Microtexture and compositional variation of alkali feldspars from the Kakkonda granitic pluton, northeast Japan: Implications to the formation processes of granitic texture

Abstract

This paper describes textures and chemical compositions of alkali feldspars to get a clue of the cooling history of the Kakkonda granitic pluton. The Kakkonda pluton of tonalite-granodiorite contains two types of alkali feldspar: one is microperthitic alkali feldspar in the lower-temperature granodiorite of 370°C at the shallower depth (sample C11, 2936-2939 m) and the other is non-microperthitic alkali feldspar in the higher-temperature tonalite over 500°C at the deeper depth (sample C13, 3726-3729 m). The two types of alkali feldspars were examined using an electron microprobe analyzer with an attached cathodoluminescence spectrometer system. The former alkali feldspar is spotted due to microscopic pores and several types of inclusions with microperthitic texture, and the latter is clear and featureless under a microscope. Concentric and/or domain Ba zoning patterns with irregular veins were newly found in both the alkali feldspars. The microperthitic texture in the lower-temperature granodiorite contains two types of Ab-rich plagioclase: one is a bead type of smaller size with rather rounded shape, and the other is a flake type of larger size with irregular shape. The appearance of the microperthitic alkali feldspar is quite different from ordinary patch microperthites with turbidity in granitic rocks described to date. Based on the microperthitic textures and Ba-distribution patterns, resorption of primary plagioclase during the growth of interstitial alkali feldspar might contribute to the formation of the microperthitic alkali feldspar in the granodiorite of C11. Barium-zoning patterns are principally magmatic (C13 alkali feldspar), but they were modified during the formation of microperthite (C11 alkali feldspar) at cooling.