Chemistry of rock‐forming minerals of the Cretaceous‐Paleocene batholith in southwestern Japan and implications for magma genesis

Abstract

Petrographic descriptions and electron microprobe analyses of minerals are presented for 35 specimens from seven suites chosen to examine the transition from magnetite series to ilmenite series granitoids along two transects across the Cretaceous‐Paleocene Inner Zone batholith of southwestern Japan. Regularities in chemical compositions of amphiboles, biotites, and feldspars suggest that fundamentally similar processes produced the magmas that formed the two series. Constant or decreasing Fe/(Fe+Mg) for biotites and amphiboles with increasing host rock silica content, coupled with the absence of early formed magnetite and sphene, suggest that magnetite series rocks may have become oxidized during crystallization near the level of intrusion, through the processes of second boiling and differential loss of hydrogen. For the Daito‐Yokota, magnetite series suite, Fe/(Fe+Mg) for biotites decreased from 0.48 to 0.37 as SiO2 content of the host rock increased from 55.3 to 75.5 wt %; for an ilmenite series suite from the Takanawa Peninsula, Fe/(Fe+Mg) for biotites increased from 0.51 to 0.77 with an increase in host rock SiO2 from 53.4 to 75.5. Detailed consideration of amphibole chemistry shows predominance of edenitic and tschermakitic substitution schemes as well as coupling between substitutions of Ti in octahedral sites and AlIV. Interrelations between amphibole and biotite chemistry show that Fe/(Fe+Mg) and Mn contents can be interpreted in terms of equilibration, whereas Ti content cannot. The chemistry of chlorites correlates well with that of biotites; primary and secondary muscovites are distinct in composition. Plagioclase in all studied suites shows igneous zoning appropriate to host rock composition; perthitic alkali feldspars in all samples have lost albite component, and temperatures based on the two‐feldspar geothermometer are low. The biotite‐apatite geothermometer is also inoperative for this group of samples because fully fluorinated apatites typically occur in biotites of modest F content. Whereas magnetites have reequilibrated, analyses of ilmenites for the representative Daito‐Yokota and Takanawa suites corroborate biotite compositional data and suggest that fO2 probably differed by 2 to 3 orders of magnitude during crystallization of silica‐rich magnetite and ilmenite series granites. Whole‐rock chemistry supports mineral chemistry in suggesting that the studied granitoids have crystallized from magmas generated in a lower crustal environment in which mantle‐derived magmas partially melted source materials with igneous characteristics.

The distribution of Fe and Mg between biotite and amphibole in granitic rocks: Effects of temperature, pressure and amphibole composition.

“…For example , in the presence of key mineral buffering assemblages, biotite Mg/Fe can be quantitative ly related to fo e of crystallization (cf. Wones and Eugster, 1965;Czamanske et al, 1977Czamanske et al, , 1981Ague and Brimhall 1987, 1988a, 1988b, which may in turn be used in evaluating if magmas were derived partially or completely from reduc ing (meta)sedimentary source rock components (cf. Chappell and White, 1974;Ishihara, 1977;Ague and Brimhall, 1988b).…”

Chemical variation in micas from the Cairngorm pluton, Scotland

“…The fluids were able to leave via these fracture systems and re-precipitate in hydrothermal systems overlying the pluton. The fracturing of the pluton roof may also have enhanced oxidation through volatile dissociation (Czamanske and Wones, 1973). All faults in the pluton are characterized by infillings of red, cryptocrystalline silica and patches of intensely hematized and silicified granite.…”

“…The Cairngorm biotites are particularly rich in A1 v~ in comparison with biotites from many other granites (e.g. de Albuquerque, 1973;Barri6re and Cotten, 1979;Czamanske et al, 1981;Dodge et al, 1969), ranging from 0.1 to 2.1 a.f.u. (atoms per formula unit, on an anhydrous basis of 22 oxygens; Table 1).…”

Iron-titanium oxide minerals of Cretaceous to Paleogene volcanic rocks in western Chugoku district, Southwest Japan. Special reference to manganese content of ilmenites.

“…Compositional ranges of ilmenites from Cretaceous granitoids of Ryoke and San'y o zones are ill ustrated by shadowed portions . Data from Tsusue and Ishihara (1974) , Tainosho et al (1979) , Czamanske et al (1981) and this study. fits better into magnetite structure than into ilmenite structure.…”