| Summary: | 碩士 === 國立東華大學 === 自然資源與環境學系 === 101 === The amphibolites exposed in the Fenniaolin area of the northern Tailuko belt show petrographic features of poly-metamorphism (Liou et al., 1981). In the nearby Yuantoushan area, mafic rocks were found as enclaves (probably xenoliths) in metagranitoids. This study discovered that garnet is more common in the enclaves than previously thought. Three types of garnet are differentiated: (1) Type I: with a matrix of biotite, quartz, plagioclase, and epidote; garnet is porphyroclastic and zoned; (2) Type II: with a matrix of calcic amphibole, chlorite, plagioclase, and epidote; garnet is also porphyroclastic and zoned; (3) Type III: with a matrix of calcic amphibole, quartz, feldspar, and epidote; garnet occurs commonly as fine-grained and euhedral aggregates. The Type I zoning implies a cooling process, whereas the Type III records a heating process. The amphiboles in the Fenniaolin amphibolites, in the boundary zone between amphibolites and metagranitoids, and in the Type 3 enclave are mainly magnesiohornblende, but, in the latter two, some amphibole rims are actinolite. Only the Type II enclave contains tschermakite and a high metamorphic temperature of 714 ± 15°C was estimated with the amphibole-plagioclase thermometer of Holland and Blundy (1994). The Type III enclave records a retrograde metamorphic stage in greenschist-facies conditions. All of the studied epidotes show zoning with Fe content increasing from core to rim. The representative one is a clinozoisite core and an epidote rim. Titanite commonly includes rutile and/or ilmenite. These petrographic features indicate a retrograde process. The An (anorthite) content of plagioclase in the Fenniaolin amphibolites decreases from core to rim, which is likely caused by a cooling process, whereas the An content of the plagioclase in the amphibolites at the amphibolite-metagranitoid contact is lower than those of the Fenniaolin amphibolites in average. These petrographic features and mineral compositional trends do not support the hypothesis of ‘contact metamorphism’ by Wintsch et al. (2011).
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