警告:不是一个火成岩岩石学家在这里,我更感兴趣的时发生的岩浆一旦他们达到表面比他们可能遇到的所有进程的路上……话虽如此,我认为这与岩浆的来源,是地幔,它本身就是镁铁质。因此,地幔的部分熔融产生的镁铁质岩浆。这些岩浆(区分)[1]更“进化”(即中间或长英质的)。但这些分化过程有局限性。你与分步结晶镁铁质矿物,Mg和Fe silica-enriched融化,离开它,但你永远不会达到100%硅熔体(毕竟镁铁质矿物仍硅酸盐,他们需要一些硅结构)。与地壳同化:大陆地壳平均只有安山(中级)([鲁德尼克1995][2]),甚至与同化的长英质的(如花岗岩)你永远不会达到100%的硅部分融化。起始物料太镁铁质。______更“哲学”的注意,我将增加,尽管没有术语“ultrafelsic”文献中,仍然可以考虑一些岩石ultrafelsic。如果你看看火山岩的化学分类(Le管家et al . 2002][[3]),这四个领域是这样的分离(wt %)二氧化硅含量:-超基性的:SiO _2 < 45 -镁铁质:45美元< SiO _2 < 52 -中间:52美元< SiO _2美元< 63 -长英质的:SiO _2 >美元63 63 wt %以上的一切SiO _2只是美元“长英质的”。 But there are some rhyolites with more than 76 wt% SiO$_2$ (some of Mono Domes for instance, see Table 2 in [Bray et al. 2017][4])! So there is no distinction between a low-silica dacite and a high-silica rhyolite: they all are just "felsic". That doesn't mean they are not different, just that we never bothered making a fifth filed. But those fields are just arbitrary, we could have split the silica spectrum into twelve fields, or just two. [1]: https://en.wikipedia.org/wiki/Igneous_differentiation [2]: https://doi.org/10.1038/378571a0 [3]: https://doi.org/10.1017/CBO9780511535581 [4]: https://doi.org/10.1007/s00445-017-1123-y