。低δEu值（0.62-1.14）暗示斜峪关群和草滩沟群火山岩初始玄武质岩浆的分离结晶程度很低、初始岩浆中斜长石分异作用很弱,大离子亲石元素明显富集,而相对N-MORB较低的高场强元素丰度,特别是明显的Ta、Nb低谷,展示了典型的岛弧火山岩的特征。同位素地球化学分析揭示,斜峪关群和草滩沟群火山岩的初始Sr比值为0.703438-0.708329,初始Nd比值为0.511948-0.512327,多集中于0.512003-0.512327间,εNd均为正值（＋5.42-＋12.81）,表明二群火山岩主要来自富集地幔源区。斜峪关群和草滩沟群火山岩的岩浆源区为混染的富集地幔源区,是俯冲板片熔融和地壳混染的产物,形成于岛弧构造环境。SHRIMP锆石U-Pb测年表明斜峪关群基性熔岩形成时代为早奥陶世（472±11Ma）,其中的古老捕晶锆石（1294±34Ma）表明斜峪关群火山岩的岩浆源区存在元古宙地壳混染。

英文摘要：

Geochemical analyses in the present paper showed that volcanic rocks in the Xieyuguan and Caotangou groups are predominated by the subalkaline series, in which basaltic lavas are mainly tholeiitic series, while the intermediate-acid rocks in the Caitangou Group are characterized by calc-alkaline series. Basaltic lavas in the Xieyuguan and Caotangou groups are slightly enriched in LREE （（La/Yb）N = 1.31-3.27）, while the intermediate-acid lavas are significantly enriched in LREE （（La/Yb）N =4.22-26.54）. Low Euvalues （δEu=0.62-1.14） of volcanic rocks in the two groups indicate a poor fractionation and very weak partition of plagioclase from a primitive basaltic magma. Typically, volcanic rocks from the two groups are characterized by selective enrichment of LILE, low abundances of HFSE relative to N-MORB, and pronounced troughs at Ta and Nb, which indicates that these volcanic rocks were formed in an arc setting. Isotopic analyses for volcanic rocks from the two groups show wide ranges of initial strotium （0. 703438 -0.708329）, and initial neodymium （0. 512003-0. 512327）. Positive end values （＋5. 42 - ＋12. 81） indicate that these volcanic rocks were derived from enriched mantle （EMI and/or EMII）, with crustal contamination. SHRIMP U-Pb analyses for zircon grains from a basaltic sample yielded a mean age of 472 ± 11 Ma （n= 14, MSWD= 1.94）, which clearly identifies an early Ordovician volcanic arc, or subductionaccretion orogeny in the early Paleozoic in the North Qinling organic belt. A xenocrystic core within a zircon grain with an age of 1294±34 Ma, together with tom ages, indicates that the primitive magma was apparently contaminated by Proterozoic crust.