METAMORPHIC FACIES OF ACID ROCKS
酸性岩的变质相

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.