Minerały ciężkie w glinach zwałowych Polski środkowej

Abstract

HEAVY MINERALS IN BOULDER CLAYS OF CENTRAL POLANDSummaryWithin the scope of a complex study on the boulder clays found to occur in the Polish Lowland area, examinations of heavy minerals have been made for the following fractions: 0.5-0.25 mm, 0.25-0.1 mm, 0.1-0.06 mm and 0.06-0.01 mm, as well as contents of opaque minerals, of glauconite, and of transparent minerals have been calculated. As concerns the transparent minerals, a detailed analysis has been made of an assemblage, and the values of weathering coefficient W have been determined to shorten the expression of the results obtained. On the whole, the analyses have been made for 160 samples of boulder clays, various in age. The analyses of heavy minerals, made for four fractions, allowed the authors to determine the variations in the frequency of these minerals, depending upon their gram size. Opaque minerals (mainly iron oxides and hydroxides, pyrite, rarely also ilmenite) make more than 50% of the entire heavy fraction of the boulder clays examined. Their amount considerably increases with the increase in grain size. On the other hand, in the older stratigraphical horizons, the quantity of the minerals rises, independently of the diameter (Table 1). Glauconite, that is most frequently found in fraction 0.25-0.1 mm, demonstrates a changing value, its frequency increasing, as a rule, in the older boulder clays. Amphiboles (mainly green hornblende) are characterized by a regular increase in their quantity with the increase in grain size. A lower frequency of this mineral is characteristic of the older boulder clays. Biotite does not show any correlation between its quantity and grain size, its contents being approximate in all the fraction examined. Chlorite also demonstrates a frequency independent of grain size (the highest frequency has been observed to appear in fractions 0.25-0.1 mm and 0.1-0.06 mm). It should be stressed here that the boulder clay of the oldest glaciations is rich in this mineral. Garnets show a variation like that of amphiboles, i.e. their frequency increases in coarser fractions. On the other hand, no quantitative changes may be observed in stratigraphical profile. Both zircon and rutile are characterized by an inverse distribution of contents, as compared with garnets, i.e. they strongly augment their part in finer and finer fractions. Staurolite differs from other minerals by its considerable concentration in fraction 0.25-0.1 mm. Tourmaline in turn shows an approximate scattering in all the fractions, its greatest concentration appearing in fraction 0.25-0.l mm. Both analysis of quantitative changes in the contents of heavy minerals and comparison of frequency of various values of weathering coefficient W permit us to explain a series of new aspects. So, the quantitative changes in the assemblage of heavy minerals have been ascertained to appear distinctly and regularly in fraction 0.25-0.1 mm. On the contrary, in the extreme fractions the contents at the individual minerals reveal lowest oscillations. In fraction 0.25-0.1 mm, the frequency of distribution of the value of the coefficient W allows us to differentiate the horizons of boulder clays, according to their ages. This cannot, however, be obtained by a comparison of the mean values of the same coefficient. The stratigraphical differentiation is expressed in the assemblage of heavy minerals in boulder clays mainly in the form of an increase in non-resistant minerals in the younger and younger boulder clays, and of a simultaneous decrease in the frequency of glauconite and transparent minerals. These regularities may be observed to occur in the general scheme when this is made for the entire area in study. In the individual sections, however, some different tendencies may be seen, this being an evidence of a regional variation of the heavy mineral assemblage in the boulder clays. All the above quantitative changes in the assemblage of heavy minerals are closely related to those in the character of grain size of the boulder clays considered.