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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,Na_2O明显偏高(最高含量为SiO_2 65％的中酸性岩),K_2O明显偏低,Na_2OK_2O。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。
The Early Paleozoic fault-zone volcanic rocks and their formation environment in the northern part of North Qinling Mountains are of great significance to discuss the tectonic evolution of the Qinling orogenic belt. The volcanic rocks are distributed in the Proterozoic Qinling Group and Kuanping group within the fault zone. The lower part is tuffaceous terrigenous clastic rocks. The local residuals are not integrated with the basal layer. The middle fine quartz quartz porphyritic sandwiches are normal sedimentary rocks. The upper part of carbonate rocks produce gastropods, corals and sea lily stem fossils. The volcanic rocks are mainly quartz-angle porphyries, and the small bijins are increased from west to east, with fewer porphyry rocks. Compared with the corresponding normal calc-alkaline volcanic rocks, Na 2 O is obviously higher (the highest content is medium-acidic rock with 65% of SiO 2), K 2 O is obviously lower and Na 2 O K 2 O is lower. The rock series is dual in nature, with two rock series of tholeiite basalts and calcareous rocks. Micro-bijin trace element and rare earth element distribution similar to island arc tholei basalt. The initial strontium isotope ratio (0.706) was significantly higher than that of the upper mantle (0.699). Based on the analysis of geotectonic features, rock assemblages and their geochemical characteristics, the authors believe that the volcanic rocks in this area were formed in the active continental margin rift zone.