10:55 〜 11:20
[T1-05] Insights into the mineralogical characteristics of Li-enriched metasomatic albitite from the Iwagi islet, SW Japan
「招待講演」
キーワード：Lithium、albitite、metasomatism
The study of metasomatic rocks is essential for comprehending the nature and origin of metasomatic agents. It might provide a clue for fluid circulation within the crust. In the Setouchi Province, metasomatic rocks are distributed along an approximately E–W trend, and these rocks are associated with Late Cretaceous granites. The Iwagi albitite is one such metasomatic rock. However, unlike other metasomatites in the area, its high lithium content (500 ppm) is unique. Detailed investigations of the mineralogical characteristics of Li-minerals have been conducted to better understand the formation and evolution of Iwagi albitites. The albitites exist as small masses, and the textures of the weakly metasomatized ones resemble those of the host adjacent granite. The transition from granite to albitite occurs gradually and can be understood through the mineral assemblages. The Iwagi albitite is known as the type locality of four Li-analog minerals: sugilite KNa2(Fe3+, Mn3+, Al)2Li3Si12O30, katayamalite KLi3Ca7Ti2(SiO3)12(OH)2, murakamiite LiCa2Si3O8(OH), and ferro-ferri-holmquistite □Li2(Fe2+3Fe3+2)Si8O22(OH)2. The former three minerals were found in the fully albitized rock, while the latter was found in the weakly albitized granite. The albitites display a variety of replacement textures due to Na–Li metasomatism, and they also exhibit noticeable strain-induced textures.
The δ7Li values of murakamiite and Li-rich pectolite show a wide range from -9.1 to +0.4‰ (ave. -2.9‰) and should have resulted from hydrothermal fluid-rock interactions at 300−600 °C. The very low δ7Li values may have originated from intra-crystalline Li isotope diffusion or involvement of deep-seated, Li–Na-enriched subduction-zone fluids with low δ7Li values. This finding highlights the significance of fluid-rock interactions in the formation of metasomatic rocks. Deformation-induced fracturing of the rock may have enhanced fluid circulation, leading to the formation of the metasomatic rocks along the E–W trending lineament.
The δ7Li values of murakamiite and Li-rich pectolite show a wide range from -9.1 to +0.4‰ (ave. -2.9‰) and should have resulted from hydrothermal fluid-rock interactions at 300−600 °C. The very low δ7Li values may have originated from intra-crystalline Li isotope diffusion or involvement of deep-seated, Li–Na-enriched subduction-zone fluids with low δ7Li values. This finding highlights the significance of fluid-rock interactions in the formation of metasomatic rocks. Deformation-induced fracturing of the rock may have enhanced fluid circulation, leading to the formation of the metasomatic rocks along the E–W trending lineament.