Protoliths and phase petrology of whiteschists

• Published in: Contributions to mineralogy and petrology Vol. 166; no. 1; pp. 255 - 274
• Main Authors: Franz, Leander, Romer, Rolf L., de Capitani, Christian
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01-07-2013; Springer; Springer Nature B.V
• Subjects: Carbon dioxide; Carbonates; Clay minerals; Climatic conditions; Continental crust; Earth and Environmental Science; Earth Sciences; Geology; Marine sediments; Metamorphism; Mineral Resources; Mineralogy; Minerals; Original Paper; Petrology; Phase transitions; Pressure measurement; Rocks; Rocks, Metamorphic; Sedimentary rocks; Spatial distribution; Germany, Berlin; Metasomatism; Hydrothermal alteration; Continental collision; Thermodynamic modeling; Palygorskite; Geochemical constraints; Sepiolite; Whiteschists
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-013-0874-5

Whiteschists appear in numerous high- and ultrahigh-pressure rock suites and are characterized by the mineral assemblage kyanite + talc (+-quartz or coesite). We demonstrate that whiteschist mineral assemblages are well stable up to pressures of more than 4 GPa but may already form at pressures of 0.5 GPa. The formation of whiteschists largely depends on the composition of the protolith, which requires elevated contents of Al and Mg as well as low Fe, Ca, and Na contents, as otherwise chloritoid, amphibole, feldspar, or omphacite are formed instead of kyanite or talc. Furthermore, the stability field of the whiteschist mineral assemblage strongly depends on XCO
2
and fO
2
: already at low values of XCO
2
, CO
2
binds Mg to carbonates strongly reducing the whiteschist stability field, whereas high fO
2
enlarges the stability field and stabilizes yoderite. Thus, the scarcity of whiteschist is not necessarily due to unusual P–T conditions, but to the restricted range of suitable protolith compositions and the spatial distribution of these protoliths: (1) continental sedimentary rocks and (2) hydrothermally and metasomatically altered felsic to mafic rocks. The continental sedimentary rocks that may produce whiteschist mineral assemblages typically have been deposited under arid climatic conditions in closed evaporitic basins and may be restricted to relatively low latitudes. These rocks often contain large amounts of the clay minerals palygorskite and sepiolite. Marine sediments generally do not yield whiteschist mineral assemblages as marine shales commonly have too high iron contents. Sabkha deposits may have too high CO
2
contents. Protoliths of appropriate geochemical composition occur in and on continental crust. Therefore, whiteschist assemblages typically are only found in settings of continental collision or where continental fragments were involved in subduction. Our calculations demonstrate that whiteschists can form by closed-system metamorphism, which implies that the chemical and isotopic composition of these rocks provide constraints on the development of the protoliths.