酸性岩的变质相

在沸石相变质条件下,花岗岩里浊沸石交代了斜长石和石英,在酸性火山岩里产生明矾石、埃洛石或高岭石。经受绿纤石-葡萄石相变质的花岗岩,其中黑云母变为钙铝榴石、帘石、绿纤石和葡萄石集合体,同时斜长石发生绢云母化。绿片岩相内酸性岩的浅色矿物有石英、微斜长石、钠长石和绿帘石,暗色矿物有绿泥石和黑云母。在角闪岩相变质的酸性岩中,开始出现中、基性斜长石,其中暗色矿物黑云母的镁铁比值要大于角闪石的镁铁比值。经受麻粒岩相变质后,紫苏花岗岩的矿物组成没有变化,但有铀、钍和钾的迁出。

METAMORPHIC FACIES OF ACID ROCKS

Granite in zeolite grade has been described in the Qilianshan region, where laumontite replaced plagioclase and quartz. Metamorphased granites with prehnite and pumpellyite show mineral association of quartz, feldspar, epidote and grossular-andradite with some hydrogrossular component. The last mineral was also found in ultramafic rocks of prehnite grade and can serve as an indicator mineral of this grade.Granites in greenschist facies contain quartz, albite, microcline, chlorite, muscovite and biotite. Biotite-granite belongs to middle greenschist facies, whereas chlorite granite, lower greenschist facies. Plagioclase in this grade still shows primary twinning and zoning. It means that the crystal hasn't been completely recrystallized in this condition. In CO2-rich condition biotite changes to chlorite with rutile in the lower greenschist facies, in CO2-poor condition, to chlorite with sphene.Primary hornblende in granite has higher Mg/Fe ratio than primary biotite, as hornblende crystallized at a temperature higher than that of biotite. After metamorphism the Mg/Fe ratio of biotite is higher than that of hornblende, due to equilibrium recrystallization of hornblende with biotite. The TiOa content of muscovite increases with the increase of metamorphic grade. In amphibolite facies the TiO2 content is higher than 1%. meanwhile in greenschist facies it is about 0.5%. The meniral composition of charnack-ite does not change after granulite metamorphism, but K, Th and Uare depleted.