[S-MP25] 変形岩・変成岩とテクトニクス

[SMP25-18] New deformation proxy for alkaline felsic rocks: Na-K zoning of alkali feldspar in syenites from the Wadi Dib alkaline ring complex, Eastern Desert, Egypt

*小澤 一仁¹、ハッサン エマン²、吉田 英人¹、クデｰル アリ² (1.東京大学大学院理学系研究科地球惑星科学専攻、2.アシュート大学地質学部)

キーワード：応力、化学的累帯構造、アルカリ長石、Na-K累帯構造

Deformation of rocks gives rise to diversities not only in microstructures but also chemical heterogeneity on various scales. These records in deformed rocks will conversely provide rich information on deformation conditions, such as strain, stress, and strain rate. Ozawa (1989) shows that Cr-Al zoning in spinel in deformed mantle peridotites formed by a diffusion creep driven by stress (kinetic demixing: Dimos and Kohlstedt 1987). The zoning is characterized by maxima of both Al and Cr on the grain surface and an intermediate value at the grain center. The shape of spinel and the strain predicted from diffusion creep is consistent: the Al maxima are found in the elongate direction and Cr on the shortest direction. The conditions for development of such zoning and morphology are: (1) notably smaller diffusivity of Cr than Al (Suzuki et al., 2008), (2) Cr and Al diffusion controls rate of deformation for given conditions of grain size and temperatures (Suzuki et al., 2008), (3) proper deformation conditions such as temperature, pressure, stress, and strain rate, and (4) deformation took place under lowering temperature to freeze the zoning pattern. If these conditions are fulfilled in other minerals and chemical components, similar chemical zoning in naturally deformed rocks may be expected. There are, however, no indisputable reports so far. We found such chemical zoning in intermediate alkali feldspar in terms of Na and K in fine-grained porphyritic trachyte and syenite from the 580Ma Wadi Dib alkaline ring complex, Eastern Desert, Egypt (Frisch and Abdel-Rahman, 1999). Deformation induced chemical zonings of feldspars have long been reported (e.g., White, 1975; Kramer and Seifert, 1991), but they mostly require involvement of open reaction processes and do not belong to kinetic demixing. The rocks from the Wadi Dib complex are composed dominantly of intermediate alkali feldspar, which underwent recrystallization by pyrometamorphism and deformation to grain size smaller than 200 microns. The above conditions (1) are satisfied because of a large difference in K and Na diffusivity in alkali feldspar by more than two orders of magnitude (Cherniak, 2010). The conditions of (2) - (4) might also be fulfilled because of high temperature and small grain size (Farver and Yund, 1995). In this talk, we will report microstructure and Na-K zoning in alkali feldspar to show that they formed by deformation, and we propose that the zoning may be a useful proxy of deformation.