摘要
本文提供了西秦岭新生代流纹岩的全岩化学、稀土微量元素和Pb、Sr、Nd同位素分析数据,以及新生代双峰式火山岩27个同位素定年结果。研究表明,西秦岭新生代双峰式火山岩具有与东非裂谷完全相同的岩石组合。K/Ar和39Ar/40Ar同位素定年确定新生代双峰式火山岩的年龄从23~7·1Ma。双峰式火山岩中的钾霞橄黄长岩与钾玄岩的87Sr/86Sr在0·704031~0·70525之间,206Pb/204Pb为18·408~19·062,207Pb/204Pb为15·476~15·677,208Pb/204Pb为38·061~39·414,εNd=0·3~5·3,几乎全部为正值,与新特提斯地幔端元地球化学域非常相似,岩浆起源于与OntongJava和FOZO地幔柱相似的亏损地幔源区。由此推测,火山岩的成因与印-亚大陆碰撞诱发的软流圈地幔流向东移动和上涌有关,也是青藏高原向东扩展的一种响应。西秦岭新生代双峰式火山岩的厘定为查明南北构造带的性质提供了岩石探针,它证明南北构造带是一条大陆裂谷。但是,将西秦岭的双峰式火山岩产出的大地构造背景,深部地球物理以及地幔热结构与贝加尔裂谷和东非裂谷的地幔相比较可以看出,南北构造带的裂谷特征不同于东非大陆裂谷,也与贝加尔裂谷不尽相同。南北构造带的裂谷成因与印-亚大陆碰撞诱发的软流圈物质向东的移动和上涌有关,其特征受西秦岭周边各个小块体间相对运动速度与方向以及东昆仑断裂-西秦岭北缘断裂运动性质的联合制约。由此推测,南北构造带是一条复杂的裂谷带,也可能是一个发展中的板块边界。
Presenting the analysis data of whole rock chemistry, REE and trace element and Sr, Nd, Pb isotopes of the rhyolite, and 27 isotopic dating of the Cenozoic bimodal volcanic rocks from West Qinling. The studies showed that the Cenozoic bimodal volcanic rocks like to East Africa rift, consisted of kamafugite, carbonatite, shoshonite, rhyolite and/or trachyte. The age of the bimodal volcanic rocks is from 23Ma to 7.1Ma determined by isotopic dating of K/Ar and 39Ar/40Ar. The 87Sr/86Sr=0.704031~0.70525, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb is 18.408~19.062, 15.476~15.677 and 38.061~39.414 respectively, εNd=0.3~5.3 of the kamafugite and shoshonite in the bimodal volcanic rocks. All of these are similar to the feature of Neo-Tethyan mantle geochemical domain, the magma origined from depleted mantle resources like Onto Java and FOZO mantle plum, suggest the origin and genesis of Cenozoic bimodal volcanic rocks related to northeastward migration and upwelling of the India-Asia collision-deduced mantle flow, and also responses to eastward expanding of the Tibetan plateau. Cenozoic bimodal volcanic rocks in West Qinling providing ideal lithoprobes for understanding N-S trending tectonic belt and support for the N-S trending tectonic belt is a continental rift. However, the rifting of N-S trending tectonic belt is not similar to Baikal rift, and is also not like East Africa rift formed in typical Craton, comparing geological setting, deep-geophysics and thermal-structure of mantle in West Qinling with the mantle beneath Baikal rift and East Africa rift. The genesis of the rift of N-S trending tectonic belt related to northeastward migration and upwelling of the India-Asia collision-deduced asthenosphere flow, the characteristics of the rift may have restrained by East Kunlun faults-north boundary of West Qinling and the move velocity and direction between the blocks. We suggest that the N-S trending tectonic belt is complex tectonic belt and is a developing boundary of plates.