Petrographic study on the high-grade metamorphic rocks from the Highland and Kadugannawa Complexes, central Sri Lanka

その他

This study focused on texture of the metamorphic rocks from the central region of Sri Lanka，representing both the Highland Complex and the Kadugannawa Complex．Petrographical investigation of some pelitic and intermediate to basic granulites from the central Highland Complex and some pelitic and mafic rocks from the Kadugannawa Complex of Sri Lanka was carried out．
Among the metapelites，garnet-absent and garnet-bearing as well as spinel-absent and spinel-bearing lithologies were studied．Garnet is completely absent from biotite gneiss，while garnet biotite sillimanite gneiss occurs with or without spinel（hercynite）．Garnet contains inclusions of kyanite and sillimanite inclusions，indicating that these rocks have passed the kyanite stability field during the prograde metamorphism．However，the formation process of Zn-rich hercynitic spinel as inclusions in garnet which is free from other mineral inclusions except ilmenite is not clear．
Metamorphosed intermediate to basic rocks include amphibole- and biotite-bearing meta granitoid，charnockitic gneisses and garnet amphibole pyroxene gneiss．Hypersthene is a common mineral in chamockitic gneiss，whereas garnet is absent from some．Rare cpx was found occuring in symplectites with plagioclase in meta granitoid．In contrast garnet contains inclusions of cpx in garnet amphibole pyroxene gneiss，where opx occurs as porphyroblasts．Inclusions of cpx + plagioclase±quartz in garnet amphibole pyroxene gneiss indicate that the rock was once equilibrated in the high pressure granulite field（ > 10 kbar）．
Pelitic gneisses in the Kadugannawa Complex are composed of biotite gneisses．These rocks mainly consist of quartz，K-feldspar，plagioclase and biotite．Mafic gneisses include garnet -bearing and -absent rocks．Garnet-absent rocks include
hornblende gneiss and migmatitic gneiss．No zoning is observed in the Kadugannawa Complex garnets，but prograde evidence is provided by several inclusion phases in garnet．