Die autometamorphe retrograde Umwandlung von Biotiten in Chlorite und Muskowite in sauren Tiefengesteinen

Abstract

The autometamorphic transformation of iron-rich biotite to chlorite and muscovite with the by-products hematite, ilmenite, sphene, and rutile has been studied in thin-sections of about 50 granitic rocks of Austria and Bavaria. From 19 granitic rock samples the magmatic biotites, their secondary chlorites, and muscovites were separated, purified by hand-picking, and investigated by optical, x-ray, and chemical methods. The silicatic products of biotite transformation were found to be iron-rich chlorites and iron-poor muscovites. The essentiall process of biotite alteration to chlorite is based in the emigration of 9,5–12, 2 Eq.wt.% Si and nearly the complete alcali. This leak is only compensated by the importation of 23,3–27,6 Eq.wt.% H2O+. The chemical changes of biotite alteration to muscovite are characterized by the emigration of 15–20 Eq.wt.% Fe, Mg, Ti and the importation of 9,3–14,2 Al, 4,6–7,1 Si, and 1,3–4,3 Eq.wt% H2O+. Owing to the transformation of biotite unit cell dimensions considerable changes. As a result of biotite replacement by chlorite c 0·sin β increases about 40%. a 0 and b 0 only extends for 0,8%. In comparison with these facts of chloritisation biotite transformation to muscovite was found to reduce a 0 and b 0 (−2,6%) much stronger than c 0·sin β (−0,7%). Modal analysis data of retrograde alterations of biotite and feldspar in the granites were connected with the observed and calculated values of main element changing. In the granitic samples about 50% of iron and titanium have been converted from silicatic into oxidic minerals like hematite, ilmenite, sphene, and rutile during the biotite transformation. 59–92% of H2O+ in these rocks fall to the share of secondary sheet silicate forming during the latest phases of granite crystallisation. The balance account of the various chemical reactions shows for the closed system a deficit of H2O and Al, and a surplus of Fe, Ti, and F. The deficit or surplus of Si, Mg, Mn, Ca, Na, K, and Li in the samples is very different and dependent on the values of biotite chloritisation, feldspar sericitisation, and so forth. At the present time quantitative control of the complete chemical exchanges during the automorphism of these granites is not possible, because the quartz-feldspar-reactions have not yet been investigated.