Metamorphic layering

E. Merino, P. Ortoleva and P. Strickholm "Kinetics of Metamorphic Layering in... 

From among the many beautiful patterns of mineral distribution, a number have been modeled via equations of phase transition kinetics and transport. Mathematical models for metamorphic layering and stylolites will be presented and their predictions summarized. Nonlinear redox waves and deposition, and porosity instabilities in aquifers are also modeled and discussed. 

Both cases, svegliabile and cognitc, are observed in our theory of metamorphic layering and stylolization to be presented here. Liesegang banding is discussed in the other article by the the author in these proceedings. 

This technique was introduced in [6] and applied there and in [7 ] to the study of the stability to the formation of textural patterns of the uniform state of a rock under stress. This is clearly a rather powerful technique for analyzing these phenomena in random elastic or other media and must be investigated in greater detail to completely unravel all the effects contained in the mechano-chemical coupling that can lead to pattern formation in stressed rocks. In the next two sections we investigate the application of these concepts to metamorphic layering and stylo-lization. 

The basement is made up of crystalline schists of the meso-metamorphic Somes Series. Sedimentation started during Permian with detritic deposits interbedded with rhyolites. The overlying Triassic deposits are unconformable and include detritic formations (Lower Triassic) and massive layers of carbonate rocks (Middle Triassic). The absence of the Upper Triassic is due to the uplift of the region during the Kimmeric tectonic phase. 

The Sambagawa metamorphic belt of central Shikoku is part of the intermediate high-pressure metamorphic belt in Japan. Marbles examined in this study were collected from Iratsu (samples Ta-01 Ta-14) and Tonaru (samples Ta-15 Ta16), as well as and epidote amphibolites masses from each area representing metamorphosed layered gabbro... 

In marbles and skarnoids, the consistent U-Ba-Mo-Pb-V-Zn association and absence of Th, the rare occurence of graphite, and the REE patterns in uraninite are evidences for a possible synsedimentary and/or diagenetic origin of the uranium and associated metals. This primary metal stock located in impure dolostone with possible evaporitic layers (as indicated by the presence of scapolite and kampfite in skarnoids) would have been more or less remoblllzed during regional metamorphism and tectonics. 

Figure 31b. Compositions of chlorites in the mixed-layered mineral facies of pelitic rocks (circles) and from the illite-chlorite facies (barred circles). Shaded area shows chlorite compositions from muscovite-chlorite metamorphic rocks.

Pelitic rocks investigated in the same areas where corrensites are formed during alpine metamorphism (Kiibler, 1970) revealed the absence of both montmorillonite and kaolinite but the illite or mica fraction was well crystallized as evidenced by measurement of the "sharpness" of the (001) mica reflection (Kiibler, 1968). This observation places the upper thermal stability of the expandable and mixed layered trioctahedral mineral assemblages at least 50°C. above their dioctahedral correlevants. This is valid for rocks of decidedly basic compositions where no dioctahedral clay minerals are present. 

Most likely, the chemical system remains closed, as far as the other components in the silicate phases are concerned, as diagenesis or low grade metamorphism becomes more evident. Although there may be transfer of calcium, it seems, from bulk chemical analysis, that there is no systematic increase in potassium nor decrease in sodium content of argillaceous sediments. The transfer of Na and K is between the two size fractions—clay and coarse fraction—or between phyllosilicates and tectosilicates. Albitization of argillaceous rocks should be a common phenomenon where mixed layered phases are predominant in clay assemblages and especially evident in the illite-chlorite zone. 

Talc and Pyrophyllile. Talc and pyrophyllite are 2 1 layer clay minerals having no substitution in eiflier the tetrahedral or octahedral layer. These are electrostatically neutral particles i t = 0) and may be considered ideal 2 1 layer hydrous phyllosilieates. Talc and pyrophyllite are found in metamorphic rocks that arc rich in Mg and Al. respectively... 

In nature the Fe-rich illites (glauconite and celadonite) appear to progress from the lMd to the 1M polytype. The Al-rich illites are predominantly the lMd and 2M varieties. If the 1M polytype is an intermediate phase, it is surprising that it is not more abundant in sediments. Recent studies of unmetamorphosed Precambrian sediments (Reynolds,1963 Maxwell and Hower,1967) have shown that the lMd polytype is relatively abundant in ancient sediments, particularly in the extremely fine fraction. The senior author has noted the relative abundance of the lMd polytype in the fine fraction of most Paleozoic rocks but has considered most of it to be mixedlayered illite-montmorillonite rather than illite. Weaver (1963a), Reynolds (1965), and Maxwell and Hower (1967) have shown that during low-grade metamorphism water is squeezed from the expanded layers and the lMd polytype is transformed into the stable 2M polytype. 

Complete unmixing of the mixed-layered minerals requires temperatures on the order of 400°C or a relatively high degree of metamorphism (phengite -> muscovite + chlorite). Velde (1964), Weaver (1965), Raman and Jackson (1966), and Weaver and Beck (1971a) have presented data to indicate chloritic layers are present in most, if not all, 10A illites. Based on chemical data, Raman and Jackson concluded that the illites they had examined contained 20—29% chlorite layers. Weaver and Beck believe that most of the chloritic layers are dioctahedral. 

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