INDICATION OF NEPHRITE TEXTURE TO ITS ORIGIN

Microstructure, as visual information, is an important record of metamorphic process and has potential values for the traceability of nephrites. Tremolites in Xiaomeiling nephrite grow in an extensional tectonic setting and crystalloblastic textures without deformations are common, such as fine granular and microgranular crystalloblastic textures. Some coarse-grained subidioblastic and xenoblastic tremolites, which are not completely replaced by microcrystalline tremolites, are distributed in the matrix of Xiaomeiling nephrite. The texture evolution of Xiaomeiling nephrite reveals that the metasomatism is an effective way to achieve the microcrystallization of coarse-grained tremolites. Affected by the metasomatism and tectonism, the pseudomorph and ductile deformations are two typical textures for Maxianshan nephrite. Although most of early coarse-grained tremolites have been replaced by late microcrystalline tremolites, the original external shapes can still be recognized. Due to shear stresses, various intragranular deformations occur in residual coarse-grained tremolites, such as intragranular slip, undulatory extinction, deformation bands and elongated filamentous tails at their edges. Unlike Maxianshan nephrite, Longxi nephrite is the product of syntectonic metasomatism and characterized by the growth lineation of tremolites. The preferential growth direction of acicular tremolites is parallel to the shear direction. There are also some idioblastic coarse-grained tremolites in Longxi nephrite. The textures of three nephrites are obviously different, which can reflect their provenances. On the other hand, the textures of nephrites have a significant impact on their toughness, embodied by the toughness of Longxi nephrite smaller than that of other two nephrites. By contrasting the microstructures of these nephrites, it can be concluded that the ductile deformation plays an important role in the toughness of nephrite. Shear stresses are able to push the filamentous tails to intertwine with those of adjacent coarse-grained tremolites and/or microcrystalline tremolites, which promotes the formation of interlocking texture with high toughness. This phenomenon belongs to the strain strengthening of rocks.