White slate
White slate is a rarely occurring metamorphic rock . Its characteristic feature is a mineral association ( paragenesis ) of talc and thistle . This paragenesis is stable under high pressure amphibolite facial and eclogite facial metamorphic conditions, ie at pressures above approx. 1 GPa (corresponds to a depth of more than approx. 30 km below the surface of the earth) and temperatures of approx. 550 to 850 ° C. Such conditions are realized in nature under low geothermal gradients , e.g. B. in the area of subduction or collision zones .
Mineralogy / rock chemistry
The chemical composition of the minerals in white slate is characterized by the MASH system named after the first letters of the components: they contain MgO - Al 2 O 3 - SiO 2 - H 2 O in varying compositions. Fe 2 O 3 , Mn 2 O 3 , MnO 2 , FeO, MnO, CaO, TiO 2 and others can be contained in smaller amounts . Elements that can be mobilized by fluids (e.g. Na, K) often only occur in low concentrations.
With a progressive (increasing) metamorphosis, a chlorite-quartz-slate is transformed into a talc-thistle-slate (white slate). Typical “late” degradation products that arise from the retrograde metamorphosis of talc and thistle include cordierite , enstatite , chlorite , aluminum-containing anthophyllite and kornerupine . In some cases, the formation of the rare mineral yoderite has been observed, which in addition to the components of the MASH system contains Fe 2 O 3 .
White slate formation takes place under both oxidizing and reducing conditions. Differences in the oxygen - fugacity cause each typical mineralogy under oxidizing conditions occur iron and manganese oxides ( hematite , Bixbyit ), while under reducing conditions, sulfides ( pyrite , pyrrhotite arise).
Parent rock
White slate is mostly formed through metasomatic processes , i.e. through the allochemical conversion of a source rock , whereby the main result is an enrichment of magnesium and a depletion of alkalis . This transformation can take place either before or with the metamorphosis (pre- or synmetamorphic). However, an isochemical conversion of a saponitic bentonite is theoretically also conceivable. In most cases, basaltic rocks were identified as protoliths (parent rock ); In a few cases, however, granitic protoliths or metasomatosis of orthogneiss caused by the circulation of highly saline water in fault zones are assumed . Sedimentary parent rocks (e.g. evaporites or pelites ), on the other hand, are rarer.
Occurrence
Compared to other types of rock, white slates are more exotic rocks. European sites are, for example, in Austria (in the Semmering window of the Eastern Alps ) or in the Simplon tunnel . They are also known from the Dora Maira massif and from Monte Rosa in the western Alps or in Norway . Outside Europe they are known from Sar-e-Sang in Afghanistan , from Mautia Hill in Tanzania , from the Dabie Shan in China , from the Zambezi Belt in Zambia , Zimbabwe and Mozambique, and from Tasmania .
See also
literature
- Timm John: Subduction and continental collision in the Lufilian Arc-Zambesi Belt orogen: A petrological geochemical, and geochronological study of eclogites and whiteschists (Zambia) . 2001 ( Online [PDF; 4.1 MB ] Dissertation at the Faculty of Mathematics and Natural Sciences at the Christian Albrechts University in Kiel ).
- T. John, V. Schenk, K. Mezger & F. Tembo: Timing and PT evolution of whiteschist metamorphism in the Lufilian Arc - Zambesi Belt orogen (Zambia): implications for the assembly of Gondwana . In: The Journal of Geology . tape 112 , 2004, ISSN 0022-1376 , p. 71-90 .
- SP Johnson & GJH Oliver: High fO 2 metasomatism during whiteschist metamorphism, Zambezi Belt, Northern Zimbabwe . In: Journal of Petrology . tape 43 , 2002, ISSN 0022-3530 , p. 271-290 , doi : 10.1093 / petrology / 43.2.271 .
- W. Schreyer: Whiteschists. Their compositions and pressure temperature regimes based on experimental, field, and petrographic evidence . In: Tectonophysics . tape 43 , 1977, ISSN 0040-1951 , pp. 127-144 .