Pelitic rocks

In thermal metamorphism, biotite occurs in the clorite-sericite facies as a dispersed phase in the argillitic matrix and is stable up to low-grade cornubianites. In regional metamorphism, biotite is typical of argillitic and pelitic rocks up to the staurolite-garnet facies (biotite, biotite-sericite, biotite-chlorite, and albite-biotite schists, and garnet-staurolite micaschists). 

Herzberg C. T. and Lee S. M. (1977). Fe-Mg cordierite stability in high grade pelitic rocks based on experimental, theoretical, and natural observations. Contrib. Mineral Petrol, 63 175-193. 

Thompson A. B. (1976). Mineral reactions in pelitic rocks, II Calculations of some P-T-X (Fe-Mg). phase relations. Amer. Jour. Set, 276 425-444. 

Fig. 4. Y+Nb vs. Rb (ppm) plot showing that dykes are mature arc rocks derived from the mantle, and from melting of crustal pelitic rocks or their incorporation in the melting process.

Figure 4. Depth-temperature coordinates of apparent kaolinite upper stability in various pelitic rock sequences of Tertiary or younger age in deeply buried sediments. D = Dunoyer de Segonzac (1969) B = Browne and Ellis (1970) M = Muffler and White (1969) P = Perry and Hower (1970) S = Steiner (1968). Arrow indicates that kaolinite is stable to the greatest depth in the sequence.

Thus the "stability sequence" is equivalent to increasing the intensity of weathering or to completing the process of equilibration under conditions of high water content, low concentration of alkali and silica ions and oxidation of iron in a soil profile. In fact the weathering process in a soil horizon sequence is much the same for pelitic rocks in all environments the dominant sequence is repeated with minor variations due to local conditions of pH or efficiency of the oxidation process. 

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.

Two phase assemblages of any of these minerals are known. It should be noted that aluminous phases, such as kaolinite, have never been reported with corrensite neither are sedimentary phyllosilicates such as 7 8 chlorite or glauconite. Non-phyllosilicates in association with corrensite frequently include diagenetic quartz, albite and dolomite. Pelitic rocks, specially associated with those containing corrensite, contain allevardite and fully expanding montmorillonite (dioctahedral). 

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. 

The stability conditions of corrensite then cover the low grade clay mineral facies (near 100°C) and extend well into the calcium zeolite-prehnite, muscovite-chlorite facies. In pelitic rocks the upper limit will be somewhat lower near the illite-chlorite zone. It is evident that composition of a rock governs the occurrence of corrensite. It can be... 

Phase Relations in Pelitic Rocks Composition Diagrams... 

Figure 50. "Facies diagram" for phyllosilicates in pelitic rocks and sediments. Zones I to VI are discussed in the text. M02 = dioctahedral montmorillonites M03 = trioctahedral, fully expandable phases ML =...

FREY (M.), 1970. The step from diagenesis to metamorphism in pelitic rocks during alpine orogenesis. Sedimentology L5, 261-79. 

VELDE (B.), 1968. The effect of chemical reduction on the stability of pyrophyllite and kaolinite in pelitic rocks. Journ. Sed. Petr. 

Loomis T. P. (1972) Contact metamorphism of pelitic rock by the Ronda ultramafic intrusion, southern Spain. Geol. Soc. Am. Butt. 83, 2449-2474. 

Thompson A. B. (1982) Dehydration melting of pelitic rocks and the generation of H20-undersaturated granitic liquids. 

Gharrabi M., Velde B., and Sagon J.-P. (1998) The transformation of ilhte to muscovite in pelitic rocks constraints from X-ray diffraction. Clays Clay Min. 46, 79-88. 

Adams (1986) dated a large suite of samples of fine-grained pelitic rocks (well cleaved slates) from the Swanson and Mackay mountains and from Mt. Passel in the Ford Ranges of MBL by the K-Ar method. These dates in Fig. 15.13a have a distinctly bimodal distribution. The older suite reflects the low-grade metamorphism and deformation of the Swanson Formation during the Late Ordovician. The younger suite records the time when the rocks were heated by the intrusion of Late Devonian granites. Part B of Fig. 15.13 contains the whole-rock K-Ar dates of slates... 

Clay as a rock term Using clay as a rock term may create confusion, because clays are generally unconsolidated while rocks are essentially rigid. Lithification, diagenesis etc. transform clays to pelitic rocks like shale, claystone etc. which are indurate and rigid, not plastic like clays. So clay should not be used as a rock term. The position of clay in rock cycle is shown in Fig. 1.5. 

When the unconsolidated, plastic clays are deposited in a sedimentary basin as pelitic sediments, they are transformed to rigid, brittle pelitic rocks by diagenesis. Claystones and mudstones are non-laminated pelitic rocks, and shales are laminated pelitic rocks. During diagenesis, some new clay minerals may appear through transformation of pre-existing clay minerals and neoformation of some non-clay silicate minerals. 

At high pressme and temperature, pelitic rocks are metamorphosed to form metapelites. Slate, phyllite and schist are produced successively as the grade of metamorphism increases. Clay minerals are not stable in the high pressure and temperature condition of metamorphism and are gradually transformed to other minerals. 

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