Relationship between the compositions and unit-cell parameters of biotites from the Taiping-Huangshan batholith in southeast China

The Taiping-Huangshan batholith in southeast China is a polygenetic composite granitoid consisting of two parts. The northern part is known as the Taiping granodiorite, genetically designated to the Syntexis-type granitoids of southeast China^1] or I-type^4]. Based on whole-rock Rb-Sr isochron dating, its age is 22 3 m. y. with a relatively low initial Sr ratio (0.7069), suggesting that it was emplaced during the Indosinian period. The southern part, the Huangshan multi-staged granite, is genetically ascribed to the transformation type of southeast China, or S-type, which includes two bodies known as the Huangshan and Shizilin granites. Both were formed in the late Yenshanian period as indicated by their ages: 131 m.y. for the Huangshan granite and 123 m. y. for the Shizilin granite, and high initial Sr ratios (0.714 for the Huangshan granite and 0.718 for the Shizilin granite) determine using whole-rock Rb-Sr isochron dating method. The biotite in the Taiping granodiorite is Mg-biotite with a high Mg/(Mg +ΣFe + Mn +^VIAl³⁺) ratio (> 0.42) in its octahedral layers, and has a longerc-axis (10.220–10.224 Å) and a larger unit-cell volume (499.5 ų), as calculated from X-ray powder diffraction data But the biotites in the Huangshan and Shizilin granies occur mostly as siderophyllite, subsequently as Fe-biotite and occasionally as zoned mica consisting of siderophyllite and musco-vite. The Mg/(Mg +ΣFe + Mn +^VIAl³⁺) ratios are low, while^VIAl³⁺/(Mg+ΣFe+Mn+^VIAl³⁺) ratios high in their octahedral layers. They have a shorterc-axis (10.128–10.196A) and a small nit-cell volume (494.5 ų on average). Comparison of their chemical compositions and corresponding unit-cell parameters for 13 samples from the batholith indicates that thec-axis length increases with increasing Mg content with a noticea ble positive correlation between them, but decreases with increasing^VAl³⁺ content, showing an obvious negative correlation. The changes of theb-axis lengths with the compositions of biotites are much less pronounced than that of thec-axis length.