大同火山群新生代火山岩成因相平衡实验约束

通过常压控制氧逸度熔融相平衡实验结果可探讨一些火山岩岩石学的基本问题及火山岩成因机制问题。在Ｏｌ－Ｄｉ－Ｎｅ三元相图上，不同大地构造环境中产出的碱性玄武岩的Ｏｌ＋ＣＰｘ－Ｐｌ饱和同结线不重合，相同构造环境中产出的碱性玄武岩的Ｏｌ＋ＣＰｘ＋Ｐｌ饱和同结线随ｆｏ＿２升高只是简单地朝Ｏｌ端移动。在ＬｅＢａＳ（１９８６）ＴＡＳ图内火山岩低压结晶作用硅─碱演化趋势呈正斜率并近似与ＴＡＳ图内火山岩系列分界线相近。由低压液相成分演化线与大同火山群岩石化学变异趋势对比，以及高压控制下的结晶作用特点的分析，大同火山群中更新世早期岩浆向中更新世晚期岩浆的演化不能用简单的低压结晶作用解释，而高压控制下的结晶作用可能是主要的控制因素。

Constraints of Phase Equilibrium Experiments on the Origin of Cenozoic Volcanic Rocks in the Datong Volcanic Cluster

Petrology and genetic mechanism of volcanic magma can be studied thro-ugh low pressure experiments.The Ol-Di-Ne tertiary phase diagram shows that the Ol-CPx-Pl saturated cotectic lines of alkaline basalts formed in different geotectonic environments do not coincide with each otber,and Ol-CPx-Pl saturated cotectic lines of alkaline basalts formed in the same tectonic enviro-nment simply move toward the Ol vertex with the increasing oxygen fugacity.The silica-alkali evolution trend of low pressure crystallization fractionation of volcanic magma exhibits positive slope and is approximately parallel to the boundary of volcanic series in the TAS diagram(Le Bas,1986).The 1-atom experimental liquid lines of descent are different from the evolution trend of the natural rocks,implying that low-pressure crystallization fractionation could not be the major mechanism for the magmatic evolution of the Datong volca-nic cluster.It seems that high pressure crystallization fractionation was the major factor controlling the evolution of the volcanic magma from the early stage to the late stage of Middle Pleistocene.