Abstract: To study the petrogenesis and tectonic setting of Langao mafic volcanic rocks in South Qinling orogenic belt, basalts and volcanic breccias are sampled from Langao County for petrography, major elements, trace elements and REE analysis. The results show that they are classified to alkaline basalts, and characterized by strong differentiation between LREE and HREE, Rb,K depletion relative to Th, Hf and no Nb, Ta, Zr, Ti negative anomalies. All of the samples display highly consistent REE patterns and trace elements spider diagrams with OIB. Petrogenenis studies reveal that they were derived from a congenetic primitive magma which was the product of low-degree partial melting of the spinel-garnet lherzolites and garnet lherzolites. These rocks mainly underwent fractional crystallization of clinopyroxene with minor plagioclases and were not accompanied by contamination of continent crustal materials during their magmatic evolution processes. Tectonic discrimination for mafic volcanics, using trace elements, and regional geological analysis demonstrate that Langao mafic volcanic rocks were most likely formed in the ocean basin plateau or ocean island setting.

Key words: petrochemistry, petrogenesis, ocean island basalt (OIB), South Qinling orogenic belt , alkaline basalts

摘要： 为了研究南秦岭岚皋地区基性火山岩的岩石成因和区域构造背景, 选择岚皋县境内的玄武岩和火山角砾岩进行岩相学、常量元素、微量元素和稀土元素的分析。通过分析发现, 它们同属于碱性玄武岩系列, 轻重稀土分异明显, Rb, K 相对Th, Hf 亏损, 不具 Nb, Ta, Zr, Ti 负异常, 整体表现出与 OIB( 洋岛玄武岩) 高度一致的稀土图谱和微量元素特征。岩石成因分析表明, 它们为同源岩浆演化序列, 其原始岩浆起源于尖晶石-石榴石二辉橄榄岩和石榴石二辉橄榄岩地幔源区的低度部分熔融, 在上升过程中基本未受地壳混染的影响。岩浆演化过程中主要经历了单斜辉石、少量斜长石的分离结晶作用。综合区域地质学和微量元素特征, 该组火山岩最有可能形成于洋盆台地或大洋岛构造环境。

关键词: 岩石地球化学, 岩石成因, 洋岛玄武岩(OIB), 南秦岭, 碱性玄武岩