Abstract

Compilation of 155 analyses gives the following average composition of all pelitic rocks in per cent, from low to high grade (standard deviations in parentheses): SiO2, 61.54 (4.68); TiO2, 0.82 (0.61); A12O3, 16.95 (4.21); Fe2O3, 2.36 (1.97); FeO, 3.90 (2.25); MgO, 2.52 (1.91); CaO, 1.76 (2.03); Na2O, 1.84 (1.18); K2O, 3.45 (1.32); H20, 3.47 (2.25); CO2, 1.67 (2.37).

After classifying the analyses into two groups (clays, shales, and slates; phyllites, schists, and gneisses) statistical discrimination tests showed no change in composition during regional metamorphism, beyond loss of H20 and CO2.

The Littleton formation of New Hampshire is a series of pelitic rocks, but is not completely representative of the group, since it forms a sedimentary petrographic province. Thus, the formation is somewhat deficient in CaO, Na2O and CO2, contains more Al2O2 and possibly TiO2, and is in a more reduced state at the low-grade level than the average pelitic rock. It is also more restricted in composition. Moreover this formation underwent minor metasomatism during metamorphism, resulting in the addition of about half a per cent of CaO and Na2O and the loss of H2O, as well as minor element changes.

Comparison of pelitic rocks with the average igneous rocks shows that the alkalies and alkaline earths are relatively concentrated in shales as follows: Li > K > Na; Sr > Ba > Mg > Ca. These relations cannot be explained solely on the basis of relative ionic potentials.

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