Biotyty z granitów strzegomskich

Maria Wichrowska




The mineralogic examinations of biotites are a continuation of the petrologic research on the criteria concerning the differentation of the Strzegom granites. Biotite is one of more important genetic indices of granites, the therefore it has been thoroughly examined both optically, and chemically. Microscope observations of thin slides of granites yielded information the development of the biotite, intergrowths, inclusions, its alteration products etc. Optical examinations allowed the author to calculate the index of refraction and of birefringence. Moreover, absolute weight of some seceded biotite samples was measured, and on the results of chemical analyses of the main components - their composition was determined.
The examinations of the biotites from, the Strzegom granites allow us to draw the following conclusions:

1. Biotites from the Strzoegom granites belong to iron-magnesium micas subdivided into two varieties:

a - Biotite from the Kostrza granites with predominance of the siderophile particle upon the annite one. Due to a considerable amount of iron and a high index of refraction it can he referred to lepidomelane.

b - Biotite from the Chwałków granite with the predominance of the annite particle upon the siderophile one. It is characterized, as compared with the previous ones, by lower indices of refraction, lighter colour, and smaller contents of colouring chemical elements Fe3+ and Ti.

2. Absolute weight of the biotite samples examined amounts to 3,13 g/cm3, on an average.

3. Chemical composition of the minerals distinctly changes according to the cation relationships in the octahedral layer. The biotites from the Chwałków granites and from the biotite-muscovite granites are grouped in one field of M. D. Forster's concentration triangle (Fig. 5), and those from, the Kostrza granites – in the other. The biotites from the Graniczna granites take the intermediate positions between these fields. Thought the biotites from the biotite-muscovite granites do not differ considerably from those of the Chwałków granites in their optical and chemical indices, the problem can be due to a low amount of samples.

4. The values of the index f and the ratio Fe2+ (Mg are higher in the Kostrza biotites, unlike the parameters al and the ratio AlVI/AlIV, which undergo augmentation in the biotites from the Chwałków granites and the biotite-muscovite granites.

The problem of the source of granitoid magma and of its differentiation into granodiorites and monzonite granites of the region of Strzegom has been already many times discussed.
Without any discussion concerning the character of magma and of its origin we can state on the basis of the result obtained that the biotite composition is characteristic of magmatic acid rocks. A. Majorowicz (1972) and S. Kural and T. Morawski (1968) are unanimous and maintain that magma which crystallized in the form of the Kostrza granite was more acid, more fluid and homogenized than this part of the magma which resulted in the production of the Chwałków granodiorites.
The research made demonstrates that the biotites from the Kostrza granite are characterized by a more “acid” composition and, generally, are less differentiated than the Chwałków biotites, as proved by the higher homogeneity of this portion of magma, which crystallized in the form of the Kostrza granite.

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