华北克拉通西部晚第四纪大同火山群火山地质与岩浆成因

North China Craton (NCC), had experienced prominent lithospheric thinning in Mesozonic, is the most seriously damaged craton among the global cratons. The previous study has indicated that the eastern and western part of NCC, segregated by Daxing’anling-Taihangshan Gravity Lineament (DTGL), has the quite different nature probably corresponding to the different geological events and evolution history. Locating in the west region of the NCC, the Datong volcanic group has the special geological setting and belongs to the rare intraplate rift volcanic groups in the loess plateau in the world. The Datong volcanic group is one of the only two late Quaternary volcanic groups in NCC most closing to Beijing. There are a number of volcanic cones in the Datong volcanic field and most of them were kept in good shapes. What’s special is that there is different volcanic landscapes and rock types in eastern and western Datong volcanic field. The eruption and overflow activities of Datong volcanoes are the newest volcanism in the western NCC. Furthermore, the mantle xenoliths entrained by the Datong volcanic rocks will provide important information for lithospheric mantle of the western NCC. The eruption history and magma process of Datong volcanoes is the key ring in the evolution cycle of the Cenozoic NCC due to the special tectonic location and recent seismic activities. This research will play the primary constraints on the petrogenesis of Cenozonic continental alkali and tholeiitic basalts as well as lithospheric evolution in the western NCC.

华北是全球古老克拉通遭受破坏最明显和最典型的地区，中生代以来岩石圈有明显减薄。已有研究表明，被重力梯度带分隔的华北克拉通东、西部岩石圈性质迥异，它们可能对应着不同的地质事件和演化历史。位于大兴安岭－太行山重力梯度带以西、华北克拉通西部的晚第四纪大同火山群构造位置特殊，属世界上少有的黄土高原区板内裂谷系火山群，也是华北克拉通内仅有的离北京最近的两个活动火山群之一。大同火山数目众多，多数火山锥保存完好，并且在不大的范围内东、西两区火山形貌及岩性截然不同，具有独特性，其喷发活动是华北克拉通西部最新的火山活动。本申请通过对大同火山群进行全面野外勘察，了解火山锥形态特征、锥体结构和喷发方式等火山地质特征，研究桑干河玄武岩下伏射气岩浆喷发活动特点与时代，确定火山群喷发活动历史和岩浆作用过程，可以为新生代华北克拉通西部陆内碱性与拉斑玄武岩的成因和岩石圈演化提供重要约束。

野外地质调查确定大同火山群由24座火山组成，火山作用次序为：第四纪伊始存在着大同-阳原古湖，早更新世晚期本区外围开始了少量火山喷发；中更新世时南部大峪口、秋林峪及龙堡村火山相继发生亚碱性拉斑玄武质喷发；接着来自深部岩浆房的碱性玄武质岩浆上升，在本区产生了两种类型的喷发，其一无水中心式喷发，在大同盆地形成众多火山锥；其二遇湖泊水发生少见的射汽岩浆爆炸喷发；之后火山区中东部又发生了大规模拉斑玄武质熔岩的溢流式喷发，向大同盆地溢流并覆盖在射汽喷发物之上。晚更新世古湖最后消失，大同盆地及周围泉水和雨水汇集在一起形成桑干河雏形。东、西两区火山喷发物虽然成分截然不同，但喷发时间上是交互喷发的。.大同火山区依陈庄-许堡断裂分为东、西两区，西区以碱性玄武岩为主，火山锥成群分布；东区以拉斑玄武岩溢流为主，少见火山锥。橄榄石和单斜辉石斑晶是主要的分离结晶矿物。火山岩均相对富集LREE、LILE和HFSE，类似OIB，微量元素研究得出大同玄武岩是石榴石二辉橄榄岩低度部分熔融的结果，碱性玄武岩约为1.5%-3%，拉斑玄武岩为4%-8%，部分熔融程度的不同控制着两种玄武岩中主、微量元素含量的差异。同位素研究表明它们都是以PREMA端元为代表的软流圈地幔低度部分熔融，并有较少比例以EMI端元为代表的古老岩石圈地幔的加入（碱性玄武岩不超过10%，拉斑玄武岩不超过20%），其同位素特征的差异则是岩石圈物质参与比例不同造成的。.本区较大范围的射汽岩浆爆炸喷发物断续出露于桑干河沿岸的大辛庄、乌龙峡及册田水库等地区，出露厚度不等，由火山碎屑、围岩碎屑、地表沉积物碎屑和火山灰胶结物等组成，形态多样。射汽喷发物碎屑颗粒粒径普遍较小，频率直方图显示单峰态，推测其堆积成因相对单一，粒度中值与分选系数成因投点图中样品大都落在射汽和空降的范围内。从产状、碎屑颗粒大小和形貌等方面判断，大辛庄处为近火口相运移堆积而成，乌龙峡及册田水库地区为远火口相空降成因。