【作者】 宋子季； 张维吉； 安三元；

【Author】 Song Ziji(Regional Geological Surveying Party,Shaanxi)Zhang Weiji and An Sanyuan(Xi′an College of Geology)

【机构】 陕西省地矿局区调队； 西安地质学院；

【摘要】 北秦岭北部早古生代断陷带火山岩及其形成环境对探讨秦岭造山带的构造演化具有重要意义。该火山岩系分布于元古界秦岭群和宽坪群之间的断陷带内。下部为含凝灰质的陆源碎屑岩,局部残留与基底层呈不整合关系,中部细碧石英角斑岩夹正常沉积岩；上部碳酸盐岩产腹足类、珊瑚及海百合茎等化石。火山岩以石英角斑岩为主,细碧岩自西向东增多,角斑岩较少。与相应的正常成分钙碱性火山岩相比,Na₂O 明显偏离（最高含量为 SiO₂ 65%的中酸性岩）,K₂O 明显偏低,Na₂O》K₂O。岩石系列具有双重性,即出现拉斑玄武岩和钙碱性岩两个岩石系列。细碧岩微量元素及稀土元素分配型式类似岛弧拉斑玄武岩。锶同位素初始比值（0．706）明显高于上地幔（0．699）。综合地质构造特征、岩石组合及其地球化学特征分析,笔者认为本区火山岩形成于活动大陆边缘裂谷带。更多还原

【Abstract】 The present paper deals with volcanic rocks and its environment formed in graben zone of northern part of the North.Qinling Mts.in Early Cenozoic Era. It is important that they are inquired into the tectonic evolution,for orogenic zone there.The volcanic rocks system is distributed among the graben zone of Qin- ling and Kuanping Formation in Proterozoic Earthem.The lower part of the system is tuffaceous clastic rocks from terrigenous and its partial residue is un- conformable with fundamental rock.The middle is spilite-quartz-keratophyre in- tercalated normal sedimentary rodks and the upper carbonatite with Gastropods, Anthozoa and Crinoidea.Quartz-keratophyre is dominant in volcanic rocks and spilite is increased from the west towards the east and keratophyre is a little. The Na₂O content is obvious on the high side （the hignest content being middle- acid rocks of SiO,65%）,the K₂O on the low side,Na₂O》K₂O,as compared with equivalent to calc-alkali volcanic rocks which are normal in composition. The rocks system has such double feature as,occurring tholeiite and calc-al- kali rocks.The pattern distributed of rare elements and REE in spilite is ana- logous to that of tholeiite in island curve.An inital ratio of Sr isotope is 0.706, which is higher than upper mantle（0.699）. By analysis of the feature of geological structure,rocks constitute and their geochemical feature,authors consider that the volcanic rocks were formed from the marginal graben zone of active continent.更多还原